Optimization of the ion-beam intensity in the cyclotrons low energy beam transport line with space charge simulations

by Antoine Beaudouin (1^st year PhD student)

The beam intensity is a crucial parameter in any particle accelerator. Improving its value represents a challenge, both theoretical with the elaboration of realistic beam dynamics models, and practical with the experimental physics issues that come with. The first part of my PhD thesis focuses on the low energy beam transport line (LEBT) I1, connecting the ECR ion source (ECRIS) to the C01 injector cyclotron. Right after the ECRIS source where the charge density is important, the repulsive electric field between particles induces a strong beam defocusing. This effect, called “space charge”, leads to an important degradation of the beam quality (emittance growth) at the very beginning of the accelerator beam line. Therefore, the transmission of the intense beams is strongly reduced.

In 2024, the I1 LEBT was improved with the addition of a quadrupole after the source. Measures with several ion beams (Ar, Xe, Kr) confirmed positive gains of intensity (30-60% for 40 Ar 9+). Simulations roughly predicted these improvements: however, many physical effects make quantitative predictions difficult to achieve.

The objective of the study is to take another look at the simulations with new codes including more realistic and precise features and models (beam optical parameters and characteristics, space charge model, space charge compensation, electro-magnetic fields map etc.).
I will present in the seminar the challenges associated with the transport of intense ion beams using two transport codes: the TRANSPORT code [1] using the matrix formalism, and the code RF-Track [2].

After a better understanding of the beam intensity improvement in I1, the next step is to propose a modification on the I2 LEBT which, according to its particularities, did not receive such an upgrade. Thus, we considered a study performed at the department of Physics of the University of Jyväskylä. Using a double Einzel lens [3], they achieved an optical improvement of the beam: the losses in the LEBT after the ECRIS were reduced, and the transmission after their cyclotron was raised up to 80-90%.

[1] S. Sinigardi and M. Camatti, « PSI Graphic Transport Framework User Manual », Tech. Report, 2012
[2] A. Latina, « RF-Track Reference Manual », Tech. Rep., 2020
[3] V. Toivanen, T. Kalvas, H. Koivisto, J. Komppula and O. Tarvainen, « Double einzel lens extraction for the JYFL 14 GHz ECR ion source designed with IBSimu », JINST 8 P05003 (2013), DOI:10.1088/1748-0221/8/05/P05003