Opening Remarks

For Intense Proton Beam Production with Compact Ion Sources: The ALISES Ion Source Family Developed at CEA Saclay

• Olivier Tuske (Université Paris-Saclay, CEA, Département des Accélérateurs, de la Cryogénie et du Magnétisme, 91191, Gif-sur-Yvette, France.)

The production of intense proton beams in CEA Saclay started in the 90’s with the development of the SILHI source to inject the IPHI accelerator. This ECR ion source is still in operation nowadays. ECR plasma is well-known heating process and plasma chamber internal dimensions were investigated in order to reduce size and maintenance time. In 2012 R&D investigations on those simple plasma chamber parameters were started with the design of the first ALISES ion source that will give birth to a new source family that still in progress today. This paper will summarize the different steps of this development.

Performance of Laser Ion Source LION Operated at Brookhaven National Laboratory

• Kazumasa Takahashi (Nagaoka University of Technology)

LION is a laser ion source that has been in operation at Brookhaven National Laboratory (BNL). It is the first laser ion source to supply ion beams stably for users at a large accelerator facility in the world. LION is located at the upstream end of the heavy ion accelerator complex at BNL and supplies singly charged ion beams of various ion species. LION has been in operation from 2014 to 2023 and is planned to be upgraded. This presentation summarizes the operational performance achieved by LION.

Visible Camera-Based Diagnostic to Study Negative Ion Beam Profiles in ROBIN Ion Source

• Sidharth Kumar Dash (1.Institute for Plasma Research, Bhat, Gandhinagar, Gujarat 382428, India 2. Homi Bhabha National Institute, Anushaktinagar, Mumbai, Maharashtra 400094, India.)

The ion beam divergence & uniformity/homogeneity apart from beam energy are two crucial parameters of a large-size ion source-based NBI (Neutral Beam Injector) for the physical interpretations of beam dynamics & control of its extraction system. Characterization of $H^-$ ion beam in ROBIN (Rf Operated Beam source in India for Negative ion) source using two orthogonally placed cameras installed at the top & a side port of the beamline, at an axial distance $\sim1.42$m & $\sim1.90$m from the grounded grid (GG) respectively are presented in this report. As the beam particles travel through the background gas, it emits photons in the visible range due to the production of excited species. The cameras characterize the beam profile based on the photon intensity level that is proportional to the beam current density. The ROBIN ion source with masked large area grid (LAG) extraction system, having only $146$ open beamlet apertures dispersed equally on two grid segments for beam extraction. The grid segments are inclined by $0.873^{\circ}$ from the vertical plane towards the forward direction. The exposed area of the grid segments focuses the beam vertically at a distance of $\sim3$m. The initial observations through lateral camera at $1.9$m shows: individual beamlet identities are lost & each segment forms a Gaussian-type beam profile which eventually merges into a super-beam with a nearly flat-top profile with Gaussian wings at the edges. The shape of the super-beam depends on the mechanical beam focussing & the space-charge blowing-up effect. The estimation of beam uniformity/homogeneity & divergence is complex & so a beam modeling activity is initiated. The total beam profile is obtained by integrating all the beamlet profiles over the whole extraction area. The correlation between the measurements & simulation of the beam is highlighted in the report. Tomographic analysis using camera data & analytical models is the future scope of action.

Production and Pre-Acceleration of Intense Light Ion Beams Based on Laser Ion Sources

• Huanyu Zhao (Institute of Modern Physics, Chinese Academ of Sciences)

The capability of laser ion sources to produce high intensity short pulse ion beams, especially from solid element, makes them promising pre-injectors for some application facilities, such as cancer therapy facilities and accelerator-based neutron sources. Aiming at these applications, some specific ions, like carbon, lithium ions, have been produced and optimized in terms of the high charge state yields and the repeatability of the ion pulses with the laser ion source at the Institute of Modern physics in the past years. Specially, the capability of 50-hour operation of the carbon ion beams have been demonstrate by the laser ion source, with the pulse-to-pulse repeatability within 15% for the main pulse parameters, the total charge, peak current and pulse duration. Furthermore, the intense C6+ ions produced by the laser ion source was accelerated by a radio frequency quadrupole (RFQ) linac based on the Direct Plasma Injection Scheme (DPIS) to the energy 586 keV/u. The peak current at the exit of the RFQ and after the dipole magnet achieved 27 and 13 emA, respectively.

Feasibility Study of High Intensity Lithium Beam Production for Directional Pulsed Neutron Flux Generation

• Masahiro Okamura (Brookhaven National Laboratory)

A compact accelerator-driven neutron generator with a lithium beam driver can generate neutrons in a forward direction, even using incident beam energy at a near-threshold energy. Especially for medical and industrial applications, the ability to suppress unwanted radiation to patients is a major advantage. However, it is difficult to supply a high-intensity lithium-ion beam, and its practical application has been considered impossible. Therefore, to solve the most important issue, the lack of ion flux, a direct plasma injection method was adopted. In this method, pulsed high-density plasma from a metallic lithium foil generated by laser ablation is efficiently injected and accelerated by a radio-frequency quadrupole linear accelerator (RFQ linac). As a result, a peak beam current of 35 mA, which is higher than that of conventional ion sources, can be obtained. This demonstrated the feasibility of constructing a neutron generator using inverse kinematics scenario.

Evolution of Transverse Phase-Space Distribution of Highly Charged Heavy Ion Beams through LEBT

• Takashi Nagatomo (RIKEN Nishina Center)

We studied how the transverse phase-space distribution (PSD) of heavy-ion beams extracted from an ECR ion source (ECRIS) changes as a function of the extraction current $I_{\rm ext}$ through the LEBT. Heavy ion beams produced by ECRIS are distributed over a certain range of charge, so that the total current extracted from the ion source $I_{\rm ext}$ is an order of magnitude larger than that of the target ion beam. Therefore, when aiming for higher intensity ion beams than the now, the $I_{\rm ext}$ will also increase, and the emittance increase due to the space-charge effect (SCE) of $I_{\rm ext}$ is expected to become a serious problem. From the view point, it is necessary to clarify how the transverse PSD changes with changes in the extraction current $I_{\rm ext}$. The emittance of Ar beams with several charges extracted from the 28-GHz superconducting ECRIS at the RIKEN was measured with a pepper-pot type emittance meter installed after the magnetic analyzer. The Ar beam was tuned to a few $10~\mu {\rm A}$ to avoid significant SCE of its own. The $I_{\rm ext}$ was varied from 1.4 mA to 7.0 mA, mainly by adjusting the amount of N$_2$-support gas and microwave power for ECR heating. It is found that the $x$ and $y$ emittances of Ar$^{10+,11+,13+}$ increase with increasing values of $I_{\rm ext}$. Furthermore, it is found that the $x$-$y$ distribution at the analyzing slit does not spread in a similar shape. These Ar beams, which are annularly distributed (hollow beam) up to $I_{\rm ext}$ ~ 2 mA, but from $I_{\rm ext}$ ~ 3mA, are gradually concentrated in a few localized spots. Finally, at 7 mA, the beams no longer have annular structure. These changes in the beam distribution are not distributions that can be simply predicted from the beam spread due to SCE only. We will discuss whether the combination of the SCE and aberrations during LEBT transport can explain these changes in the transverse PSD, or whether we need to include phenomena inside the ECRIS.

Roadmap for the Increase of Beam Brilliance from ECRIS and Microwave Discharge Ion Sources

• Lorenzo Neri (INFN)

The requirements for future accelerator chains need to increase the injected beam brilliance significantly, still keeping high the beam quality in terms of reliability, reproducibility and stability. A roadmap for ion source development may consist of several steps: plasma simulation, multiphysics simulation of each system component, high-level control system, plasma characterisation, beam characterisation, data analysis and, again, plasma simulation. The cycle starts and ends with plasma simulation because it is the instrument that shows how different phenomena take part in the plasma and beam formation and because, in such a way, the accuracy grows with each cycle. Commercial multiphysics simulation tools are essential for adequately designing all ion source equipment: magnets, intense electrostatic field regions, microwave propagation and coupling, thermal dissipation and vacuum. The dependence of source performances from source parameters (magnetic field profile, gas pressure, microwave power) has been widely investigated using a high-level control system able to test tens of thousands of source configurations without human interaction. This kind of characterisation allowed us to identify a new magnetic configuration, High Stability Microwave Discharge Ion Sources, that produces a beam with high stability, intensity and brilliance. The plasma simulation tool we developed discloses the role of two types of electrostatic waves in plasma formation and their correlation to stability. The simulation provides a complete view of ions and electrons energy and density distributions, the formation of the plasma meniscus and the beam extraction. The paper will present the results obtained with this development procedure on Microwave Discharge Ion Sources and how we started to apply it to the Electron Cyclotron Resonance Ion Sources development.

The Child Langmuir Illusion

• Dan Faircloth (ISIS-RAL-STFC-UKRI)

It is a commonly held belief that the beam current extracted from an ion source varies with the applied extraction voltage with a V^(3/2) power law as defined by the space charge limited Child Langmuir equation. However, recent experiments and modelling work have shown that the reason for the apparent V^(3/2) relationship is not caused by space charge limited extraction, but instead the experimentally observed power law is caused by changes in the shape of the plasma meniscus and collimation on the extraction electrode. At lower extraction voltages the measured beam current is divergence limited. Due to computational limitations previous attempts to model this effect either rely on analytical equations to represent the plasma (e.g IBSimu), or they rely on combining a Particle in Cell (PIC) model of the plasma with a beam tracing model to track the beam through the extraction gap. Combining models or relying on analytical equations always leave questions about the reliability of the overall results. Here we present a single PIC model that is capable of modelling both the plasma meniscus and beam transport in the extraction gap in a single model using a variable density mesh in VSim. A single PIC model for extraction and beam transport provides strong proof that the origin of the V^(3/2) relationship for plasma ion sources is caused by meniscus focussing and collimation, not by space charge limited extraction. Excellent correlation between the PIC model, IBSimu and experiment is shown for low current Ar+ beams. The maximum beam current and plasma densities that are currently computationally feasible to simulate are investigated for 2D3V Axisymmetric and full 3D variable density mesh PIC models in VSim.

Photo-Assisted Negative Ion Production in Caesium Sputter Ion Source

• Akbar Hossain (University of Jyväskylä)

The production of negative ions in caesium sputter ion sources occurs on the surface of a cathode, which contains the ionized material and is covered by a thin layer of caesium that enhances the negative ion yield by lowering the work function. We have recently demonstrated that the negative ion beam currents can be enhanced by exposing the ion source cathode to a laser beam [1-3]. In this paper, we present new results on the laser-assisted enhancement of Br$^-$ beam current. These results have been obtained by systematically varying the ion source parameters, most importantly the ionizer and Cs oven temperatures, and measuring the effect of a 15 W, 445 nm laser on the beam current with different cathode holder materials (Al, Ti, Ni, C or Cu). We discuss two findings that are relevant for the ion source operation. First, it is shown that the laser allows reaching higher beam currents at low ionizer temperatures compared to running the source without the laser but at higher ionizer temperature. Second, we sometimes observe a "priming effect", i.e. applying a number of laser pulses at certain ion source settings causes the beam current measured without the laser to increase significantly and remain high after ceasing the laser pulsing. These observations suggest that the photo-assisted effect is related to changes of the cathode caesium coverage. Finally, we report the results of our first attempts to sustain a stable beam current at the elevated level, achieved through active control of the laser power. This last experiment is relevant for applications of the caesium sputter source, e.g. in accelerator mass spectrometry. REFERENCES: 1. Tarvainen O et al 2020 J. Appl. Phys. 128 094903 2. Tarvainen O et al 2021 AIP Conf. Proc. 2373 020001 3. A Hossain et al 2022 J. Phys. D: Appl. Phys. 55 445202

3D Self-Consistent Full Wave – PIC Models for Investigating Space-Resolved ECR Plasma Properties

• Bharat Mishra (INFN-LNS and University of Catania)

Simulations are a powerful method to study the correlation between output beams and internal dynamics of ECR plasmas, which involve a complex interplay between injected power, RF frequency, gas type and pressure. Modelling the microscopic properties of such plasmas is essential not only for fundamental research into the operation of ECR ion sources but also for applications like the PANDORA facility which aims to utilise the stellar-like laboratory plasma to measure in-plasma $\beta$-decay rates. We present here some details on a 3D full wave-PIC code suite originally developed to model electron dynamics self-consistently with EM field propagation in the plasma under the cold electron approximation, but now extended to include ion dynamics as well. The coupled PIC codes have been updated to include stepwise ionisation, self-generated potential dip, and atomic excitation and can now furnish space-resolved information on charge density, energy and charge state distribution (CSD). The models are currently being upgraded to include thermal plasma effects through evaluation of the hot plasma dielectric tensor to first order in temperature, thus including collisionless wave damping mechanisms. Furthermore, efforts are also underway to include collisional excitation and spontaneous emission in ions to obtain atomic level distributions along with CSD. Preliminary runs of the simulation show an encouraging match on comparison with experimental data and offer several perspectives for future improvements.

Plasma Homogeneity over One Extraction Beamlet Group at the Half Size ITER Negative Ion Source at ELISE Test Facility

• Dimitar Yordanov (Max Planck Institute for Plasma Physics)

The ion source for neutral beam injection at ITER has to provide high intensity and low divergence negative hydrogen and deuterium ion beam. Extracting the negative ions from the plasma is inevitably accompanied by co-extraction of electrons, which can limit the source performance, especially in deuterium. Reducing the co-extracted electrons is done by applying a magnetic filter field and a positive bias on the first grid of the extraction system (plasma grid) and on a window-frame plate in front of the plasma grid (bias plate) with respect to the source walls. On the one hand, the filter field reduces the electron temperature and the amount of co-extracted electrons, on the other hand, it introduces **E** x **B** drift. This drift creates a vertical plasma inhomogeneity in front of the extraction area and consequently a strong inhomogeneity of the co-extracted electrons. Vertical plasma inhomogeneity close to the extraction area may have an impact on the extracted negative ion beam uniformity. This motivated detailed studies on the plasma inhomogeneity of a beamlet group in the half-ITER-sized ELISE ion source using a movable Langmuir probe. The investigations are done in deuterium at different filter field configurations and with different biases on the plasma grid and on the bias plate. It is demonstrated that the vertical distribution of the plasma potential changes together with the plasma grid sheath – from repelling to attracting sheath. A repelling sheath reduces the flux of electrons from the plasma towards the PG surface and consequently increases the co-extracted electron current, while the surface-produced negative ions are accelerated towards the plasma volume. The attracting sheath collects the electrons on the grid and reduces the flux of negative ions leaving the grid surface and moving towards the plasma volume.

Production of Intense Vanadium Ion Beam for Super-Heavy Element Research Experiments

• Yoshihide Higurashi (RIKEN)

Recently, production of highly charged medium mass heavy ion beams, such as Titanium (Ti), Vanadium (V), and Chromium (Cr) ions, are strongly demanded for the synthesis of new elements. At RIKEN, especially, production of intense 51V13+ ion beam for long term was strongly required for synthesis of the new element (Z=119). For this purpose, we constructed new superconducting electron cyclotron resonance (ECR) ion source and produced intense 51V13+ ion beam. For production of intense beam effectively, optimization of both consumption rate of the material and microwave power are very important. Therefore, we carefully studied the effect of not only microwave power, but also material consumption rate on the beam intensity systematically. Consequently, we produced about 1 emA of V13+ ion beam at the injected microwave power of about 3.5 kW at the extraction voltage of 12.6 kV. Additionally, to optimize the transmission efficiency of the beam in the accelerator, we studied the emittance size under the various conditions. In these test experiments, we observed that the emittance of V13+ ion beam was strongly dependent on the extraction current. It may be due to the space charge effect. In this paper, we report the experimental results of V13+ ion beam production under the various conditions (support gas pressure, consumption rate, microwave power) and the emittance measurements.

Announcements

Innovative Cesiation Deriving Incredible 145 mA Beam from J-PARC Cesiated RF-Driven Hˉ Ion Source

• Akira Ueno (Japan Atomic Energy Agency (JAEA))

In NIBS2022, the stable 8-hour operation of the J-PARC cesiated RF-driven Hˉ ion source in a test-stand with a 69.9 keV 120 mA beam and a beam duty factor of 4 % (1 ms x 40 Hz) was reported. However, the Cesiation condition was produced after many times and rather large amount of Cesium and H2Os injections. The necessary plasma electrode temperature (TPE) of 254 ${}^\circ$C was also much higher than those not only for the J-PARC source (about 70 ${}^\circ$C) but also for the standard cesiated Hˉ ion sources (180 ~ 200 ${}^\circ$C). In this paper, the novel Cesiation procedure (how to inject H2Os and Cesium), which reproduces the Cesium effects lasting not only for the high TPE but also for the high-density plasma bombardments, is presented. The innovative Cesiation derived a 76.5 keV 145 mA beam from the J-PARC source in the test-stand. The measured results of the 145 mA beam, extraction electrode current and RF waveforms, parameter trends of an 8-hour 145 mA operation and the transverse emittances are also presented. The available Hˉ ion intensity for the J-PARC source operation energy of 52.5 keV was increased from 72 mA to 83 mA, which was consistent with the 1.5 power law on the beam energy compared with 145 mA for 76.5 keV.

Physics and Engineering Design of the 500 keV Beam Source for the BEST Neutral Beam Injector

• Jianglong Wei (Institute of Plasma Physics, Chinese Academy of Sciences)

Burning plasma Experimental Superconducting Tokamak (BEST) will be a new magnetic confinement fusion device (major radius ~3.6 m, minor radius ~1.1 m, plasma current <7 MA, toroidal field <6.1 T) located at Hefei, China. BEST aims to research and develop the physics and technology of the fusion power to generate electricity. Neutral beam injection is one of the auxiliary heating and current drive systems for the BEST to ignite and sustain the D-T burning plasma. The BEST NBI system is now under final design, which has one injector with one beam source. The BEST beam source is required to generate a 500 keV and 20 A beam of deuterium negative ions for the first phase (800 keV and 31.25 A for the second phase). This paper presents the important details of the physics and engineering design of the 500 keV BEST beam source, which mainly consists of a four-driver RF plasma source and a three-stage electrostatic accelerator. The concepts, structures and parameters of the design are determined and supported via a series of numerical analyses, experimental activities, and also the R&D experience of the negative ion beam source worldwide.

Work Function of the Caesiated Converter Surface at the BATMAN Upgrade H- Ion Source at Different Source Parameters

• Adrian Heiler (Max-Planck-Institut für Plasmaphysik (IPP))

Negative hydrogen ion sources for neutral beam injection (NBI) systems for nuclear fusion experiments rely on the surface production of negative hydrogen ions on a low work function converter surface. The state-of-the-art technique for the generation of low work function surfaces is steady evaporation of the alkali metal Cs into the ion source. As the Cs layers are affected by residual gases from the background pressure (typically $10^{-7}–10^{-6}$ mbar) during vacuum phases as well as by reactive hydrogen particles and energetic photons during plasma phases, non-pure Cs layers are present and are subject to temporal dynamics. In consequence, the achievement of a stable and reliably good ion source performance (i.e., high extracted ion current and technically manageable co-extracted electron current) is challenging and in particular an issue for long pulse operation (1 h required in the case of ITER). To control the work function and get insight into temporal dynamics in different operational scenarios, a work function diagnostic has been developed that is suitable for harsh ion source environments. High power fiber-coupled LEDs are used to irradiate the surface with different photon energies in vacuum phases between plasma pulses, and the resulting photocurrents are measured to evaluate the absolute work function according to the Fowler method. The diagnostic is successfully benchmarked at a dedicated laboratory experiment and is applied at the BATMAN Upgrade test bed at IPP, which is equipped with an ion source 1/8 of the size of the ITER NBI source and has recently been upgraded for long pulse operation. It is shown that the work function is subject to pronounced temporal dynamics and is far below the one of bulk Cs in a well-conditioned source. Investigations are performed both in H$_2$ and D$_2$ for operational scenarios with different pulse lengths to identify correlations between the work function, parameters such as the Cs density and ion source performance.

Optimizing the ITER NBI ion source by dedicated RF driver test stand

• Isabella Mario (Consorzio RFX (CNR, ENEA, INFN, UNIPD, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy)

The experimental fusion reactor ITER will feature two heating neutral beam injectors (NBI) capable of delivering 33(50) MW of power into the plasma. NBI consists of a plasma source for production of negative ions (extracted negative ion current up to $\mathrm{329\,A/m^2}$ in H and $\mathrm{285\,A/m^2}$ in D) then accelerated up to 1 MeV for one hour. The negative ion beam is neutralized, and the residual ions are electrostatically removed before injection. The beam divergence has to be 3 to 7 mrad. The ion source in ITER NBIs relies on RF-driven, Inductively-Coupled Plasmas (ICP), based on the prototypes developed at IPP Garching; RF-driven negative-ion beam sources have never been employed in fusion devices up to now. Compared to filament sources, beams extracted from RF sources exhibit larger divergence, which might result in unexpected heat loads over the accelerator grids. Previous studies verified that such beam features are not due to the electrostatic grids shape, so it is believed plasma differences between the two source types play a role. Recent results of SPIDER, the full size ITER NBI ion source operating at NBTF in Consorzio RFX, Padova, highlighted non-uniformities, which are then projected into the properties of beam. One RF driver, identical to the ones used in SPIDER, installed in a relatively small-scale experimental set-up, more flexible than large devices, is starting operations to carry out experiments on the properties of RF-generated plasmas, to contribute to the assessment of negative ion precursors, and their relationship with the plasma parameters, particularly when enhancing plasma confinement. The scientific questions that have arisen from SPIDER operation guided the design of the test stand, which is described in this contribution together with the diagnostic systems and related simulation tools. The test stand will also allow testing technological developments and optimised engineering solutions related to the ICP design at the NBTF.

Plasma Properties in Giant Negative Ion Sources for Fusion

• Emanuele Sartori (Consorzio RFX - Università degli Studi di Padova)

Giant negative ion sources are used for neutral beam injectors in fusion devices. A high density of cold negative hydrogen ions is required over the large extraction area of the caesium-seeded plasma source, to provide the required negative ion current, distributed uniformly over thousands of extraction apertures. In this regard, it is expected that the expansion of plasma and neutrals from the driver region provides as uniform as possible plasma properties at the extraction region, for adequate compensation of the space charge of such large negative ion density, and relatively slow precursors for the negative ion conversion at caesiated surfaces. These conditions are difficult to achieve in the presence of the transverse magnetic field necessary to filter the diffusion of electrons to the extraction region. The driver region can be either a large volume multi-cusp filament-arc plasma, or an inductively-coupled plasma discharge realised in multiple drivers with external radiofrequency antennas: neutral beams based on filament sources for negative ions reached impressive performances in the recent decades, and an intense development program is in progress for the rf-driven source plasma to bridge the gap in view of the ITER neutral beam injector. The optimization of the ITER beam source plasma, aiming at extracting 350/290 A/m$^2$ of H$^–$/D$^–$ with low-divergence at the low filling pressure of 0.3 Pa, is challenging. A review of the ITER beam source physics is provided, based on experimental measurements obtained until now also on the one-to-one prototype SPIDER, and on results of numerical models. This is in the line of the massive work done until now towards the development of negative ion sources, based on both filament arc and rf sources. An overview of the ongoing R&D physics program for SPIDER is also proposed and results of experiments performed at other test facilities are presented.

First Results from the Hybrid RF-FA Ion Source at NIFS

• Katsuyoshi Tsumori (National Institute for Fusion Science)

Reduction of beamlet divergence angle is one of the most challenging issues in negative ion sources driven with Radio-Frequency (RF). Minimum divergence of the beamlet accelerated from Filament-Arc (FA) driven source is ~5 mrad, while that from RF driven source is ~12 mrad, which is larger than the beamlet divergences of 7 mrad required for ITER HNB and DNB. The reason why the beamlet divergences are different in FA and RF discharges is not clear so far. To investigate the reason, a RF system was added to the present FA system at the NIFS NBI Test Stand (NIFS NBTS) and a FA-RF hybrid system, which is available to compare the beamlet characteristics using the same geometric, magnetic configuration and accelerator, was composed; (oscillator) and several components needed for the RF plasma generation (RF-driver, Faraday Shield, coil, etc) have been provided by IPP Garching. The ion source installed at the NBTS is modified to a FA-RF hybrid source by replacing the backplate to attach the RF driver on it. The NIFS NBTS equips several diagnostic devices to measure the source plasmas and accelerated beamlets and has a possibility to reveal the reason why the beamlet divergence is so different in the FA and RF driven modes. In the presentation we are going to discuss the characteristics of the RF generator, matching box, distribution of the plasma parameters such as electron density, temperature and plasma potential, comparison of beamlet profiles in FA and RF modes and COMSOL simulation results. •The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

Towards ITER-Relevant CW Extraction at Negative Ion Sources for Fusion

• Dirk Wünderlich (Max-Planck-Institut für Plasmaphysik)

Negative hydrogen or deuterium ion sources for neutral beam injection (NBI) systems used at fusion devices are based on the surface production process at a caesiated low work function converter surface. The ITER NBI system will deliver a large beam (≈2 m × 1 m, extraction area ≈0.2 m$^2$) with an accelerated current of 40 A negative deuterium ions (46 A in hydrogen) with a high homogeneity, low divergence and being stable over one hour. While producing a stable and homogeneous negative ion beam is not an issue, during long pulses typically a significant increase in the co-extracted electrons is observed, limiting the pulse length or the achievable performance. This effect is particularly pronounced in deuterium and it is attributed to an increasing work function of the converter surface. A major and still open challenge is to develop conditioning techniques providing a homogeneous and stable low work function of the converter surface. In the last years the negative ion source test facilities at IPP Garching, BATMAN Upgrade (using the small prototype source) and ELISE (using a source of the same width but only half the height of the ITER NBI source) have been converted into CW machines. While BATMAN Upgrade focusses on basic physics investigations, aim of ELISE is to demonstrate that the ITER targets can be achieved by ion sources with an ITER-relevant size. The contribution presents the status of both test facilities as well as latest results achieved during long pulse operation in hydrogen and deuterium. Investigations are performed on homogenizing and stabilizing the co-extracted electrons by either actively modifying plasma properties close to the converter surface or by modifying the flux of Cs onto the converter. For example, vertical plasma asymmetries, caused by plasma drifts, can be affected by biasing surfaces or introducing additional surfaces into the plasma and Cs can be evaporated in direct vicinity to the converter for a more controllable caesiation.

Beam Divergence of RF Negative Ion Sources for Fusion

• Christian Wimmer (Max-Planck-Institut f. Plasmaphysik)

Neutral beam injectors (NBI) for fusion facilities have strict requirements on the beam divergence (7 mrad for the ITER NBI at 1 MeV). Measurements of the single beamlet divergence of RF negative ion sources (at lower beam energy < 100 keV) show significantly higher values (12-15 mrad), also larger than filament arc sources at similar beam energies. This opened up questions whether the higher divergence is a problem at all after full acceleration, whether it is caused by different measurement or evaluation techniques, or whether it is a direct cause of the RF source, e.g. due to a higher temperature of negative ions. In a joint effort modeling and diagnostic capabilities at NNBI test facilities have been massively extended and evaluation methods benchmarked. Particularly challenging is the strong increase in beamlet divergence at a lower filling pressure, seen both in filament arc and RF sources. More energetic negative ions in the source at lower filling pressure might be the reason, hints given by beam simulations [1]. Beside the source and beam investigations carried out in SPIDER (with selected, isolated apertures rather than the total of 1280 apertures) at Consorzio RFX [2], the IPP test facilities ELISE (640 apertures) and BATMAN Upgrade (70 apertures) contribute to the physics understanding of the beam optics in RF sources. ELISE is capable to determine beam properties and uniformity on a global scale. BATMAN Upgrade offers an extended set of diagnostics for measuring and correlating the single beamlet divergence to fluxes and energy distributions of the parent particles atomic hydrogen and positive ions in the source plasma. This contribution summarizes the present beam divergence understanding gained from experimental measurements and beam optics simulations. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. [1] N. den Harder et al., contribution to the IAEA FEC 2023 [2] E. Sartori et al., this conference

Recent Research and Development of RF-Driven H- Sources at CSNS (Negative Ion Sources and Sources for Fusion Facilities)

• Weidong Chen (Institute of high energy physics, Chinese academy of sciences (CAS))

The RF-driven ion source has been put into commissioning on China spallation neutron source (CSNS) accelerator since September of 2021. It has a service life time of more than 310 days and availability of almost 100%. To fully meet the requirements of CSNS project phase-II (CSNS-II), the beam intensity should be enhanced and the transverse emittance should be minimized. This report covers the recent research and development of the RF-driven H$^-$ source, including the impurities elimination from the hydrogen plasma, the transverse emittance optimization, and space charge compensation study. A new test bench consisting of an ion source and an LEBT is constructed to carry out these measurements and research. A featured function of the LEBT is the electrostatic beam chopping. The influence of chopping electric field to the space charge compensation is also experimentally studied.

Recent Advancements in the H- Injector Performance for the Spallation Neutron Source Operation and Upgrade

• Baoxi Han (Oak Ridge National Laboratory)

The Spallation Neutron Source (SNS) located at the Oak Ridge National Laboratory is an accelerator-based, pulsed neutron scattering facility utilized for a broad range of scientific research applications. For the past decade, the facility has operated at its original design beam power of 1.4 MW, and it is currently undergoing an upgrade to double its power to 2.8 MW within the next several years. A 65-keV H- injector, which comprises an rf-driven H- ion source and an electrostatic low energy beam transport system, delivers the required high-current, time-structured H- beam to the accelerator. At present, the H- injector can reliably provide 50-60 mA beam current at 6% duty-factor (1 ms, 60 Hz) for a 3-4 month service cycle on the accelerator front-end. To ensure sufficient operational margins for the SNS routine production runs and ongoing upgrade requirements, the injector system has been continuously improved on the R&D test facility. This paper presents the recent advancements in the injector system performance including enhancing the ion source beam output capability to 80 mA, improving the low energy beam transport and diagnostics, and upgrading the beam chopper system.

The CUBE-ECRIS Prototype - Towards a 100 GHz ECRIS

• Taneli Kalvas (University of Jyvaskyla, Department of Physics)

The vast majority of the ECRIS are based on magnetic confinement with solenoid and sextupole field components. The development of such structure using present superconducting wire technology is limited to 56 GHz, whereas the minimum-B quadrupole ARC-ECRIS topology has been theoretically shown to enable up to 100 GHz operation. The CUBE-ECRIS is a recently commissioned permanent magnet implementation of the quadrupole topology and it has been used to demonstrate the applicability of the concept as a high charge state ion source. This work introduces technological challenges specific to the concept, especially the slit beam extraction system necessary due to beam formation at a line-shaped plasma loss. We present the most recent performance measurements of the ion source with measured charge state distributions for helium, argon, krypton and xenon. We show emittance measurements and analyze them together with transmission efficiency measurements with the aid of beam formation and transport simulations. Optional beam transport solutions for the slit beam are also presented. An outlook is presented with the next steps of the CUBE project being beam transport studies with a permanent magnet dipole and a possible upgrade to a 14 GHz permanent magnet CUBE.

Positive Ion Sources for Supplying with Mono/multi Charged Ions the C400 Cyclotron Devoted to the CYCLHAD Hadrontherapy Center at Caen (Applications of Ion Sources)

• Laurent Maunoury (Normandy Hadrontherapy)

Normandy Hadrontherapy (NHa) and Ion Beam Application (IBA) are collaborating to develop a full hadrontherapy treatment solution based on a new multiparticle cyclotron. C6+ and He2+ ions will be accelerated up to 400 MeV/u and (H2)+ up to 260 MeV/u. Three different ion sources will be carried out for each accelerated particle: the mono-charged ion sources (H2)+ and low charged ion source He2+ are provided by the Polygon Physics (PP) company. The carbon ion source is under development at NHA in collaboration with IBA and PP. The (H2)+ ion source is an industrial Tubular Ecr Source (TES) type one fitted for the needs of the NHa C400 cyclotron (60µA of (H2)+). The He2+ ion source is a classic 10GHz ECR type one with a new concept because the complete source is set inside a vacuum chamber and it runs under 10-6 mbar of gas residual pressure. The 12C6+ ion source is also an ECR type ion source operating at 14.5GHz frequency. Its design is under progress to produce a beam of naked carbon with a high stability and reproducibility. The article will present the External Injection System of the NHa C400 cyclotron, hence it will focus on the experimental results obtained with the (H2)+ ion source and preliminary outputs from the He2+ ECRIS. A presentation of the multicharged ECRIS design dedicated to the 12C6+ production will be done.

ASTERICS, a New 28 GHz Electron Cyclotron Resonance Ion Source for the SPIRAL2 Accelerator

• Thomas Thuillier (LPSC)

A new A/Q=7 injector is under development for the SPIRAL2 accelerator at Caen, France (NEWGAIN* project). A new 28 GHz superconducting electron cyclotron resonance ion source (ECRIS) named ASTERICS**, located on a high voltage platform, is under design for this project. The source features a modern cryostat allowing for the saving of helium during a quench and a large plasma chamber (90 mm radius and 600 mm length). The physical and technical motivations for a larger plasma volume are detailed and estimates of expected beam intensities enhancement discussed. The source will mainly produce metallic ion beams: the concept of a temperature-controlled liner (up to ~1000°C) will be presented. The design of the ion source and the high voltage platform, with a focus on the ECRIS superconducting magnet [1] will be detailed. * NEWGAIN is the acronym for NEW GAnil Injector. Project funded by Agence Nationale de la Recherche. ** ASTERICS is the acronym for Advanced Spiral Two Electron cyclotron Resonance Ion source at Caen with Superconducting magnets [1] D. Simon et al., "Design of ASTERICS: A Superconducting 28 GHz ECR Ion Source Magnet for GANIL," in IEEE Transactions on Applied Superconductivity, vol. 33, no. 5, pp. 1-5, Aug. 2023, Art no. 4002905, doi: 10.1109/TASC.2023.3260779.

Initial Experimental Results of Producing Multicharged Ions Efficiently by Lower Hybrid Resonance Heating with Exciting Helicon Waves on Electron Cyclotron Resonance Ion Source

• Yushi Kato (Osaka Univ.)

For many years, we have been promoting basic and applied research on multiply-charged ion generation in an electron cyclotron resonance (ECR) ion source (ECRIS). Based on these experimental results, we considered the accessibility conditions for wave propagation in the magnetized plasma in ECRIS, and proposed various microwave feeding methods to improve the efficiency of multiply-charged ion generation by ECR, and tried them experimentally. As a typical original application case, the lowest-order Bernstein wave (BW) mode of conversion from electromagnetic (EM) to electrostatic wave (ES) mode in extra-ordinary mode (X-mode) introduction of higher frequency microwaves than ECR ones had been tried to induce upper hybrid resonance (UHR) heating. Next, we firstly excite helicon waves by introducing electromagnetic waves with frequencies lower than those of ECR and UHR into the ECRIS. Then, we generate the lower hybrid resonance (LHR) in the ECRIS by the electric field of the X-mode of the helicon wave which additionally heat the electrons, and then it was conducted to improve the efficiency of multiply-charged ion generation experimentally. As a result, due to the introduction of the X-mode electric field by the helicon wave introduction, under the critical condition where the LHR condition is satisfied, the LHR efficiently contributes to the electron heating, and the efficient generation of multiply charged ions is achieved, and then multicharged ion current were increased successfully. Under this condition, the electron energy distribution function obtained by the electrostatic probe measurement shows an increase in the high-energy region in the corresponding region, supporting occurrence of the resonance heating. In this paper, for the first time we will describe the initial experimental results on enhanced production of multiply-charged ions by the LHR resonance phenomenon of the low-frequency RF electromagnetic waves introduced to the ECRIS.

European Effort to Improve Highly Charged Heavy Ion Beam Capabilities with ECR Ion Sources

• Hannu Koivisto (Accelerator Laboratory, Department of Physics, University of Jyväskylä)

The European Electron Cyclotron Resonance (ECR) ion source community has more than 20 years of experience working together in various EU-funded projects. In the recent project, called ERIBS (European Research Infrastructure – Beam Services), we will focus on improving ion beam services for the EURO-LABS (European-Laboratories for Accelerator Based Sciences) research infrastructures. The EURO-LABS is a four-year project funded by the Horizon Europe program of the European commission for years 2022 - 2026. In ERIBS collaboration the best expertise, know-how and practices of our community will be exploited and transferred between the partners to take full advantage of the European ion source infrastructure. The aim is to extend the beam variety available for the European user community by developing beam production methods and techniques. This development includes further improvement of technologies related to high temperature ovens, axial sputtering and MIVOC method for all the participating laboratories. We will also aim to improve both short- and long-term plasma and beam stability, as well as methods for online monitoring of these conditions. This can be realized by optical emission spectroscopy, identifying kinetic plasma instabilities by means of hard x-ray detection and using online beam current monitoring systems. An example of the recent developments is the new service provided by the CNRS-IPHC team to synthesize enriched MIVOC compounds for the other ERIBS partners. For example, the team successfully prepared an enriched Chromocene compound, which was needed to produce $^{54}$Cr and $^{50}$Cr beams for the JYFL and GANIL nuclear physics programs, respectively. During the project the efforts will also continue to further advance the European ion beam database for beam preparation practices.

Technical Approach towards the 4th ECR Ion Source FECR and the Latest Progress

• Liangting Sun (Institute of Modern Physics, CAS)

Aiming to the production of 1 emA U3x+ intense highly charged ion beam, the 4th generation ECR ion source 45 GHz FECR (First 4th generation ECR ion source) is under development at IMP. Fundamental progresses have been made towards this challenging machine. The first superconducting ECR ion source magnet based on Nb3Sn technology is going to be in place to provide sufficient magnetic confinement to the 45 GHz microwave heated plasma. A new microwave coupling method called the Vlasov launcher has been proposed and successfully tested that can enhance the peak performance by 20% or higher. High power plasma chamber incorporated with the micro-channel cooling concept has been successfully developed and put in operation to handle the very challenging localized over-heating with the power dump density of ~10 MW/m2 or beyond, which enables the long-term stable operation of an ECR ion source at 10 kW power level. Careful tuning of a 24 GHz ECR ion source operating at afterglow mode has made very intense pulsed highly charged ion beams. Typical ion beams such as 503 eμA Xe30+, 266 eμA Xe34+, 169 eμA Xe38+ and 50 eμA Xe42+ at the FWHM pulse length of ~10 ms have been produced. Intense ion beams have been already used for routine operation at IMP, such as the continuous operation of Kr26+ or Xe32+ at the intensities of 200~300 eμA. This paper will present the status of the FECR and the key technologies to make it work at high microwave power and high intensity ion beam production.

Time-Resolved Measurement of Optical Emission Line Profiles from Electron Cyclotron Resonance Ion Source Plasma

• Oskari Timonen (Accelerator Laboratory, Department of Physics, University of Jyväskylä)

Optical emission spectroscopy provides a noninvasive method to probe the properties of hot and highly charged magnetically confined plasmas. The optical emission line profiles enable, for example, identifying the different species and characterizing the relative population densities and temperatures of the ions and neutrals forming the plasma. The feasibility of this approach has been demonstrated at University of Jyväskylä accelerator laboratory by measuring the light emitted by Electron Cyclotron Resonance Ion Source (ECRIS) plasma with a high-resolution spectrometer setup POSSU (Plasma Optical SpectroScopy Unit). In these previous studies the emission line profiles were measured by scanning the desired wavelength range by rotating the diffraction grating of the spectrometer. This process is slow compared to many interesting plasma phenomena, thus limiting the applicability of the setup. Recently, POSSU has been upgraded by changing the light sensor from a photomultiplier tube to a position sensitive imaging sensor. As a result, it is possible to measure simultaneously a 1 - 2 nm wavelength range, with a spectral resolution in the order of picometers, without moving the grating. This enables time-resolved study of the optical emission line profiles. The measured wavelength region can be chosen between 370 nm and 870 nm, which covers the visible light spectrum, by turning the grating. The time evolution of optical emission line profiles emitted from the JYFL 14 GHz ECRIS plasma, during shifting plasma conditions induced by changing the gas balance, has been measured to demonstrate this new capability. The temporal evolution of temperatures and emission intensities of selected ion and neutral species, correlated with extracted ion beam currents, are presented.

Excursion

Upgrade and Improvement of the TRIUMF Charge State Booster ECRIS

• Joseph Adegun (TRIUMF)

The RF system of the TRIUMF electron cyclotron resonance ion source charge state booster (CSB) was recently upgraded for the implementation of two-frequency heating using a single waveguide. The injection and extraction optics as well as the injection and extraction systems of the CSB were systematically modelled and optimized. The quadrupole scan technique was developed for beam emittance measurement. The implementation of the two-frequency heating and systematic optimization were conducted to improve the efficiency and beam quality of the charge state booster. Under the single-frequency heating regime with well-optimized plasma and beam optics, the maximum charge state of 133Cs isotope that could be produced was 27+ with an efficiency of 1.5 % and the peak of the efficiency distribution was on Cs23+ with an efficiency of 8.8 % but with the two-frequency heating, the maximum charge state of cesium that could be produced shifted to 32+ with an efficiency of 0.02 % and the peak of the efficiency distribution shifted to Cs26+ with an efficiency of 9.1 %. For the tests with 238U isotope and under the single-frequency heating regime, the maximum charge state that could be produced was 36+ with an efficiency of 0.03 %. This could be improved to a 39+ charge state under the two-frequency heating with an efficiency of 0.02%. Although the peaks of the efficiency remain the same for both heating regimes at 28+, the efficiency of U28+ under the two-frequency heating regime was more than a factor of 2 higher. Another significant effect of the two-frequency heating on the CSB was that the total beam emittance was about a factor of 2 smaller than the total emittance measured under the single-frequency heating counterpart due to the increase in the electron energy most especially in the plasma core.

Design of a Novel ECR Ion Trap Facility for Fundamental Plasma Processes and Nuclear Physics Studies

• Eugenia Naselli (Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud (INFN, LNS))

An innovative ECR ion trap facility, called PANDORA (Plasma for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry), was designed for fundamental plasma processes and nuclear physics investigations. The overall structure consists of three main pillars: a) a large (70 cm in length, 30 cm in diameter) ECR plasma trap with a fully superconducting B-minimum magnetic system (Bmax = 3.0 T) and an innovative design to host detectors and diagnostic tools; b) an advanced non-invasive plasma multidiagnostic system to locally characterize the plasma thermodynamic properties; c) an array of 14 HPGe detectors. The PANDORA facility is conceived to measure, for the first time, in-plasma β-decaying isotope rates under stellar-like conditions. The experimental approach consists in a direct correlation of plasma parameters and nuclear activity by disentangling - through the multidiagnostic system that will work in synergy with the $\gamma$-ray detector array - the photons emitted by the plasma (from microwave to hard X-ray) from $\gamma$-rays emitted after the isotope $\beta$-decay. In addition to nuclear physics research, fundamental plasma physics studies can be conducted in this unconventional ion source equipped with tens of detection and diagnostic devices (RF polarimeter, OES, X-ray imaging, space and time-resolved spectroscopy, RF probes), with relevant implications for R&D of ion sources for accelerator physics and technology. Several studies have already been performed in downsized nowadays operating ECRIS. Stable and turbulent plasma regimes have been described quantitatively, studying the change of plasma morphology, confinement, and dynamics of losses using space resolved X-ray spectroscopy, also measuring locally the plasma density and temperature. A fully virtual experiment joining PIC plasma simulations, dynamics of metallic isotopes injection in plasma and GEANT4 simulations modeling the response of the HPGe array and diagnostics tools is ongoing.

FRIB ECR Ion Sources Operation and Future Development

• Junwei Guo (Facility for Rare Isotope Beams)

The Facility for Rare Isotope Beams (FRIB) has become operational as a user facility in April 2022. The primary beam power available on target routinely now reaches 5 kW and preparations for operation at 10 kW are underway with an ultimate design goal to deliver 400 kW within the next 6 years. FRIB front end currently operates two ECR ion sources: a room temperature ECR ion source operating at 14 GHz, ARTEMIS, in operation since 2016, and a High-power superconducting Nb-Ti ECR ion source (HP-ECR) in operation at 18 GHz since January 2023. The HP-ECR has demonstrated the magnetic field necessary for operation at 28 GHz and preparations are underway for high power operation, including the installation of a 28 GHz gyrotron microwave system, upgraded radiation shielding, improved new plasma chamber, high temperature oven and so on. In addition, to mitigate single point failure for intense highly charged ion beam and maintain high availability, a second 28 GHz ECR ion sources based on a Nb3Sn sextupole is being developed in collaboration with the superconducting magnet group at Lawrence Berkeley National Laboratory (LBNL) that developed the original cold mass for the 28 GHz HP-ECR. This paper presents progress made in some of these areas as well as summarize the current state of the FRIB ion sources and future development. This work was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under the Cooperative Agreement DE-SC0000661

First Ion Source at ISOL@MYRRHA with an Improved Thermal Profile - From Prototype to the First Experimental Validation

• Sophie Hurier (SCK CEN & KULeuven)

MYRRHA will be the world’s first large-scale Accelerator Driven System at power levels scalable to industry. In ISOL@MYRRHA, Radioactive Ion Beams (RIBs) will be produced using the Isotope Separation On-Line (ISOL) technique, with increased isotope production through high-intensity primary beam and long irradiation times. High-quality RIBs are to be maintained over that period, up to 4 weeks. As an initial source, a surface ion source, or hot cavity, was chosen for its reliability and simple design. This ion source type was studied theoretically and experimentally by Kirchner [1990, Nucl. Instrum. Methods Phys. Res. A 292] and the temperature was clearly identified as a key element to the ion source operation, but it was assumed to be constant along the cavity. However, a set of finite element thermal-electric simulations performed with ANSYS as well as measurements conducted by researchers at SPES [Manzolaro 2017, Rev. Sci. Instrum. 88] have shown a temperature inhomogeneity along the cavity, where the temperature is the highest in the middle of the cavity and decreases towards both edges. A new ion source design was studied to reduce this inhomogeneity: a modified heating system was proposed with an Active Thermal Screen (ATS). The formerly passive thermal screen from SPES and ISOLDE is now an actively heated part which will heat up the cavity end. As a first validation of this design, simulations were performed in an earlier work [Hurier 2020, J. Phys. Conf. Ser. 2244] and the calculated thermal profile showed a significantly reduced temperature drop at the cavity’s end compared to SPES results. A first prototype was constructed and tested at CERN to validate the ANSYS thermal-electric simulations. Secondly, the surface (and laser) ionisation properties of this prototype were tested at the ISOLDE Offline II at CERN. These experimental results will be presented in this contribution.

Radioactive Molecular Ion Beams at CERN-ISOLDE

• Mia Au (CERN)

The ISOLDE facility at CERN provides ion beams of nuclides produced in reactions between 1.4-GeV protons and thick targets. Molecules have been studied as a method to deliver beams of release-limited refractory elements by forming volatile molecules [1-5]. Molecular sideband extraction is also used as a technique to improve beam purity. Molecular beams additionally provide opportunities for fundamental physics studies [6-11]. We present our work on molecular ion beam development at ISOLDE and beam composition studies using: the ISOLTRAP Multi-Reflection Time-of-Flight Mass Spectrometer (MR-ToF MS) [12] for identification by ToF mass measurements, online gamma-ray spectroscopy at the ISOLDE tape station [13,14], and off-line alpha- and gamma-ray spectrometry of ion-implanted samples. This project has received funding from the European’s Union Horizon 2020 Research and Innovation Program (grant number 861198 project ‘LISA’ MSC ITN) and support by the German Federal Ministry of Education and Research (BMBF) (grant numbers 05P18HGCIA, 05P21RDFNB, and 13E18CHA, Wolfgang Gentner Program) [1] R. Eder et al., Nucl. Inst. and Meth. B 62, 535 (1992) [2] R. Kirchner, Nucl. Inst. and Meth. B 126, 135 (1997) [3] H. Frånberg et al., Rev. Sci. Inst. 77, 03A708 (2006) [4] J. Ballof et al., Eur. Phys. J. A 55, 65 (2019) [5] U. Köster et al., Eur. Phys. J. Special Topics 150, 293 (2007) [6] G. Arrowsmith-Kron et al., arXiv, DOI 10.48550/arXiv.2302.02165 (2023) [7] T. A. Isaev et al, Phys. Rev. A, 82, 052521 (2010) [8] T. A. Isaev et al., arXiv, DOI 10.48550/arXiv.1310.1511 (2013) [9] M. Safronova et al., Rev. Mod. Phys. 90, 025008 (2018) [10] N. Hutzler et al., arXiv, DOI 10.48550/ARXIV.2010.08709 (2020) [11] R. Garcia-Ruiz et al., Nature 581, 396 (2020) [12] R. N. Wolf et al., Int. J. Mass Spec. 123, 349 (2013) [13] S. Stegemann et al., Nucl. Inst. and Meth. B. Conf. Proc. EMIS XIX (2022) [14] R. Catherall et al., J. Phys. G : Nucl. Part. Phys., 44, 094002 (2017)

Ion Sources for Radioactive Ion Beam Delivery at CERN-ISOLDE

• Sebastian Rothe (CERN)

The ISOLDE radioactive ion beam facility is located at CERN’s Proton Synchrotron Booster, where thick targets are irradiated with 1.4-GeV protons. Over 1000 different radioisotopes with half-lives down to milliseconds can be delivered to low-energy experimental setups at up to 60 keV, or post accelerated using the REX and HIE ISOLDE linear accelerators. One important factor for planning experiments at ISOLDE and other RIB facilities alike is the yield or production rate of the desired isotope. When deciding on the target and ion source combination different aspects must be considered: the reaction cross-section for the isotope production in the target matrix, the diffusion and effusion properties of the element, and the optimal ionization mechanism. In addition to the absolute rate of a certain isotope of interest, the unwanted isobaric contaminants often must be addressed by choosing additional purification techniques. Examples are use of a neutron converter, quartz transfer lines, creation of radioactive molecules, or the use of the Laser Ion Source and Trap (LIST). Here we will introduce the ISOL process and illustrate the aspects to be considered when selecting target and ion source combinations to optimize the yield and purity of the radioactive ion beam for the user. The ongoing and future ion source developments will be discussed and development facilities will be introduced.

Review of the High-Current Capabilities and Upgrades of the EBIS/T Charge-Breeding System of the Reaccelerator at the Facility for Rare-Isotope Beams

• Alain Lapierre (FRIB/MSU)

The Reaccelerator (ReA) of the Facility for Rare-Isotope Beams (FRIB) at Michigan State University (MSU) uses a helium gas-filled Radio-Frequency Quadrupole (RFQ) ion trap and an Electron-Beam Ion Trap (EBIT) as a charge-breeding system. Rare isotopes produced via projectile fragmentation or in-flight fission are selected by the Advanced Rare Isotope Separator of FRIB and stopped in a helium gas cell before transport at low energy to ReA. Continuous ion beams are injected into the RFQ trap, which cools and bunches the ions, that are then ejected as ion pulses. The ion pulses are injected into the EBIT, captured, charge bred, ejected, and accelerated by the superconducting linear accelerator of ReA to several MeV/u. The electron current of the EBIT (300 - 600 mA) limits its capacity to ~2E10 charges, yielding maximum rates of less than ~2E10 pps for light ions. An upcoming upgrade to the EBIT electron gun is expected to provide 2 A in current. In parallel, a high-current electron-beam ion source (HCEBIS) is being commissioned. In its present configuration, the HCEBIS can provide an electron current of 2 A. An upgrade will increase the current to 4 A. The implementation of these two upgrades is expected to allow for maximum rates of ~2E11 pps (light ions), compatible with FRIB projected rates. This will also provide redundancy of the breeders. We present the status of the upgraded EBIT and HCEBIS. We also review the expected high-intensity capabilities of the future ReA frontend considering the maximum capacity of the RFQ trap. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics and used resources of the Facility for Rare Isotope Beams (FRIB), which is a DOE Office of Science User Facility, operated by Michigan State University, under Award Number DE-SC0000661.

Brightness Award Talk

• Valentin Fedosseev (CERN)

Novel ECR Magnets in Development and Under Construction at LBNL

• Mariusz Juchno (Lawrence Berkeley National Laboratory)

Future heavy-ion driver accelerators will require high charge state ECR sources operating beyond the currently achievable heating radio frequency of 28 GHz. Lawrence Berkeley National Laboratory is exploring two technological paths that can enable reaching this goal. The first approach is by using Mixed Axial and Radial field System (MARS). This novel coil system allows the use of the Nb-Ti superconducting coil system to reach the 45 GHz resonant heating while operating within the field limit of 9 T at 4.2 K. This project aims at building a new ion source at the 88 inch cyclotron facility at LBNL. The second approach is by using a traditional sextupole-in-solenoid coil configuration, similar to VENUS magnet, but with sextupole coils that are wound using the Nb3Sn superconductor that has a much higher field limit of 22 T at 4.2 K. This project, in collaboration with FRIB facility at Michigan State University, aims at building a 28 GHz ion source but it allows to develop coil fabrication techniques that will be critical for the next generation ion sources as well. This article describes the design of both magnet systems and summarizes coil fabrication challenges, the development and prototyping efforts so far. This work was supported in part by the U.S. DOE, Office of Science, Office of Nuclear Physics under contract number DE-AC02-05CH11231 (MARS) and DE-SC0000661 (FRIB).

The PI-LIST: High-Resolution Crossed-Beams Laser Spectroscopy inside the ISOLDE Laser Ion Source

• Asar AH Jaradat (CERN)

Laser resonance ionization spectroscopy is a sensitive tool for investigating nuclear structures [1], but its spectral resolution is limited by Doppler broadening, which becomes significant at ion source temperatures of approximately 2000°C. This can limit resolution to 1-10 GHz, making it difficult to measure nuclear magnetic and quadrupole moments. A new laser ion source design has been implemented at ISOLDE to provide in-source spectroscopy capabilities with higher resolution than previously achievable. It is based on the high beam purity Laser Ion Source and Trap (LIST) [2, 3], featuring spatial separation of the hot cavity where potential ion beam contamination can arise from non-laser related ionization mechanisms such as surface ionization. This design features the Perpendicularly Illuminated LIST (PI-LIST) [4], which uses a crossed laser/atom beam geometry for spectroscopy, resulting in only the transverse velocity spread of the atom ensemble contributing to the experimentally observed Doppler broadening. This allows for spectral resolutions down to 100-200 MHz, an order of magnitude below usual limitations. This technique was used to study neutron-rich actinium isotopes with the highest spectral resolution ever achieved for in-source resonance ionization spectroscopy at ISOLDE, CERN.[5] Technical implementation challenges and limits, such as efficiency loss, will also be discussed. [1] V. Fedosseev et al., J. Phys. G: Nucl. Part. Phys. 44 084006 (2017) [2] D. Fink et al., Nucl. Instr. Meth. B, 317 B, 417-421 (2013) [3] D. Fink et al., Phys. Rev. X 5, 011018 (2015) [4] R. Heinke et al., Hyperfine Interact 238, 6 (2017) [5] E. Verstraelen et al., Phys. Rev. C 100, 044321 (2019)

Commissioning and First Operation of East Japan Heavy Ion Center at Yamagata University

• Hikaru Souda (Yamagata University)

A world smallest carbon ion radiotherapy facility, East Japan Heavy Ion Center, Faculty of Medicine, Yamagata University, started treatment operations in February 2021. The treatment system consists of an ECR ion source, an RFQ and IH-type drift tube linac, a 430 MeV/u synchrotron and a spot-scanning irradiation system. It has two treatment rooms, one is a fixed horizontal beam port and the other is a rotating gantry beam port with superconducting magnets. The ECR ion source is the 6th Kei2-type permanent magnet 10 GHz ECR ion source with a maximum field of 0.8 T. All the improvements of the previous facilities are applied to the ion source, such as helium gas-assisted operation and changes in the shape of an anode electrode and a cathode electrode to reduce the discharge. Owing to these improvements, the ion source has been operated stably during the commissioning and clinical operation. The irradiation system of Yamagata University eliminated the plastic block range shifter to realize a compact gantry, and has 600 energy levels to control the beam range in step of 0.5 mm. In order to optimize the beam transport parameters quickly, automation tools for orbit correction were developed. Machine commissioning and clinical commissioning were carried out in parallel to allow treatment to begin earlier. First, the horizontal fixed beam port was commissioned by verifying the dose distribution of the treatment planning system. Then, the gantry beam port was commissioned by full beam distribution measurement for representing beam angle of 90 degree, and other angles were sequentially released after the compatibility measurement of beam position, beam size, and two-dimensional uniformity. In September 2022, we were able to accept all treatment sites in the gantry port with angle step of 30 degrees. By March 2023, 890 cancer patients had been treated and 24 angles with a 15 degree step were available. Further improvements to increase beam efficiency are ongoing.

HVE Ion Sources for Medium and High-Energy Accelerator Systems

• Arun Tejaswee Annaluru (High Voltage Engineering Europa B.V.)

Since decades, High Voltage Engineering (HVE) manufactures particle accelerator systems for research and industry. HVE’s product line includes Singletron and Tandetron accelerator systems with terminal voltages of up to 6 MV. They are dedicated to a wide range of applications, including ion implantation and irradiation, Ion Beam Analysis (IBA), Accelerator Mass Spectrometry (AMS), and neutron calibration. In this paper, we give an overview of the different positive and negative ion sources that are applied in these systems. We focus especially on the recent development and the performance of a compact 2.45GHz permanent magnet ECR bipolar ion source (HVE Model SO-160), used for negative light-ion injection into tandem accelerators. It generates high-current, low-emittance light-ion beams at 30 keV energy. The novelty in the design of this ion source is that it combines direct negative extraction for H- with positive extraction for He+ that is followed by charge exchange to He- in a Na-based electron donor canal. The switch-over between H and He operation is fully automated and computer controlled, including the change-over of the required power supplies, and does not require any mechanical changes on the source head. The source produces in excess of 300 euA of H- and more than 25 eµA of He-. It is expected the design allows for substantially higher currents, especially for H-.

Compact Monoenergetic Proton Generator in MeV Region Using NANOGAN

• Tetsuhiko Yorita (RCNP, Osaka Univ.)

For simple applications, such as the calibration of a charged particle detector, a multi-MeV proton generator may be preferable to cyclotrons or electrostatic accelerators such as Van de Graaff generator. Thus, a compact proton generating system, consisting of 10Ghz ECR ion source NANOGAN and a deuteron target, was developed at the Research Center for Nuclear Physics at Osaka University. A $^3$He$^{2+}$ beam was generated by the NANOGAN with the acceleration voltage of 20~40 kV in an experiment that utilized the fusion reaction $^3$He + deuteron (D) → proton(P) + $^4$He. The monochromatic protons with energies of 14.67 MeV were successfully obtained at the atmosphere side of the target in the experimental setup, when a novel target base with a thin metal foil and Polyimide film window are used.

A New Proton Injector Based on PKU-type 2.45GHz PMECR for BNCT Facility

• Shixiang Peng (Peking University)

The first fully domestically boron neutron capture therapy demonstration device located in Yangtze River Delta region based on an accelerator neutron source will be build recently. It is held by "Xi'an Jiaotong University-Huzhou Neutron Science Laboratory" joint lab established by Xi'an Jiaotong University and Huzhou Industrial Group. The BNCT facility requires a proton beam of 30 mA@40 keV at RFQ entrance, with its beam duty cycle between 0.5%-100%, and its normalized root mean square emittance less than 0.2 π.mm.mrad. The device development cycle is 10 months. Ion source group of Peking University (PKU) is in charge of the proton ion source and its low energy beam transportation section (LEBT). A PKU type compact permanent magnet 2.45 GHz ECR ion source(PKU-Type PMECR) and a two solenoid LEBT is under development for this purpose. Further, this LEBT integrates beam chopper, absorption area for the chopped beam, ACCT, electron trap in a vacuum tube with the length of 210 mm after the second solenoid. So far, the project has completed work such as scheme demonstration, system mechanical design, ion source conditioning, and peripheral component procurement. Detail will be presented in this article.

A Review of State-of-the-Art Ion Source Plasma Diagnostics

• Olli Tarvainen (UK Science and Technology Facilities Council)

There are a huge variety of ion source types employed to produce charged particle beams for discovery physics and accelerator applications such as neutral beam heating of thermonuclear fusion plasmas and spallation neutron sources. In most ion sources the ion beams are extracted directly from a plasma sustained by a DC or RF energy source. What is common to all ion sources is the need for diagnostics to quantify their performance and develop them further. The purpose of this review paper is to describe the state-of-the-art plasma diagnostic methods used for ion source development, operations, and monitoring. We present examples of diagnostics techniques, such as optical emission spectroscopy and x-ray diagnostics, applied to negative and positive ion sources highlighting similarities of the diagnostic needs and individual diagnostic challenges pertaining to specific types of ion sources, e.g. microwave sources (including ECR ion sources), RF-driven sources, and arc discharges. The overarching message is that further advances in ion source performance, deeper understanding of the underlying physics, and validation of novel ion source concepts require complimentary plasma diagnostics.

Implicit PIC Development for Bounded Plasmas

• Nicolas Savard (TRIUMF, D-Pace)

Particle-in-Cell (PIC) codes used to study plasma dynamics within ion sources typically use an explicit scheme. These methods can be slow when simulating regions of high electron density in ion sources, which require resolving the Debye length in space and the plasma frequency in time. Recent developments on fully-implicit PIC models in curvilinear geometries have shown that these spatial/time scales can be significantly decreased/increased respectively, allowing for notable speed-ups in simulation time, and thus making it a potential tool for studying the physics of ion sources. For this purpose, a charge and energy conserving implicit PIC code has been developed in 1D to show its potential for simulating bounded plasmas. In this paper, we use this model to simulate a 1D analytical benchmark of a bounded plasma with fixed power input and Maxwellian electron distributions. The results are shown to compare well to analytical theory and to the results using an explicit PIC code. We demonstrate the ability of the implicit PIC code to speed-up simulation time by nearly a factor of 10x compared to explicit PIC, which would correspond to a speed-up of up to 1000x in 3D.

Investigation of Plasma Chamber Erosion in an RF Ion Source

• Anand Mathai George (TRIUMF/D-Pace)

D-Pace is developing a 13.56 MHz RF ion source capable of producing negative ions (H$^-$, D$^-$). The ion source is a hybrid design between the TRIUMF-licensed filament ion source and the RADIS ion source licensed from the University of Jyväskylä. An Inductively Coupled Plasma (ICP) is generated inside the plasma chamber using a planar spiral external antenna, powered by an RF power supply. The RF power is coupled between the antenna and the plasma through an Aluminum Nitride (AlN) dielectric window. The RF ion source is expected to offer a 'maintenance-free' operation compared to a filament ion source due to the absence of any filaments that erode in the plasma. However, the RF ion source is challenged by the erosion of the copper plasma chamber during the operation. This leads to the deposition of copper layers on the plasma-exposed surface of the AlN window. These metal deposits decrease the power coupling from the spiral antenna to the plasma and deteriorate the beam current from the ion source. Different experiments were performed to control the deposition of copper on the dielectric window in an H$_2$ plasma. The copper deposition can be reduced by maintaining the surface of the dielectric window at high temperatures. Operation of the ion source at high H$_2$ gas pressures ($\geq 20$ mtorr) can also reduce the formation of copper layers considerably. Moreover, it was found that the erosion is directly related to the value of the electric potential applied to the plasma electrode during the beam extraction. A reduction in the ratio of the plasma exposed area of the plasma electrode to the grounded surface area of the plasma chamber to $\approx$ 0.03, reduces or nearly completely eliminates the formation of copper layers on the dielectric window. A combination of these solutions is incorporated into the operation of the RF ion source and the latest results are presented in the paper.

Determining Absolute VUV Fluxes for Assessing the Relevance of Photon-Surface Interaction in Ion Sources

• Roland Friedl (AG Experimentelle Plasmaphysik (EPP), University of Augsburg)

The ion source discharge influences the plasma-facing surfaces by impinging fluxes of ions, radicals and photons. This plasma surface interaction is particularly important for, but not limited to, caesiated ion sources for negative hydrogen ions due to its influence on the surface work function and possible photo-emission of electrons into the plasma sheath. While the role of ions and radicals is widely accepted in the course of plasma surface interaction and quantitative measurements have been performed throughout the literature, VUV photons (< 200 nm) are often disregarded. The reason is mostly the complicated setup to measure such fluxes absolutely and energy-resolved. However, they impact the surface with at least 6.2 eV and several studies have already confirmed that comparable fluxes to the ion fluxes can easily occur [Barton2000, Fantz2016]. To overcome the drawback of complex VUV-spectroscopic systems including their calibration, several alternative solutions have been applied in the past, where a VUV-sensitive photo diode with optical filters was already employed in the ion source community [Komppula2015]. This contribution takes up the basic concept and enhances it by direct in-house absolute calibration of the system down to 46 nm against a VUV spectrometer [Fröhler-Bachus2021]. The system is calibrated for energy-resolved absolute VUV flux measurements up to photon energies of 27 eV in a variety of gases and gas mixtures, including Ar, H2, O2 and N2 [Friedl2023]. Demonstration of the system is presented at the ion source of the BATMAN Upgrade facility. The system was used to measure the VUV fluxes of the hydrogen plasma in the driver region as well as in the region close to the extraction surface. Using solid angle calculations and a ray tracing code, the total energy-resolved flux onto the plasma grid was determined. Comparable fluxes to the impinging ion flux could be shown, and the major contribution to this flux originates from the driver plasma.

Using Machine Learning Algorithms for ECR Tuning and Physics Studies

• Damon Todd (Lawrence Berkeley National Laboratory)

The superconducting ECR ion source VENUS remains one of the world’s highest-performing ECRs for producing high-current, highly-charged ion beams. However, VENUS has up-to-twenty control parameters defining an operation space that is largely unexplored and inside which record beams are still produced via dedicated tuning time. This size of operation space has proven manageable for computer algorithms such as Bayesian Optimization. Therefore, we are endeavoring to apply this and related machine learning techniques to the problems of maximizing VENUS performance, improving stability control, and understanding the underlying physics dictating source performance. We have implemented algorithms that vary all control parameters and maximize beam currents for gas-fed plasma beams. We have added a cost function to find optimized solutions more efficiently and we are working to develop a neural network based on collected source data. We will report on our progress, present our results, and discuss both the challenges we have overcome and those that still remain.

X-Ray Diagnostics of ECR Ion Sources

• Richard Racz (Institute for Nuclear Research (Atomki))

ECR (Electron Cyclotron Resonance) Ion Sources (ECRIS) are widely used to provide highly charged heavy ions for high energy accelerators. In their plasmas, ECRISs are able to generate electron population having enough high energy to effectively ionize the atoms up to their inner shells. The quality (intensity, emittance, stability) of the ion beam used by the end-stations of the accelerators is strongly determined by the general plasma conditions, therefore plasma diagnostics techniques were implemented and developed during the more than 50 years colorful history of ECRISs. One of the most commonly used non-destructive diagnostic methods is based on the investigation of spectral and/or spatial distribution of the X-ray photons emitted by the plasma and plasma chamber complex. The interaction of warm and hot electrons with the plasma atoms, ions and plasma chamber walls results intense X-ray emission and by applying X-ray plasma diagnostic technics meaningful, quantitative plasma parameters (density, temperature) can be revealed. During my talk I will go through the main features of the ECRIS related X-ray measurement techniques in the context of their contribution to the understanding of ECR Ion Sources.

Fifteen Years of Developments of Isotopic MIVOC Metallic Beams

• Benoît Gall (IPHC / Université de Strasbourg)

Some metallic ion beams such as titanium are quite difficult to produce at the intensity level required in order to deliver several particle micro-amperes on target for nuclear physics experiments. At the University of Jyväskylä (JYFL), Matti Nurmia proposed to use organometallic compounds giving birth to the so-called MIVOC method [1]. Ferrocene is the archetype of metallic beams procced via this Method. J Arje, H. Koivisto and their colleagues identified several organometallic compounds that were successfully accelerated at JYFL [1]. In order to make the first prompt spectroscopy of 256Rf (Z=104), an intense metallic 50Ti beam was necessary. This implied chemistry with isotopically enriched compounds. We started to develop MIVOC compounds of titanium-50 (92+%) at the University of Strasbourg. We could successfully prepare pentamethylcyclopentadienyl trimethyl titanium that was accelerated at JYFL cyclotron [2]. This beam was then used in many laboratories such as (GANIL, RIKEN, DUBNA), leading to significant improvement of our process. This was the start of 15 years of isotopically enriched MIVOC compound preparations leading to rather intense and versatile isotopic beams of titanium vanadium and chromium. It is proposed in a first part of this presentation to detail the MIVOC developments done and ongoing in Strasbourg. A special focus will then be given to the limitations observed for very high intensity metallic beams. The last part of the talk will be devoted to the development of an inductive oven performed in order to feed the ECR plasma directly with metallic vapors. [1] H. Koivisto et al., NIM B 187 (2002)111, [2] J. Rubert et al., Nucl. Instr. & Meth. Phys. Res. B 276 (2012) 33–37.