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Accelerator Physics on 4GLS

Accelerator Physics on 4GLS. Peter Williams Cockcroft Institute & ASTeC, Daresbury Laboratory CI All Hands Meeting, 30 th March 2007. A Fourth Generation Light Source. Study of molecular processes require photons – intense, focussed SR is only practical way to produce

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Accelerator Physics on 4GLS

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  1. Accelerator Physics on 4GLS Peter Williams Cockcroft Institute & ASTeC, Daresbury Laboratory CI All Hands Meeting, 30th March 2007

  2. A Fourth Generation Light Source • Study of molecular processes require photons – intense, focussed • SR is only practical way to produce • Third generation sources (DIAMOND) enable exploration of structure • Fourth generation sources will allow study of dynamics • Requires short [fs], bright [1030 photons/s/(mm mrad)2/(0.1% bw)] pulses • Storage rings cannot deliver – beam quality degrades, low E -> Linacs • High average flux requires energy recovery • Some of the most exciting applications demand combination of sources (pump-probe) • 4GLS proposal unique: combines energy recovery, undulators and Free-Electron Lasers • Capability to produce and combine multiple, tuneable, synchronised femtosecond pulses from THz to soft X-ray.

  3. A Fourth Generation Light Source • Status: Working towards TDR production in March 2008 Today’s talks on 4GLS – tasters from current work • Introduction and Accelerator Physics Issues (Peter Williams) • High Average Current Photoinjector (Julian McKenzie) • VUV Free-Electron Laser (Neil Thompson) • Energy Recovery Linac Prototype Commissioning & Simulation (David Holder) Collaborating Institutions

  4. ERLP: A Prototype for 4GLS Technology • Nominal Gun Energy 350 keV • Injector Energy 8.35 MeV • Circulating Beam Energy 35 MeV • Linac RF Frequency 1.3 GHz • Bunch Repetition Rate 81.25MHz • Max Bunch Charge 80 pC • Bunch train 100 ms • Max Average Current 13 µA

  5. 4GLS Beamlines Schematic • Dual High Peak Current XUV-FELs • High Average Current ID Loop & High Peak Current VUV-FEL Share main linac (three beams!) • Dual High Peak Current IR-FELs

  6. 4GLS Beamlines: XUV-FEL

  7. 4GLS Beamlines: High Average Current Loop

  8. 4GLS Beamlines: IR-FEL

  9. Peak Brightness Comparison

  10. Peak Flux Comparison

  11. Detailed 4GLS Layout

  12. AP Issues on 4GLS (not comprehensive!) • HACL Path Length Adjustment (Peter Williams) • XUV Bunch Compression Scheme (Hywel Owen & Peter Williams) • HACL Bunch Compression Scheme (Hywel Owen) • HACL 156° & 48° Arc Lattice Designs (Bruno Muratori) • XUV Synchronisation / Timing Jitter (Graeme Hirst, Hywel Owen & Peter Williams) • XUV Microbunching Instability Suppression (CSR, LSC, Linac Wakefields) (Peter Williams) • Three-Beam Beam Break-up Calculations (Bruno Muratori & Emma Wooldridge) • XUV / HACL Spreader (Hywel Owen) • XUV Splitter to FELs (Hywel Owen) • Wakefield Calculations (Duncan Scott) • Start-to-end Simulations (Peter Williams) • Shielding & Beam Loss Specification (Hugh Rice)

  13. 1 nC 77 pC ~9° 180° Path Length Adjustment for 4GLS HACL • Figure shows bunch placement on main linac RF waveform • We must energy recover the 77pC HACL bunches • Ensure π out of phase for deceleration • Reliably introduce path length retardation of up to 1 wavelength ~23cm • Must do achromatically, ie without introducing transverse dispersion (ηx), desirable to do this independently from longitudinal dispersion (R56), possible to achieve isochronously (R56=0) • In ERLP this will be done by physically moving first arc – impractical in 4GLS due to size • We propose novel system, purely magnetic, dedicated section just before re-entry to main linac, independent from rest of machine

  14. Path Length Adjustment for 4GLS HACL • Two non-dispersive doglegs girder mounted such that they move transverse to the beamline introducing extra path, coupled by set of bellows that expand accordingly • These introduce negative longitudinal dispersion (R56) (higher energy particles retarded with respect to bunch centre) • Compensate for this with classical dispersive chicane, introduces positive R56 • Chicane need not physically move, dispersion between centre dipoles ensures only small angle deviation compensates → achieve by ramping magnets

  15. Path Length Adjustment for 4GLS HACL Dispersion in Doglegs – Maximal Displacement R56 in Doglegs – Maximal Displacement Matched β-functions in Chicane + Doglegs – Maximal Displacement Dispersion in Chicane + Doglegs – Maximal Displacement R56 in Chicane + Doglegs – Maximal Displacement Ratio of Path Length Introduced to RF Wavelength As a Function of Dogleg Displacement Angle

  16. Path Length Adjustment for 4GLS HACL Engineering Layout of Doglegs

  17. Bunch Compression • 4GLS photoinjectors will produce ‘long’ bunches ~1-5 ps, whilst the FEL’s require short bunches ~200 fs or even less • Compression is achieved in two stages: • Imprint a ‘chirp’ on the bunch (non-zero R65): accelerate off RF crest → energy depends on electron position in bunch • Pass through system with non-zero R56: eg negative chirp and positive R56 → higher energy electrons are behind bunch centre, they take shorter path through classical chicane and ‘catch up’ Off-crest acceleration (area/energy spread has reduced because of acceleration) Initial toy distribution All 3 plotted together: the normalised area is conserved Compression Bunch compression is often referred to as ‘phase space rotation’. Strictly speaking this is incorrect (particles top-left stay top-left); geometrically compression is a pair of shears

  18. Bunch Compression in 4GLS XUV • Compression best achieved in two stages – minimise RF imposed curvature in second acceleration stage & avoid wakefield induced bunch degradation in 180° XUV arc • Similar solutions adopted in TTFII, BESSY-FEL, FERMI, LCLS • Nevertheless, turns out that RF non-linearity major factor in achieving required fs bunch lengths • Two solutions, T566 (sextupoles) or 3rd harmonic cavity

  19. Bunch Compression in 4GLS XUV, T566 or 3rd Harmonic? • 3rd harmonic better, but expensive • If you want to linearise using sextupoles, then you have to keep your input bunch length short, < 3ps at exit of injector • Challenging, but achievable specification for 4GLS XUV injector

  20. Progressive Bunch Compression in 4GLS HACL VUV-FEL Golden Rule: beam ‘compression’ (i.e. shear) only goes one way. Mode 1: All Short (Probable wakefield limit) Mode 2: Centre Short Beam Propagation Mode 3: VUV Short

  21. 4GLS HACL 156° & 48° Arc Lattice Designs ID straights

  22. XUV Synchronisation / Timing Jitter • For a seeded FEL to lase, seed laser pulse and arrival of electron bunch at FEL must be tightly synchronised, ≤50 fs for 4GLS XUV • Initial estimate using 1D model, treat accelerator elements as mappings on bunch – compare with full start-to-end simulation in due course • Shown below are contributions to the timing jitter (fs rms) between seed photon pulses and electron bunches at the entrance to the XUV-FEL

  23. XUV Synchronisation / Timing Jitter • Model predicts timing sensitivity to % RF amplitude noise of 14.5 ps/% • RF amplitude noise will be maintained below 0.01% → 145 ps  • Clearly, RF amplitude jitter must be reduced • Feedback control system is possible solution

  24. Energy l l t Current modulation Gain=10 10% 1% t Microbunching Instability in 4GLS XUV • Small modulations in density induce modulations in energy through impedance Z(k) • Bunch compression (path length dependence on energy) converts this to more density modulation → amplification and the beam quality is degraded significantly → energy spread at FEL too much for lasing • Three sources of impedance: Coherent Synchrotron Radiation induced in compressors, Longitudinal Space Charge throughout & Linac Wakefields

  25. 800 nm laser pulse 10 cm 50 cm 2 cm 10 period undulator q 5.7º 10 cm ~120 cm Microbunching Instability in 4GLS XUV • Saldin proposed solution to problem at TTF, increase incoherent energy spread prior to compression (within FEL tolerance), Landau damping – a ‘laser heater’ • LCLS concept shown below (from Z. Huang, SLAC) • Similar solution likely to be needed for 4GLS XUV – under study

  26. Detailed 4GLS Layout

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