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

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

New Light Source project : FEL source requirements Repetition rate 1 kHz with an upgrade path to 10 kHz - 1 MHz Photon energy range and tunability 3 FELs: FEL1 50-300 eV; FEL2 250-850 eV; FEL3 430-1000 eV The required tuning range of FEL3, with the choice of undulator type (APPLE-2) and minimum gap (8 mm) Pulse length & pulse energy GW power level in 20 fs (upgrade path to sub-fs pulses) Transverse and longitudinal coherence Fully variable polarisation for FEL1 & 2, at least horizontal and circular (R/L) polarisation over the full range for FEL3 => CW Superconducting RF => Machine energy of 2.25 GeV => Laser HHG seeded, cascade harmonic FEL These are the baseline specification as found in the published ODR for the single pass machine. www.newlightsource.org

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 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

Layout of NLS single pass option This part is common for re-circulation option High brightness electron gun, operating (initially) at 1 kHz photoinjector 3rd harmonic cavity BC1 BC2 BC3 laser heater accelerating modules collimation diagnostics spreader FELs IR/THz undulators gas filters experimental stations 2.25 GeV CW superconducting linac 3 free-electron lasers covering the photon energy range 50 eV–1 keV, Total 18 SC modules

Re-circulation linac layout Laser Heater Collimation + beam switchyard (same as single-pass) Linac (2 modules) BC1 Injection dogleg Linac (8 modules) Extraction/spreader Gun 3w BC3 Merger BC2 ~30m 180˚ arc 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

Layout comparison Single pass Recirculating

Injector, linac module, LH, 3HC, BC1 3rd harmonic Cavity Bunch compressor BC1 2 CW SCRF modules based on the TESLA module Diagnostic Gun L-band NCRF Possible laser heater 200 pC,18 ps FW, 135 MeV Normalised emittance 0.3 mm. mrad 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.

Injection dogleg sextupoles Need to merge with the high energy beam; Ratio of energies = 6 Achromatic and isochronous dogleg design Optimised number, locations and strengths of energy chirp ~0.9%)using ‘Simplex algorithm’ to minimise the emittance growth due to chromatic effects. sextupoles Last dipole of high energy chicane

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. __ bx __ by __ bx __ by

Matching to arcs and extraction 1.2 GeV Extraction Dipole after Linac 10˚@1.2GeV, 5.45˚@2.2GeV 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 0.002 mm for extraction. Optimised locations and strengths of sextupoles in matching to the arc to minimise emittance. 1.2GeV 2.2GeV

Arc design Considered arcs from 4GLS, BESSY, LUX & DLS low-alpha – 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

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

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 Arc 1 : TBA Kicker Septum Arc 2 : TBA Diagnostics line FEL1  FEL2  FEL3  20 40 60 2 4 6 8 Take-off section of FELs Details of design of one spreader line

Machine optics (start to exit of the spreader) Arc Arc Linac -2nd pass Spreader Linac -1st pass Coll BC1 Dogleg Return FODO

Simulation results : projected emittance Extraction, BC3 Collimation Return FODO BC1 Dogleg Linac -1st pass Match to arc, arc arc,BC2, merger Linac -2nd pass Spreader Final normalised projected emittances in x/y ~ 0.6/0.43 mm.mrad

Emittance Start to End of spreader Recirculation x/y ~ 0.6/0.43 mm.mrad Single pass x/y ~ 0.6/0.3 mm.mrad

Parameters exit spreader (Nov 09) Tracked 100k particles. The data is binned in 1fs slices.

Luus-Jaakola optimisation (Feb 10)

Parameters exit spreader (Feb 10) Tracked 100k particles. The data is binned in 1fs slices.

Parameters exit spreader (Nov 09) Tracked 100k particles. The data is binned in 1fs slices.

Bunch parameters at the exit of spreader (Single Pass) Longitudinal current distribution Current (A) rms relative energy spread Normalised emittance Normalised emittance(m) rms relative energy spread(%) R. Bartolini

Longitudinal current distribution Single pass Recirculation

Emittance Single pass Recirculation

Relative energy spread Recirculation Single pass

Comparison with single pass time-dependent results Comparison between radiation profiles of the re-circulating and single pass cases at a comparable power level

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.

R. Bartolini Diamond Light Source Ltd. 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