Modelling of the ALICE Injector

1. Modelling of the ALICE Injector Julian McKenzie ASTeC STFC Daresbury Laboratory IOP Particle Accelerators and Beams Group Status and Challenges of Simulation and Computation for Accelerators 25 February 2011

2. ALICE Overview • Nominal Gun Energy 350 keV • Injector Energy 8.35 MeV • Circulating Beam Energy 35 MeV • RF Frequency 1.3 GHz • Bunch Repetition Rate 81.25 MHz • Nominal Bunch Charge 80 pC • Average Current 6.5 mA (Over the 100 ms Bunch Train)

3. ALICE Injector Layout cathode buncher Booster cavities solenoid solenoid 0.23 m 1.3 m 1.67 m 2.32 m 3.5 m 5 m Buncher = 1.3 GHz, single cell, normal conducting Booster = 1.3 GHz, 2 x 9 cell, super-conducting Corrector coils in each solenoid, plus another set of correctors. 2 BPMs, one YAG screen (just after buncher)

4. Current ALICE gun – JLab FEL gun clone • 350kV DC photocathode gun • 80pC bunch charge • 25mm GaAs photocathodes • Activated in-situ • 532 nm photoinjector laser

5. Current gun 3D electrostatic model (CST)

6. ALICE Injector Layout cathode buncher Booster cavities solenoid solenoid 0.23 m 1.3 m 1.67 m 2.32 m 3.5 m 5 m

7. ASTRA simulation history • Initial design before build: • 80 pC, 350 keV gun, 8.35 MeV total • Re-modelled before commissioning: • Takes into account apertures in the machine (particularly small in the buncher) and more realistic laser parameters • Current modelling for comparison to real machine • 20-80 pC, 230 keV gun, 6.5 MeV total

8. Firstly: There is NO fixed parameters so far • ALICE operations have used a large variety of injector parameters • Caused by different user needs: FEL, THz, EMMA… • Different bunch charges: 20 – 80 pC • Requires different solenoid strengths, different buncher power • Different bunch lengths: • Requires different buncher power, BC1 phase • Different energy spreads: • Different BC2 phase • Over time even operated at different energies: 8.35 MeV  4.8 MeV  6.5 MeV

9. More changes… • Solenoid 1 fixed at a certain strength for a long time to missteer a strong field emission spot • Laser transverse profile changes over time • For simulations assume 4mm flat-top transverse and 28 ps flat-top temporal profiles • This year, in an attempt to standardise the setup: • Fixed BC1 phase has been fixed at -20 • Energy after BC1 fixed at 4 MeV • Energy after BC2 fixed at 6.5 MeV • Still a lot of variables remain, BC2 phase, bunch charge, buncher power, solenoid strengths…

10. Transverse dynamics Settings used match those used in a recent shift. Not sent up to minimise emittance. No thermal emittance included. However, doesn’t add linearly – 1 mm mrad thermal adds < 1 mm mrad to final

11. Longitudinal dynamics 1

12. Longitudinal dynamics 2 BC2 phase used to compensate energy spread from first cavity Note: at such low gun energy, almost decelerate the beam completely at start of BC1

13. Beam profiles Note again that beamline settings taken from recent shift, not design values.

14. Code comparison • ASTRA (red) • GPT (green) • 10,000 macroparticles • Only slight difference in transverse emittance. • GPT is fully 3D • ASTRA is 2.5D • cylindrically symmetric grids. • 3D ASTRA available uses difference space charge model and no image charges on the cathode

15. Booster issues Booster uses two 9 cell TESLA cavities Each cavity approx 1m long Designed for ultra-relativistic beam We inject with 230 keV beam, accelerate to 4 MeV in first cavity Therefore there is much phase slippage in the cavity How do we define the phase of the cavity?

16. BC1 cresting issues at 40 pC Experimental data ASTRA simulation Blue = maximum energy Red = central energy Green = minimum energy

17. Cresting at 1 pC Central (red) energy now has a symmetric distribution. This crest matches the crest as found by ASTRA’sautophase procedure

18. Offset injection into booster • In the real machine, we are never on-axis in the injector beamline. • We start with an offset laser spot and then enter a solenoid. • Plus further effects from stray fields etc. • We have 3 sets of correctors to steer the beam before the booster. Using GPT, offset the beam from 0 to 5 mm on entrance to the booster:

19. Offset injection into booster (Barely noticeable changes to bunch length and energy spread) Not much change in beam size But large change in emittance…

20. Offset injection into booster For an offset beam, different parts of each beam see different transverse field from cavity, this leads to the emittance increase observed 1 mm offset probe particle 3 mm offset probe particle

21. Laser image as input distribution Previous simulations have always assumed a circular laser spot – often far from reality. We can use a laser image to create an initial distribution for simulations. Image of laser spot on cathode (note, not direct image, many reflections etc) Convert to 8bit greyscale Input into GPT as initial beam distribution

22. Elliptical vs round laser spots Red = round beam Green = elliptical laser image, x Blue = elliptical laser image, y Note, start with a laser spot with larger y, but beam gets rotated 90 degrees by two solenoids so x is bigger

23. Elliptical vs round laser spots Red = round beam Green = elliptical laser image, x Blue = elliptical laser image, y

24. Elliptical laser beams Create basic ellipses to test how emittance varies as a function of ellipticity.

25. Comparison of emittance measurements A large variety of emittance measurements have been carried out in the ALICE injector using different methods and different tools to analyse the same data. One problem is that the measurements have not been made with the same injector setups. The different methods do not agree but the measurements have always been much larger than simulations (which have always assumed a round laser spot) have suggested. Using the elliptical distribution and measuring both x and y emittance shows a clearer agreement.

26. However, in 2011, beam is circular In the 2010/2011 shutdown, much work was done on the photoinjector laser. The beam is now fairly circular and same initial size as model

27. Elliptical beam 4.65mm 10mm However, beam on first screen is still elliptical. Simulations obviously suggest we should have a round beam, however, dimensions roughly match that of the screen image. Entering solenoid off-centre still produces round beam Need assymmetric field…

28. Stray field measurements Background fields measured at every accessible pre-booster. Measured above, below, and on either side of the vacuum vessel. Ambient level also taken in the injector area. Lots of interpolation done from these measurements to create a 3D fieldmap for input into GPT. Lots of errors however, simulations still show the effect of random field errors. Magnetic field [mT] Distance from cathode [mm]

29. Stray field simulations 1 No stray fields (red),stray fields (green), stray fields with corrections (blue) Simulations performed on the design baseline of 80 pC, 350 keV  8.35 MeV We have three correctors pre-booster Used these to centre on the screens before and after the booster

30. Stray field simulations 2 No stray fields (red),stray fields (green), stray fields with corrections (blue) Note: effect larger at the lower gun energy we currently use

31. Elliptical beam 2 4.65mm 10mm Back to the elliptical beam on screen 1 Introducing stray fields along the injector produced a beam on the first screen which is approx 15 x 8 mm. Clearly elliptical. Therefore are stray fields a reason for our elliptical beam?

32. Thanks to all the ALICE team!

33. Bunch length measurements versus simulations Buncher power 130 W ~ 0.6 MV/m Buncher power 510 W ~ 1.2 MV/m Low energy tail observed in machine, BC2 zero-cross method gave 12.7 mm full width Measurements using BC2 zero-cross method gave 2.1 and 1.9 mm full width