ALICE Beam Simulations Deepa Angal-Kalinin On behalf of

ALICE Beam Simulations Deepa Angal-Kalinin On behalf of

ALICE Beam Simulations Deepa Angal-Kalinin On behalf of ALICE simulation team F. Jackson, J. Jones, J. McKenzie, B. Muratori, Y. Saveliev, P. Williams, A. Wolski FLS2012, Jefferson Lab, 5th -9th March 2012 Accelerators and Lasers In Combined Experiments An accelerator R&D facility @Daresbury Laboratory based on a superconducting energy recovery linac EMMA 1st arc (translatable) bunch compressor chicane ALICE THz beamline photoinjector TW laser laser

beam dump superconducting linac 2nd arc (fixed) DC gun 500KV PSU superconducting booster 2 ALICE Machine Description RF System Superconducting booster + linac 9-cell cavities. 1.3 GHz, ~10 MV/m. Pulsed up to 10 Hz, 100 S bunch S bunch trains Beam transport system. Triple bend achromatic arcs. First arc isochronous Bunch compression chicane R56 = 28 cm

Undulator Oscillator type FEL Variable gap DC Gun + Photo Injector Laser 230 kV GaAs cathode Up to 100 pC bunch charge Up to 81.25 MHz rep rate TW laser For Compton Backscattering and EO ~70 fS duration, 10 Hz Ti Sapphire Diagnostics YAG/OTR screens + stripline BPMs Electro-optic bunch profile monitor THz, FEL

BAM 3 ALICE : Operational Parameters Parameter Design Operating Units Bunch charge 80 20 - 80 pC Gun energy 350

230 kV Booster energy 8.35 6.5 MeV 35 27.5 MeV 81.25 16.25 - 81.25 MHz

Linac energy Repetition rate ALICE operates in variety of modes for different experiments : FEL, THz, EMMA, etc differing in requirements for Beam energies, Bunch lengths, Bunch charges, Energy spread, etc Gun voltage limited by ceramic replaced recently Linac energy and bunch repetition rate is limited by beam loading, replacing cryomodule with new DICC module towards end of this year. 4 ALICE Injector Layout Layout restricted by building Long (~10m) transport line between booster and linac 5

Injector Layout DC electron gun JLab FEL GaAs photocathodes buncher solenoid 0.23 m 1.3 m Booster cavities solenoid 1.67 m 2.32 m 3.5 m 5m

6 ALICE Simulations - ASTRA Initial ASTRA simulation of injection line measurements ASTRA vs. measurements in injector diagnostics line 40 35 BUNCH LENGTH @ 0.1, mm ASTRA was used in the design stage of ALICE (then called ERLP) injector1 (2003-2004) 80 pC, 350 keV gun, 8.35 MeV injector, 35 MeV Linac Re-modelled before commissioning taking into account apertures in the machine (particularly small in the buncher) and more realistic laser parameters During injector commissioning (2007) diagnostics line was used for dedicated measurements and comparison with ASTRA2

Only cathode booster exit was simulated initially (i.e. no dipoles) 125ps 30 25 20 dZ dZ-fit dZ-ASTRA Mapping Zero-cross 15 10 28ps 5 0 0 10 20

30 40 50 60 70 80 BUNCH CHARGE, pC C. Gerth et al Injector Design for the 4GLS Energy Recovery Linac Prototype, EPAC 04 2 Y. Saveliev et al Characterisation of Electron Bunches from ALICE (ERLP) DC Photoinjector Gun at Two Different Laser Pulse Lengths, EPAC 08 1 7 ALICE Simulations - ASTRA

ASTRA (without dipoles-replaced with quads) and GPT (with dipoles) compared for space charge effects in the injection line1. Start-to-end simulation used ELEGANT to track ASTRA results from booster exit through FEL to final beam dump2 Current modelling for comparison to real machine3,4,5 ASTRA-ELEGANT start-to-end simulations Energy spread and bunch length 20-80 pC, 230 keV gun, 6.5 MeV injector, 27.5 MeV Linac 1. B. Muratori et al, Space charge effects for the ERL prototype injector line at Daresbury, EPAC2005 2. C. Gerth et al, Start-to-end Simulations of the Energy Recovery Linac Prototype FEL, FEL 04 3. F. Jackson et al, Beam dynamics at the ALICE accelerator R&D facility, IPAC11 4 J. McKenzie et al, Longitudinal Dynamics in the ALICE Injection Line, ERL11 5 Y. Saveliev et al, Investigation of beam dynamics with not-ideal electron beam on ALICE ERL, ERL11

8 Energy spread Bunch length ~28 ps laser pulse formed by stacking 7ps Gaussian pulses Laser temporal profile in 2008 6 5 4 Red = after BC1 Blue = after BC2 Doesnt provide ideal flat-top

3 2 1 0 70 80 90 100 110 120 BC2 phase used to compensate energy spread from first cavity by rotating the chirp in longitudinal phase space. 9 The effect of varying the buncher and

BC1 phase on the longitudinal dynamics in the injector BC1 Phase -20deg -10deg -5deg The beam is not highly-relativistic in first cells of BC1, and the bunch sees a different phase in each cell as it is accelerated. This leads to non linear effects in the longitudinal phase space, and a hook developing at phases close to crest. Although shorter bunch lengths are achieved near crest, the intrinsic energy spread is poorer due to these effects. 10 Compression in booster to linac transport line

Total R56 of injection line ~30mm Very small compared to 28cm in chicane However, it is of the right sign to compress bunch if chirp not fully compensated by BC2 (For bunch compression setups tend to leave some positive energy chirp from BC2 (+10 to +40deg)) ELEGANT simulations can show compression but dont take into account all effects, space charge still important at 6 MeV 11 Elegant Simulations Unchirped bunch Black = After booster Red = Before linac Chirped bunch 12 Elegant with LSC on Unchirped bunch Chirped bunch

Black = After booster Red = Linac, no LSC Blue = Linac, with LSC 13 Beam optics: Arc1-to-Arc2 Undulator ARC 1 compression chicane ARC 2 ARC 2 for R56=28cm, would need linac phase of +10deg but need to compensate energy chirp in the bunch coming from injector from 0 to +5 deg; hence overall off-crest phase (for bunch compression) ; +15 / +16deg Sextupoles in AR1: linearization of curvature (T566)

14 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: Barely noticeable changes to bunch length and energy spread Not much change in beam size But large change in emittance 15 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 16 Laser image as input distribution Previous simulations have always assumed a circular laser spot often far from reality. Used 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 17 Elliptical vs round laser spots

Note, start with a laser spot with larger y, but beam gets rotated 90 degrees by two solenoids so x is bigger Red = round beam Green = elliptical laser image, x Blue = elliptical laser image, y Red = round beam Green = elliptical laser image, x Blue = elliptical laser image, y 18 However, in 2011, beam is circular In the 2010/2011 shutdown, much work was done on the photoinjector laser. The beam now fairly circular and same initial size as model 19 Elliptical beam

However, beam on first screen still elliptical. Simulations obviously suggest we should have a round beam, however, dimensions roughly match that of the screen image. 4.65mm 10mm Entering solenoid off-centre still produces round beam Need asymmetric field 20 Stray field measurements Background fields measured at

every accessible pre-booster. 0 0 0 0 -0 -0 -0 -0 -0 -0 -0 200 Ambient level also taken in the injector area. Lots of interpolation done from these measurements to create a 3D fieldmap for input into GPT. Magnetic field [mT] Measured above, below, and on

either side of the vacuum vessel. 0 0 0 0 0 0 -0 -0 -0 -0 -0 200 x 400 600 800 y

400 600 x y 800 0 z 0 z 0 Lots of errors however, simulations still show the effect of random field errors.

0 -0 -0 -0 200 400 600 800 Distance from cathode [mm] 21 Stray Field Simulations Simulations performed on the design baseline of 80 pC, 350 keV 8.35 MeV Used three correctors pre-booster to centre on the screens before and after the booster No stray fields (red), stray fields (green), stray fields with corrections (blue)

Note: effect larger at the lower gun energy we currently use 22 Elliptical beam 2 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? 4.65mm 10mm 23 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. 24 ALICE Simulations - ASTRA ASTRA continues to be used to reoptimise injector for realistic machine parameters during commissioning. ASTRA gave guidance on correct buncher and booster parameters required for small energy spread and bunch length, essential for FEL

and THz operation ASTRA global optimisation of injector parameters for optimum beam with realistic constraints Line ASTRA Dot - Expt Individual parameter scans in ASTRA + measurements 25 ALICE Simulations - ASTRA These simulations + experimental experience highlighted the importance of effects like velocity de-bunching and non-zero R56 in the injector. But ASTRA simulation of the whole injection line (including dipoles), to include all effects together, has not been achieved so far. Velocity debunching (ASTRA) and magnetic compression (ASTRA+ELEGANT) 26 ALICE Simulations - ASTRA Problems implementing full injector line with bends in

ASTRA, mainly due to the global co-ordinate frame used in ASTRA Makes beamline geometry difficult to define and beam trajectory is sensitive to geometry errors Also makes diagnostic screens difficult to simulate since ASTRA screen orientation w.r.t. beam axis difficult to define correctly 27 ALICE Simulations - GPT Gun and injector line design has been modelled in GPT, and compared to original ASTRA model ASTRA GPT Analysis shows that ASTRA and GPT agree very well Differences mainly due to space-charge meshes, as well as small differences

between different versions GPT model also includes full injector (cathode to linac) ASTRA GPT Comparisons between GPT and MAD/Elegant show relatively good agreement without space-charge Re-matched injector (in GPT) with spacecharge also shows good agreement 28 ALICE Simulations - GPT GPT model post-linac has issues Analysis of focusing in extraction chicane dipoles does not agree between MAD and GPT

Comparison between Real machine settings and GPT model agree reasonably well in the injector Slight tweaks to post-booster matching quadrupoles improve agreement Low gun voltage (230kV) and gun beamline steering suspected to account for most of the differences Space charge off for comparison Agreement quite good in longitudinal plane as well not shown here 29 ALICE Modelling - GPT Gun beamline taken from ASTRA model Injector design mapped automatically from MAD model Dipole fringe-field parameters taken from fitting 2D field maps Dipole magnetic lengths optimised to minimise steering

effects from fringe fields Quadrupole fields can be taken directly from the machine Based on measured calibration curves of Field vs. current 30 ALICE Modelling - GPT Bunch-length vs. Linac Phase FEL y (m) x (m) GPT linac model different to MAD model (Space charge on in injector, off in rest of the machine) Post-linac extraction chicane dipoles differ between MAD/GPT Re-match in MAD post-extraction

chicane: Energy Spread vs. Linac Phase 31 Conclusions The nature of ALICE accelerator R&D and experiments require different operating regimes. Injector dynamics complicated by reduced gun energy, long multi-cell booster cavity and long transfer line. Simulations/measurements still not fully understood more investigations under way Significant effort recently to simulate full machine with ASTRA and GPT. Non trivial to use dipoles. Making good progress with GPT. Need another code for comparison? (PARMELA , IMPACT) During this commissioning period, ALICE will operate at higher gun voltage (350 KV) with new photocathode. Some additional beam diagnostics will also be available which will help to understand some beam dynamics issues. We hope to progress on validating 6D machine model this year. 32 Thanks to all the ALICE team!


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