The ALICE Electron Test Accelerator - Challenges, Achievements, and Future Plans

1. The ALICE Electron Test Accelerator - Challenges, Achievements, and Future Plans Professor Jim Clarke ASTeC, STFC Daresbury Laboratory & Cockcroft Institute JAI Lecture, 17th March 2011

2. Contents • Introduction to ALICE • Major Subsystems • Experimental Highlights • EMMA • Free Electron Laser • Future Plans • Summary

3. ALICE • Accelerators and Lasers In Combined Experiments • An R&D facility dedicated to accelerator science and technology • Offers a unique combination of accelerator, laser and free-electron laser sources • Enabling studies of electron and photon beam combination techniques • Provides a range of photon sources for development of scientific programmes and techniques

4. Reminder: 4GLS ERLP Funded in 2003 • Energy Recovery Linac Prototype • To develop skills and technologies for 4GLS: • Operation of photo injector electron gun • Operation of superconducting electron linac • Energy recovery from a FEL-disrupted beam • Synchronisation of gun and FEL output

5. ALICE Parameter Value Gun Energy 350 keV Injector Energy 8.35 MeV Max. Energy 35 MeV Linac RF Frequency 1.3 GHz Max Bunch Charge 80 pC

6. ALICE Milestones (Champagne Moments…) Oct 08: First Booster Beam Dec 08: Full Energy Recovery Feb 09: Coherently Enhanced THz Nov 09: CBS X-Rays Feb 10: IR-FEL Spontaneous Em. Mar 10: EMMA Injection Line Beam Apr 10: First THz Cell Exposures Aug 10: EMMA Ring 1000s turns Oct 10: IR-FEL First Lasing Aug 06: First Electrons

7. ALICE parameters

8. Photoinjector Gun ceramic was major source of delay (~1 year) Alternative ceramic on loan from Stanford was installed to get us started – still in use today! Limits gun voltage to 230 kV (cf 350 kV) Original ceramic is on shelf waiting for opportunity to be installed First electrons August 2006

9. Photoinjector Vacuum • XHV needed for good lifetime of cathode (GaAs) • UHV is not good enough! • A new in-situ bakeout procedure was developed which monitored the ratio of gas species in the vacuum system during the bake. • Evidence suggests that partial pressures of any oxygen containing species (CO, CO2 and H2O) should be < 10-14 mbar. Standard Bake Optimised Bake

10. Photoinjector upgrade • Never need to let up gun vacuum • Photocathode activated offline • Reduced time for photocathode changeover, from weeks to mins • Higher quantum efficiency • Allows practical experiments with photocathodes activated to different electron affinity levels • 15% achieved in offline tests (red light) • Allows tests of innovative photocathodes • Installation? Photocathode preparation facility

11. Activation chamber Loading chamber Hydrogen rejuvenation chamber

12. Superconducting Linacs • Both linacs were procured from ACCEL (now Research Instruments) • They each contain two 9-cell ILC type cavities (as used by XFEL) – 1.3 GHz • Linacs only designed to operate in pulsed mode (20Hz) • Would not be suitable for 4GLS or NLS type, high-rep rate, facilities

13. Linac Collaboration • International initiative led by ASTeC to develop linac module suitable for CW operation as required by a high rep rate facility (eg NLS) • Higher power and adjustable input couplers • Higher beam currents, CW operation • Piezo actuators provide improved stability control • Improved thermal and magnetic shielding • Better HOM handling • 7 cell cavities so space for HOM absorbers • Same footprint as ACCEL linac so can install in ALICE easily • Validation with beam

14. Linac Collaboration Current Module Will be installed into ALICE in 2011 New Module

15. Linac Collaboration 7 cell cavity Input coupler testing Outer cryomodule assembly HOM absorber

16. Camera: DicamPro Compton Scattering X-rays 800 nm pulses, ca. 70 fs duration, 500 mJ pulse power @ 10 Hz Scintillator Generation of short x-ray pulses by interacting a conventional laser with a low energy electron bunch Be window To linac and beam dump Laser beam deflection and focussing mirrors Horizontal beam size: 27 µm RMS Dipole magnet Interaction region Camera: Pixelfly QE Vertical beam size: 39 µm RMS Quadrupole-04 Correctors Quadrupole-03 DIAGNOSTICS ROOM ~40pC/bunch, 29.6 MeV Electron beam

17. Head on Collisions First data November 2009 Time delay Background: Electron beam ON Laser OFF Electron beam ON Laser beam ON Evidence points to mis-alignment DIAGNOSTICS ROOM Only 2 days of actual experimentation

18. Use of THz • CSR generated in THz region because bunch length ~1 ps • Output enhanced by many orders of magnitude (N2) • Dedicated tissue culture lab • Effect of THz on living cells being studied • Source has very high peak intensities but very low power – so no thermal effects!

19. EMMA • Fixed Field Alternating Gradient accelerators are an old idea (invented in 1950s) • They use DC magnets with carefully shaped pole profiles • The beam orbit scales with energy so the magnet apertures are large

20. EMMA • Non-Scaling Fixed Field Alternating Gradient accelerators are a new idea (invented in 1990s) • They use simple DC magnets (eg quadrupoles) • The beam orbit changes shape with energy enabling the magnet apertures to be small • EMMA is the first of this type – a proof of principle

21. Non-scaling FFAG • Born from considerations of very fast muon acceleration • Breaks the scaling requirement • More compact orbits ~ X 10 reduction in magnet aperture • Betatron tunes vary with acceleration (resonance crossing) • Parabolic variation of time of flight with energy • Factor of 2 acceleration with constant RF frequency • Serpentine acceleration • Can mitigate the effects of resonance crossing by:- • Fast Acceleration ~15 turns • Linear magnets (avoids driving strong high order resonances) • Or nonlinear magnets (avoids crossing resonances) • Highly periodic, symmetrical machine (many identical cells) • Tight tolerances on magnet errors dG/G <2x10-4 Novel, unproven concepts which need testing Electron Model => EMMA!

22. EMMA Goals Graphs courtesy of Scott Berg BNL

23. Understanding the NS-FFAG beam dynamics as function of lattice tuning & RF parameters Tune plane Lattice Configurations • Example: retune lattice to vary resonances crossed during acceleration Time of Flight vs Energy • Example: retune lattice to vary longitudinal Time of Flight curve, range and minimum Graphs courtesy of Scott Berg BNL

24. ALICE Provides the Beam EMMA

25. EMMA Parameters Diagnostics Beamline Injection Line

26. EMMA Ring Cell 65 mm Field Clamps 55 mm D D F Low Energy Beam • 42 identical doublets • No Dipoles! Cavity Beam stay clear aperture High Energy Beam 110 mm Magnet Centre-lines 210 mm Independent slides

27. Injection Septum Kicker Kicker Septum Power supply

28. Realisation of EMMA August 2010

29. First Data ... Second Turn First Turn September 2010 - beam circulates more than 1000 turns Aug 2010 - First turns

30. Extraction (07/03/11) • Going clockwise towards extraction • Yellow = Inj. Kicker1 • Pink = Ext. Kicker1 • Green = Ext. Kicker2 • Blue = beam • Action of injection kicker too early to be seen • Spikes = turns • Effect of extraction clearly visible • Image seen on first YAG screen in extraction / diagnostic line 

31. Trina Ng Optical Clock Distribution Scheme Highly stable clock distribution across large scale facilities is important for the synchronisation of beam generation, beam manipulation components and end station experiments. Optical fibre technology can be used to combat the stability challenges in distributing clock signals over long distances with coaxial cable. An actively stabilised optical clock distribution system based on the propagation of ultra-short optical pulses has been installed on ALICE. Femtosecond pulses emerging at the far end are currently used to implement a beam arrival monitor. However, the clock signals could also be integrated into other diagnostic systems such as electro-optical beam diagnostics. Fibre Stabilization Interferometer Feedback Loop Circuitry Fibre Stretcher • Link Operation • 60 fs pulses are distributed to BAM sites around ALICE. • Half the pulse power will be reflected back at the far end to enable detection of optical path length changes. • Timing is actively stabilized with a fibre stretcher and delay line. • The other half of the timing stabilized pulses will be used to measure the arrival time of electron bunches and other diagnostics. EOM Mode-Locked Fibre Ring Laser (81.25 MHz) Single Mode Distribution Fibre (100m) Accelerator Area Beam Arrival Monitor Faraday Rotating Mirror (50:50) Detector RF pickup Beamline

32. S.P. Jamison ALICE Electro-optic experiments • Energy recovery test-acceleratorintratrain diagnostics must be non-invasive • low charge, high repition rate operation typically 40pC, 81MHz trains for 100us Spectral decoding results for 40pC bunch • confirming compression for FEL commissioning • examine compression and arrival timing along train • demonstrated significant reduction in charge requirements

33. Laser-electron Beam Interactions • New concepts & proof-of-principle tests • Developing technique for direct phase-space manipulation of electrons with longitudinally laser & unipolar THz pulses. • Aim to adjust phase-space without need for modulators/chicanes EM Source development and testing ALICE experiment in final stages of preparation ... propagation direction

34. Oscillator FEL Process

35. ALICE IR-FEL • Dec 2009/Jan 2010: FEL Undulator and Cavity Mirrors installed and aligned. • Throughout 2010: FEL/THz/CBS programmes proceeded in parallel with installation of EMMA. One shift per day of beamtime for commissioning. • Of available beamtime, FEL programme gets ~15%. • Progress: • Feb 2010: First observations of undulator spontaneous emission. Stored in cavity immediately. • But no lasing could be found. Problem was that we were limited to 40pC: above 40pC @ 81.25Mz beam loading prevented constant energy along 100µs train. • On 17th October 2010 we installed a Burst Generator to reduce laser repetition rate from 81.25MHz to 16.25 MHz and increased bunch charge to 60pC. • A week later, on 23rd Oct 2010 achieved first lasing @ 8µm • Shutdown Nov/Dec 2010 • Jan/Feb 2011: Lasing from 8.0-5.7µm • Mar 2011: IR transported out of ALICE area to beyond shield wall

36. FEL SYSTEMS + Transverse/Longitudinal Alignment LASER TRACKER ALIGNMENT WEDGES ALIGNMENT WEDGES ALIGNMENT MIRROR ALIGNMENT MIRROR ALIGNMENT MIRROR ALIGNMENT MIRROR (OPTICAL TARGET) ELECTRONS ELECTRONS OPTICAL TARGET OPTICAL TARGET REFERENCE AXIS FEL-M1 POWERMETER POWER METER FEL-WIG-TRANS-01 DOWNSTREAM FEL MIRROR DOWNSTREAM FEL MIRROR INFRA-RED UPS-LAM-01 UPS-LAM-02 DWN-LAM-02 DWN-LAM-01 OPTICAL TARGET OPTICAL TARGET UNDULATOR ARRAYS UNDULATOR ARRAYS MCT DETECTOR HeNe HeNe HeNe HeNe (OPTICAL TARGET) CCD VIEWER CAMERAS CCD VIEWER CAMERAS CCD VIEWER CAMERAS SPECTROMETER MCT DETECTOR FEL-M2 ALICE FEL Systems Schematic 5. Electron Beam steered to Alignment Wedges 3. Downstream Mirror aligned using Upstream HeNe 1. Undulator Arrays and Optical Targets surveyed onto Reference Axis with Laser Tracker 2. Alignment Wedges and Downstream Mirror aligned optically using Theodolite 4. Upstream Mirror aligned using Downstream HeNe 6. Cavity length scanned looking for enhancement of spontaneous emission, then LASING. SPECTROMETER

37. FEL Overview DOWNSTREAM MIRROR UNDULATOR UPSTREAM MIRROR BUNCH COMPRESSOR

38. FEL Undulator

39. FEL Resonator UPSTREAMMIRROR DOWNSTREAM MIRROR

40. FEL Local Diagnostics LASER POWER METER DOWNSTREAM ALIGNMENT HeNe FEL BEAMLINE TO DIAGNOSTICS ROOM PYRO-DETECTORon Exit Port 2 SPACE FOR DIRECT MCT DETECTOR SPECTROMETER Based upon a Czerny Turner monochromator MCT (Mercury Cadmium Telluride) DETECTOR on Exit Port 1

41. Spontaneous Emission as a Diagnostic 1. Spectrum used to optimise steering in undulator Spontaneous emission a useful diagnostic February 2010: 1st Observation Shortest wavelength + Narrowest Bandwidth when beam on reference axis 2. Coherent enhancement used to set minimum bunch length 3. Interference of coherent SE used to set correct cavity length Intensity enhancement at maximum bunch compression Intensity Oscillations at λ/2 in cavity length indicating round trip interference

42. ALICE IR-FEL: First Lasing First Lasing Data: 23/10/10 Simulation (FELO code)

43. Results from First Lasing Period (23-31 October 2010) Implies electron bunch length ≈1ps, in agreement with previous EO measurements of a similar ALICE setup

44. Results from First Lasing Period (23-31 October 2010) NB: No optimisation done at higher charges (just turned up the PI laser power (to 11)) • Gain determined from cavity rise time • From one pulse train to the next (@10Hz) the gain jitters • Cause under investigation. Phase jitter in pulsed RF? Laser jitter?.... • On average the gain is lower than we want: • rms Energy spread of 0.6% is too big: degrades the gain significantly • Aim to halve energy spread and double gain • Can then change to outcoupler with larger hole • Can set up beam to achieve this (set injector to deliver shorter bunch to linac) but haven’t yet lased with this setup – still to be understood! Should work, but doesn’t!

45. Results from February 2011: Gap Tuning

46. ALICE FEL Future Plans • Improved electron beam set-ups with reduced energy spread and jitter. • Transport of FEL beam to diagnostics room, then full output characterisation. • Slightly reduced Mirror ROC to improve gain, plus selection of outcoupling hole sizes to optimise output power. • Plan to run at 27.5MeV (5-8µm) and 22.5MeV (7-12µm) • Beyond that depends on funding being obtained for specific exploitation programmes. • But ALICE itself will not run indefinitely. • We are now thinking beyond ALICE…. Simulation results

47. The Future ... • Concepts for post-ALICE future hundred-MeV-scale electron test accelerators are currently under development in consultation with other stakeholders (including JAI!). • Potential topics of interest: • Ultra-Cold injectors (low emittance, low charge, velocity bunching, fs bunches…..) • Novel acceleration (laser plasma….) • Compact FELs (short period undulators….) • Attosecond FEL pulse generation (slicing, modelocking…) • Novel FEL seeding schemes (HHG, self-seeding, EEHG….) • FEL pulse diagnostics • Will be a national and international collaboration taking ~12 months to develop the plans in more detail.

48. Summary • ALICE is an extremely versatile and flexible test accelerator • We have gained practical experience/skills of several key accelerator technologies • Photoinjectors • SRF & 2K cryo • High power laser/electron interactions • FELs • Timing & Synchronisation • Energy Recovery • Coherent SR • ..... • EMMA is currently being commissioned (using ALICE as the injector) • Plans are being drawn up for future test facilities – please join in the discussion!

49. Acknowledgements • Thanks to the following for providing slides and other material • Neil Thompson • Bruno Muratori • Elaine Seddon • Neil Bliss • Rob Edgecock • Steve Jamison • Peter McIntosh • Susan Smith • Keith Middleman • Trina Ng