1. Simon Jolly UKNFIC Meeting 25th April 2008
RFQ Simulations in GPT
Simon Jolly
UKNFIC Meeting
25th April 2008

2. Simon Jolly, Imperial College
RFQ Optimisation
Need to optimise RFQ: beam transmission, energy, bunching etc.
This is not easy! Requires fine modifications to vane modulations.
Need to model RFQ beam properties and measure beam properties at RFQ exit.
Start with simple RFQ model and work our way up...
25/4/08
Simon Jolly, Imperial College

3. Simon Jolly, Imperial College
GPT RFQ Simulations
General Particle Tracer is a particle tracking package: sophisticated particle tracking but only simple beamline components.
Need to model RFQ as time-varying E and B field map: track particles through field map and measure beam properties.
Start with RFQ cold model (simple case with simple field) and work up to full 4m FETS RFQ channel.
25/4/08
Simon Jolly, Imperial College

4. Cold Model Simulation in GPT
Cold model field map produced by CST (Ajit).
Large, coarse and simple (no vane modulations):
100x100x100 mesh points.
z: -50 to 450mm with 5mm spacing.
x/y: -162 to 162mm with 3.3mm spacing.
Simulate with standard FETS parameters:
Input beam: 60mA, 65keV, x/y = 2mm, x’/y’ = 100mrad, ex/ey = 0.2p mm mrad, beam converging.
5000 particles, 0.2ns timestep, 100% 2Dtree space charge.
How much beam is transmitted? Where do losses occur?
Use Matlab to analyse data, colour-code losses, construct movie.
25/4/08
Simon Jolly, Imperial College

5. Cold Model Simulation: Initial Conditions
25/4/08
Simon Jolly, Imperial College

6. Cold Model Simulation: Trajectories
25/4/08
Simon Jolly, Imperial College

7. Transmitted Particles: Phase Dependence
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Simon Jolly, Imperial College

8. Simon Jolly, Imperial College
FETS Layout
Ion Source
Beam Diagnostics
Laserwire Tank
LEBT
RFQ
MEBT/ Chopper
25/4/08
Simon Jolly, Imperial College

9. From Cold Model to Full Simulation
Field map for full 4m FETS simulation created by Alan Letchford:
11 x 11 x 3110 mesh points.
x/y: -3.5 to 3.5mm (fixed mesh).
z: 0 to 4.1m (variable mesh).
Includes transverse and longitudinal field modulations.
Input conditions:
Input beam: 60mA, 65keV, x/y = 2mm, x’/y’ = 100mrad, ex/ey = 0.2p mm mrad, beam converging.
10,000 particles, 0.3ns timestep (freq/10), 100% 3Dtree space charge.
Single bunch at injection with 3D space charge.
Vary current from 0-240mA.
Measure beam transmission, bunching and energy.
25/4/08
Simon Jolly, Imperial College

10. RFQ Transverse Field Map
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Simon Jolly, Imperial College

11. Simon Jolly, Imperial College
RFQ On-Axis Ez Field
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Simon Jolly, Imperial College

12. RFQ On-Axis Ez Field (0-0.5m)
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Simon Jolly, Imperial College

13. Full FETS Simulation: Initial Conditions
25/4/08
Simon Jolly, Imperial College

14. Full FETS Simulation: Z-Y
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Simon Jolly, Imperial College

15. Simon Jolly, Imperial College
Final Beam Energy (60mA)
25/4/08
Simon Jolly, Imperial College

16. Simon Jolly, Imperial College
RFQ Beam Transmission
25/4/08
Simon Jolly, Imperial College

17. RFQ Transmitted Current
25/4/08
Simon Jolly, Imperial College

18. Simon Jolly, Imperial College
Conclusions
RFQ simulations extremely promising: we can see bunching, acceleration and losses.
RFQ design is almost optimal! Already transmitting ~85% of beam within 10% of target energy (3MeV).
Need to measure acceptance of RFQ: input very large beam and see what survives.
Also measure beam transmission with measured beam parameters from FETS ion source, tracked through LEBT.
Optimise RFQ field and layout.
25/4/08
Simon Jolly, Imperial College