深圳产业光源单束团不稳定性研究

Background]: This study aim to analyze the single bunch thresholds based on Shenzhen Innovation Light Source Facility (SILF), which has very high beam-coupling impedance, low natural emittance and low momentum compaction factor. [Purpose]: To suppress the beam instabilities, a 3rd harmonic cavity (3HC) is under consideration. This study investigates the impact of 3HC and the effect of active / passive 3HC on the single-bunch instabilities. [Methods]: The broadband impedance was represented by resistive-wall and the geometric parts modeled and analyzed by ECHO and CST respectively. The instabilities were evaluated based on macro-particle simulation by ELEGANT. Three different scenarios were analyzed, no 3HC, active 3HC and passive 3HC. The investigation of the first two scenarios was applied with single bunch using the optimum voltage and phase configurations at zero current. For the last scenario, multi-bunches with uniform gaps and 5 - 20 % filling rate were used to conserve the low current (about 3060 mA). It was accomplished by two steps. Firstly, the passive 3HC was detuned to achieve the desired bunch-lengthening factor (~4). Secondly, the impedance was included with the same detuning. [Results]: The results show that both 3HCs can suppress microwave instabilities greatly, increase TMCI threshold remarkedly with positive chromaticity; when chromaticity is +3, the threshold is 2.5 nC and 2.4 nC for active and passive 3HC respectively; without 3HC, the threshold maintains 1.0 nC when chromaticity is larger than +1; the single bunch charge thresholds satisfy 300 mA requirement for all. [Conclusions]: 3HC is essential in SILF when the requirement is more than 1 nC for single bunch. The difference between active and passive 3HC is negligible especially when chromaticity is less than or equal to +3.