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Pre -Chopper Beam Dynamics

Pre -Chopper Beam Dynamics. JB. Lallement Linac4 BCC - 29/07/2010. LEBT – pre -chopper Layout. Timing Diagram. From R. Scrivens. 1 st Stage. 2 nd Stage. 3 rd Stage . > 100 µs Increasing c urrent from the source Pre -chopper ON No SC compensation (20, 40, 60, 80 mA). 2 µs

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Pre -Chopper Beam Dynamics

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  1. Pre-Chopper Beam Dynamics JB. Lallement Linac4 BCC - 29/07/2010

  2. LEBT – pre-chopper Layout

  3. Timing Diagram From R. Scrivens 1st Stage 2nd Stage 3rd Stage • > 100 µs • Increasingcurrentfrom the source • Pre-chopper ON • No SC compensation • (20, 40, 60, 80 mA) • 2 µs • 80mA from the source • Pre-chopper switching OFF • No SC compensation • (10, 7.5, 5, 2.5, 0.1, 0 kV) • 50 µs • 80mA from the source • Pre-chopper OFF • SC compensation recovering • (0, 25, 50, 75, 90%)

  4. Simulations context • Approximations : • Constant beam distribution (independentfrom the source current) • No Space Charge compensation map • LEBT Solenoids and MEBT quads and bunchercavitiesare set to nominal values (80-8 mA in the LEBT – 65 mA in the MEBT) • Simulation codes: • Travel – Parmteq – Travel • TraceWin – Toutatis – TraceWin SC comp 90% Variable SC comp

  5. LEBT transmission Pre-chopper ON Pre-chopper ON-OFF SC recovering Source output current LEBT output current > 100 µs 2 µs 50 µs

  6. RFQ transmission Pre-chopper ON Pre-chopper ON-OFF SC recovering LEBT output current RFQ output current > 100 µs 2 µs 50 µs

  7. RFQ transmission Pre-chopper OFF Beamcurrent 80 mA SC compensation 75% Pre-chopper ON Pre-chopper ON-OFF SC recovering LEBT output current Pre-chopper ON Beamcurrent 80 mA No SC compensation RFQ output current > 100 µs 2 µs 50 µs

  8. Losses distribution in the RFQ Pre-chopper ON Beamcurrent 80 mA No SC compensation 7.5 mA lost – Localized !!!

  9. Losses distribution in the RFQ Pre-chopper OFF Beamcurrent 80 mA SC compensation 75% 12 mA lost - Distributed

  10. Chopper-line transmission (Chopper ON) Pre-chopper ON Pre-chopper ON-OFF SC recovering RFQ output current Beamcurrentafter the Chopper > 100 µs 2 µs 50 µs

  11. Conclusion • The pre-chopper extinguish factor is not 100%. • Using the 3MeV chopper is required to limit the transmitted current in the high energy part (especially for the transition stages). • In order to reduce localized losses in the RFQ, the pre-chopper could be improved by increasing the kick (higher voltage / longer plates) or its effects (move it upstream). • The presented results really depend on the space charge compensation model !!!

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