Beam emittance measurements in the SPC2 injection beamline at iThemba LABS

A dissertation submitted in fulfilment of the requirement for the Degree of Master of Science in the Department of Physics, Faculty of Science, Agriculture and Engineering, at the University of Zululand, 2020.

Beam emittance is a property of a charged particle beam in a particle accelerator that is used to estimate the beam quality. It is length-based measure of the average spread of particle coordinates in position and momentum phase space. This attribute makes emittance a crucial beam parameter that one requires when tuning the beam into smaller spaces in a beamline and matching the beam with the acceptance of an accelerator. At iThemba LABS, the low energy beam transport (LEBT) system required a system that measures transverse beam emittance. This beamline connects the two electron cyclotron resonance ion sources (ECRIS) to the second solid pole injector cyclotron (SPC2) and is composed of a set of solenoid and quadrupole magnets that are used to focus the beam. This layout makes it possible to use solenoid and quadrupole magnets to carry out beam emittance measurements. This thesis is aimed at presenting a developed system that can be utilized for measuring transverse beam emittance of beams emerging from the ECRIS. An analytic explanation of the techniques used and tools needed for these measurements are given. The Gauss approximation program and the emittance calculator were developed and used. During the development process, the system was validated using different beams simulated using a computer program TRANSPORT [1]. The beam widths obtained from TRANSPORT were used on the developed emittance calculator to establish if one could get the injected beam emittances. From the calculations it was clear that there is an agreement between injected and deduced emittances. This was an indication that the emittance calculator is working as expected. Thenceforth, the real beam emittance measurements were performed using 4He2+ beam with kinetic energy Ek = 12.7 keV. After analysing the helium data it was decided that a second set of measurements should be performed and this was done using 32Si5+ beam with kinetic energy Ek = 8.35 keV. The obtained results from both helium and silicon measurements were analyzed in order to compare the emittance measured in the AX-line with the one measured in the Q-line. From both set of measurements it was observed that the emittances obtained in the two beamlines had some disagreement. This disagreement between the two sets of transverse emittances violated Lioville’s theorem. In order to probe the source of disagreement the helium measurements were further investigated by performing beam widths comparison. To do this, the measured emittances were injected into TRANSPORT and the beam widths gathered were then compared with the measured widths. From the comparison results it was clear that only the beam widths from AX-line are comparable while there was disagreement in Q-line widths comparison. It was evident from these observations that one could ii rely on beam emittances obtained in the AX-line than those obtained from Q-line beamline. Upon further investigation, it was concluded that reason behind this could be associated with the fact that for AX-line measurements, instead of performing minimum number of measurements (i.e three) required to deduce the emittances, five set of measurements were utilized instead. This choice of number of measurements makes the system robust in such a way that a small error on measured beam widths will have a negligible effect on the deduced emittances. For Q-line measurements, however, the solenoids scan was performed with minimum number of beam widths measurements (i.e ten) required. This may have resulted in the solenoid scan being sensitive to errors that could occur when measuring beam widths. For much more reliable emittance measurements in the Q-line the system needs to be upgraded in order to accommodate utilization of more beam widths of measurements.

National Research Foundation