Pre chopper beam dynamics
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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

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Pre chopper beam dynamics

Pre-Chopper Beam Dynamics

JB. Lallement

Linac4 BCC - 29/07/2010


Pre chopper beam dynamics

LEBT – pre-chopper Layout


Pre chopper beam dynamics

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%)


Pre chopper beam dynamics

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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