ICIS2023 - 20th International Conference on Ion Sources September 17-22, 2023

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

18 Sept 2023, 11:50

20m

Victoria Conference Centre

Contribute Oral

Speaker

Takashi Nagatomo (RIKEN Nishina Center)

Description

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 $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$ 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 $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$ 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 $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$ will also increase, and the emittance increase due to the space-charge effect (SCE) of $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$ 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 $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$.
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 $10\mu \mathrm{A}$ to avoid significant SCE of its own. The $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$ 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 $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$. 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 $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$ ~ 2 mA, but from $I_{\mathrm{e}\mathrm{x}\mathrm{t}}$ ~ 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.