Re-circulating Linac Option

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1 FLS 2010, ICFA Beam Dynamics Workshop, SLAC, 1-5 th March 10 Re-circulating Linac Option Deepa Angal-Kalinin, Peter Williams & D. Dunning ASTeC, STFC, Daresbury Laboratory & The Cockcroft Institute

2 Contents Introduction to NLS Re-circulation lattice design Dogleg Linac matching Matching to Arcs and extraction Arc design Return path Switchyard/Spreader Start to End simulations Comparison with single pass Re-circulating Linac Option FLS, SLAC, 1 st -5 th March

3 New Light Source project : FEL source requirements Repetition rate 1 khz with an upgrade => CW path Superconducting to 10 khz - 1 MHz RF Photon energy range and tunability 3 FELs: FEL ev; FEL ev; FEL ev The required tuning => range Machine of FEL3, energy with the of 2.25 choice GeV of undulator type (APPLE-2) and minimum gap (8 mm) Pulse length & pulse energy => Laser HHG seeded, cascade GW power level in 20 fs (upgrade path to sub-fs pulses) Transverse and longitudinal harmonic coherence FEL Fully variable polarisation for FEL1 & 2, at least horizontal and circular (R/L) polarisation over the full range for FEL3 Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

4 Design optimisation of the facility Beam energy 2.25 GeV, bunch charge 200 pc Need constant slice parameters on a length of 100 fs or longer to accommodate present seed pulse and jitter (See Riccardo Bartolini talk Monday) No residual (or very small) energy chirp Baseline option of CW SCRF linac (Outline Design Report) Alternative option of re-circulating linac for cost savings Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

5 Re-circulating linac design Re-circulation Reduces capital cost & running cost Can extract at different energies (1.2 & 2.2 GeV) Provides natural upgrade path to higher energies Opportunities for feedback Additional issues compared to straight through option Combining and separating different energy beams CSR and ISR in the arcs, mergers/combiners & extraction Bunch compression and linearisation scheme restricted Jitter tolerances due to extra transport Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

6 Layout of NLS single pass option High brightness electron gun, operating (initially) at 1 khz photoinjector 3 rd harmonic cavity laser heater BC 1 BC GeV CW superconducting linac accelerating modules BC3 collimation diagnostics spreader IR/THz undulators This part is common for re-circulation option experimental stations gas filters FELs Total 18 SC modules 3 free-electron lasers covering the photon energy range 50 ev 1 kev, Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

7 Re-circulation linac layout Linac (2 modules) Gun Laser Heater 3ω BC1 Injection dogleg Linac (8 modules) Collimation + beam switchyard (same as single-pass) Extraction/ spreader BC3 Merger ~30m 180 arc BC2 180 arc FODO + possible path length corrector 0m 50m 100m 150m 200m Inject at ~200 MeV, two passes through 1 GeV Total 10 SC modules Compared to 18 in single pass Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

8 Layout comparison Single pass Recirculating Re-circulating Linac Option FLS, SLAC, 1 st -5 th March

9 Injector, linac module, LH, 3HC, BC1 3 rd harmonic Cavity 2 CW SCRF modules based on the TESLA module Bunch compressor BC1 Gun L-band NCRF 200 pc,18 ps FW, 135 MeV Normalised emittance 0.3 mm. mrad Diagnostic Possible laser heater Choice of beam energy at injection > 200 MeV for minimising longitudinal space charge & optimum accelerating modules. Laser heater is a place holder. Detailed microbunching studies not yet carried out. Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

10 Injection dogleg Last dipole of high energy chicane Need to merge with the high energy beam; Ratio of energies = 6 Achromatic and isochronous dogleg design Optimised number, locations and strengths of sextupoles energy chirp ~0.9%)using Simplex algorithm to minimise the emittance growth due to chromatic effects. Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

11 Re-circulation linac FODO between modules optimised at two different energies to minimise CSR blow up in first extraction dipole whilst simultaneously keeping 1.2 GeV twiss sensible. β x β y β x β y Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

12 Matching to arcs and extraction Extraction Dipole after Linac 1.2 GeV 2.2 GeV First dipole after the Linac separates 1.2 and 2.2 GeV beams. Re-designed to bring down the betas in both lattices R56 = 5 mm for matching to arcs and mm for extraction. Optimised locations and strengths of sextupoles in matching to the arc to minimise emittance. 1.2GeV 2.2GeV Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

Arc design Considered arcs from 4GLS, BESSY, LUX & DLS lowalpha BESSY best Four triple bend achromats, low dispersion Wider footprint, non-isochronous (R56~8 mm, same

13 Arc design Considered arcs from 4GLS, BESSY, LUX & DLS lowalpha BESSY best Four triple bend achromats, low dispersion Wider footprint, non-isochronous (R56~8 mm, same sign as BCs) ISR emittance growth from both arcs 3.6% Two sextupole families to correct the chromaticities Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

14 Return pass Return transport connecting arcs is simple FODO-cells with phase advance of 45 (can be increased to reduce number of magnets). Possible to include few screens to study the beam properties at 1.2 GeV. Plan to have isochronous path length adjuster. 4GLS design, based on moving girders - would give independent phase control on second pass of linac (this control has been assumed in the simulations) Path length corrector Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

15 Beam switchyard/spreader Beam switchyard design based on fast kicker and scheme like LBNL has been chosen as it provides easy path - to include more FELs - to switch the required bunches (or all the bunches) to any one particular FEL branch using a DC dipole at kicker location First branch is dedicated diagnostics branch for tomography 8 Take-off section of FELs Arc 1 : TBA 0 Kicker Septum Arc 2 : TBA Details of design of one spreader line Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

16 Machine optics (start to exit of the spreader) Arc Arc Linac - 2 nd pass Spreader BC1 Dogleg Linac - 1 st pass Coll Return FODO Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

17 Simulation results : projected emittance Return FODO Extraction, BC3 Collimation BC1 Dogleg Linac - 1 st pass Match to arc, arc arc,bc2, merger Linac - 2 nd pass Spreader Final normalised projected emittances in x/y ~ 0.6/0.43 mm.mrad Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

18 Emittance Start to End of spreader Recirculation x/y ~ 0.6/0.43 mm.mrad x/y ~ 0.6/0.3 mm.mrad Single pass Re-circulating Linac Option FLS, SLAC, 1 st -5 th March

19 Parameters exit spreader (Nov 09) Tracked 100k particles. The data is binned in 1fs slices. Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

20 Luus-Jaakola optimisation (Feb 10) Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

21 Parameters exit spreader (Feb 10) Tracked 100k particles. The data is binned in 1fs slices. Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

22 Parameters exit spreader (Nov 09) Tracked 100k particles. The data is binned in 1fs slices. Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

23 Bunch parameters at the exit of spreader (Single Pass) Longitudinal current distribution Normalised emittance(m) Current (A) Normalised emittance rms relative energy spread(%) rms relative energy spread R. Bartolini Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

24 Longitudinal current distribution Single pass Recirculation Re-circulating Linac Option FLS, SLAC, 1 st -5 th March

25 Emittance Single pass Recirculation Re-circulating Linac Option FLS, SLAC, 1 st -5 th March

26 Relative energy spread Single pass Recirculation Re-circulating Linac Option FLS, SLAC, 1 st -5 th March

27 Comparison with single pass time-dependent results 22.5m 17.5m Comparison between radiation profiles of the recirculating and single pass cases at a comparable power level Re-circulating Linac Option FLS, SLAC, 1 st -5 th March

28 Summary Many interesting beam dynamics challenges have been understood and solved for the NLS re-circulation option. The design is very close to achieving simultaneous bunch properties required for the FEL operation. Continuing with the automatic optimisation process to tailor bunch to flat top at 1kA and obtain less energy chirp on the bunch. First results show that the optimisation process gives a flat region (but energy chirp remains rather high) The design will be documented in the planned Conceptual Design Report of NLS (to be published in May 10) to be re-visited in the future if required. Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

29 Thanks to J. Jones (ASTeC/STFC) B. Fell (EID/STFC) R. Bartolini Diamond Light Source Ltd. The NLS Physics and Parameters Working Group & D. Douglas, G. Krafft (JLab) for useful discussions Re-circulating linac LinacOption FLS, SLAC, 1 st -5 th March

UK FEL R&D Plans Jim Clarke

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