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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

Pre-Chopper Beam Dynamics

JB. Lallement

Linac4 BCC - 29/07/2010

slide3

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%)
slide4

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

lebt transmission
LEBT transmission

Pre-chopper ON

Pre-chopper ON-OFF

SC recovering

Source output current

LEBT output current

> 100 µs

2 µs

50 µs

rfq transmission
RFQ transmission

Pre-chopper ON

Pre-chopper ON-OFF

SC recovering

LEBT output current

RFQ output current

> 100 µs

2 µs

50 µs

rfq transmission1
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

losses distribution in the rfq
Losses distribution in the RFQ

Pre-chopper ON

Beamcurrent 80 mA

No SC compensation

7.5 mA lost – Localized !!!

losses distribution in the rfq1
Losses distribution in the RFQ

Pre-chopper OFF

Beamcurrent 80 mA

SC compensation 75%

12 mA lost - Distributed

chopper line transmission chopper on
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

conclusion

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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