THE LATE NGLS: OVERVIEW OF LINAC DESIGN, BEAM DYNAMICS Marco Venturini LBNL Sept. 26, 2013 1 ─ M. Venturini, Sept. 26, 2013, SLAC Outline Guiding principles for choice of main parameters, lattice design, bunch compression RF vs. magnetic compression Single vs. multiple stage magnetic compression Description of layout, lattice, working point baseline Preservation of beam quality and beam dynamics issues (single bunch) Longitudinal dynamics CSR-induced emittance growth The microbunching instability Transverse space-charge effects in the low-energy section of the linac Impact of availability of passive de-chirping insertion on machine design Lowering degree of RF (velocity bunching) compression 2 ─ M. Venturini, Sept. 26, 2013, SLAC Requirements informing choice of linac design All bunches exiting the linac have same design characteristics, are adequate to feed any of the FEL beamlines (1keV photon /energy) Different kinds of beam tailored to specific FEL beamlines are a speculative possibility. Not investigated yet. As high as possible peak current consistent with: Flat current profile Flat energy profile Minimal degradation of transverse emittance (both slice and projected) Sufficiently small energy spread Sufficiently long bunches to support two- (three-?) stage HGHG external-laser seeding 2.4 GeV beam energy Q=300 pC/ bunch 3 ─ M. Venturini, Sept. 26, 2013, SLAC RF vs. magnetic compression At cathode of proposed gun bunch current is very low I ~ 5-6A Substantial compression is needed Magnetic compression: Energy chirp at exit of last compressor CSR effects Low-frequency SC RF structures would be needed for acceptance of very long initial bunches RF compression in injector (velocity bunching): Less than ideal current profile Space-charge effects, emittance compensation Adopted approach: do both RF and magnetic compression Right balance depends on various factors (e.g. how much chirp can be removed after compression) RF compression to 40-50A range has shown overall best results 4 ─ M. Venturini, Sept. 26, 2013, SLAC Single vs. multiple stage magnetic compression Overall magnetic compression ~ 10 or higher. One-stage compression: Minimizes microbunching instability Two-stage compression: More favorable to preservation of transverse emittance Better beam stability Three-stage compression Adds complexity; may aggravate microbunching instability Adopted approach: Two-stage compression with flexibility for single-stage compression (disabling second chicane). 5 ─ M. Venturini, Sept. 26, 2013, SLAC Machine layout, highlights of linac settings Linearizer off Magnetic compressors are conventional C-shaped chicanes BC1 @ 215MeV (Sufficiently high to reduce CSR effects on transverse emittance) BC2 @ 720MeV (There may be room for optimizing beam energy) Potential harm from large angle (36 deg) between linac axis and FELs (CSR) 6 ─ M. Venturini, Sept. 26, 2013, SLAC Large dephasing to remove energy chirp Baseline beam out of injector (used in Elegant simulations of linac) Out of injector beam (ASTRA simulations) flat core curvature head Physics model in Elegant simulations (next 4 slides) includes: • 2nd order transverse dynamics • Ideal (error free) lattice • Longitudinal RF wakefields (using available models for TESLA cavities) Ipk~45 A Not included: • LSC, RW wakes, transverse RF wakes head slice e┴ ≤0.6 mm proj. e┴ =0.72 mm • CSR relatively long tail is a signature of velocity compression 7 ─ M. Venturini, Sept. 26, 2013, SLAC Elegant tracking: Longitudinal dynamics through BCs BC1 exit (factor ~2 compression) head BC2 exit (~5 compression) Curvature of energy profile, to cause current spikes, harm radiation coherence if we compressed much more Ipk~90 A Ipk~500 A head flat current profile as desired (current not very high but adequate) 8 ─ M. Venturini, Sept. 26, 2013, SLAC substantial portion of bunch is in the tail Elegant tracking: Longitudinal dynamics through linac and Spreader Exit of linac head Entrance to FEL beamlines CSR long. wake in spreader helps somewhat with energy chirp removal head Energy profile relatively flat within beam core Note: tracking done through fast-kicker based spreader 9 ─ M. Venturini, Sept. 26, 2013, SLAC Flat core is > 300fs long Projected emittances through spreader Careful lattice design keeps projected emittance almost unchanged by the exit of spreader (<0.8mm) (two-stage compression) horizontal horizontal vertical vertical x/z and x’/z sections Slice* x-emittance (exit of spreader) head = . head 10 ─ M. Venturini, Sept. 26, 2013, SLAC *slice is 5mm 10 10 Aside on setting of linearizer wakefields (RF, CSR) generate energy chirp w/ positive quadratic term within bunch Exit of BC1 Turning on linearizer would add to positive quadratic chirp, pushing beam tail forward upon compression, and causing current spike* Exit of BC2 Exit of linac head head Elegant simulations for baseline working point; linearizer off 11 ─ M. Venturini, Sept. 26, 2013, SLAC *Details depend on machine settings One-stage compression causes 25% growth of projected emittance Longitudinal phase space is comparable to that of 2-stage compression BC1 at beam energy ~ 250MeV; BC2 off Linearizer on (20MV), decelerating mode Reduced dephasing of L3S (20 deg) head Exit of spreader Projected emittances through spreader horizontal head angle rad R56 m BC1 0.0955 0.0856392 BC2Voltage0. MV phase Indeterminate deg E MV Acc. Grad MV m L1 201.6 30. 174.591 13.8763 L2 549.998 0. 549.998 12.619 HL 20. 180 20. 8.25971 L3 1706.67 20. 1603.75 13.0524 12 ─ M. Venturini, Sept. 26, 2013, SLAC vertical no. modls 2 6 1 18 Transverse space charge effects in low-energy section of linac Space charge affects: , −matching y x w/o space charge (solid lines) some effects in section between Laser Heater (~95MeV) and BC1 (~210MeV) IMPACT simulations (Ji Qiang) Emittance growth not large (~10%) but a portion of it is slice rather than with space charge (dashed) Exit of injector Entrance of L1 emittances with space charge (dashed) y projected emittance growth. Possible remedy: Increase beam x energy at exit of injector 2 vs 1 cryomodules? E=94 MeV 13 ─ M. Venturini, Sept. 26, 2013, SLAC w/o space charge The microbunching instability can damage the longitudinal phase space Linear gain for 2-stage compression Seeded by shot noise and perturbations at the source (e.g. non-uniformity in photo-gun laser pulse) Consequences Slice energy spread (penalty on lasing efficiency) Slice average energy (penalty on radiation spectral purity, in particular in externally seeded FELs beamlines) Modeling primarily by IMPACT; simulations w/ multibillion macroparticles to minimize numerical noise. Current profile Longitudinal phase space 2-stage compression head 10keV 10keV 5keV not Venturini, fully compressed Sept. 26, 2013, SLAC 14Note:─beamM. Compare various degree of heating (rms) 5keV head Microbunching seeded by shot noise Two-stage compression: Slice energy spread is minimum for sE=15keV Slice energy along bunch heating Variations of slice energy are on the order of the energy spread (~200keV). Too big? sE =15keV heating One-stage compression: Instability is effectively suppressed for sE=10keV heating IMPACT simulations final E keV Slice* energy spread vs. Heater setting 450 400 350 300 250 200 150 100 DE~200keV Two-stage compression 5 10 15 20 Laser Heater keV E 15 ─ M. Venturini, Sept. 26, 2013, SLAC *Slice is 1mm ~ coop length Microbunching seeded by sinusoidal current perturbation at cathode (I) Initial current profile w/ perturbation Amplification of modulation depends strongly on period of perturbation Current profiles at exit of linac (Two-stage compression) 5% perturbation, 3.4ps period 5% perturbation, 0.8ps period head 16IMPACT ─ M.simulations Venturini, Sept.z26, 2013, SLAC (mm) head z (mm) Microbunching seeded by sinusoidal current perturbation at cathode (II) Slice energy along core of bunch (exit of Spreader) 5% amplitude perturbation on current at cathode 0.8 ps period sE=15keV heating Energy profile for two-stage compression shows ~200keV ripple (comparable to instability Seeded by shot noise) Energy profile for one-stage compression remains Relatively smooth IMPACT simulations 17 ─ M. Venturini, Sept. 26, 2013, SLAC 17 Specs for Heater with sE~15keV heating power are not too demanding PM Undulator gap vs. e-beam energy @LH ~0.16 MW laser peak power for ~15keV rms energy spread ~2.2 /laser pulse ~2.2 W laser average power @1MHz (at LH undulator) Dedicated laser system Commercially existing, high-repetition rate, shortpulse, high-power laser Laser peak power* requirement for sE=12keV Accurate simulation of 3D laser-beam interaction w/ collective forces (“trickle” effect) still missing. 18 ─ M. Venturini, Sept. 26, 2013, SLAC *Neglecting diffraction effects lu 5.4 cm lL 1.064 mm Eb 94 MeV s┴ 160 mm How could availability of passive “dechirping” insertions affect the linac design? Longitudinal Phase Space L3 on crest add 5-m long de-chirper (r = 3 mm) …or 35-deg off crest P.Emma Save on no. of cryomodules in last linac section (or allow for higher beam energy) 1. 5m long, r=3mm corrugated pipe would do the dechirping job (L3S on crest) Allow for compression through the spreader lines (a bit far fetched…) 2. Different FEL lines with differently compressed bunches Increase amount of magnetic compression relative to RF compression as a way to increase beam quality 3. Deliver beams with more compact current profile and possibly higher peak current 19 ─ M. Venturini, Sept. 26, 2013, SLAC 19 Tracking the origin of the long bunch tail: longitudinal dynamics in the injector (kinetic E) Fig. from C. Papadopoulos Current profile Energy profile s 1.2mm E MeV 0.58 0.56 0.54 head head 0.52 0 1 2 z mm 3 20 ─ M. Venturini, Sept. 26, 2013, SLAC 4 20 A walk down the injector (1): half-way through the gun Fig. from C. Papadopoulos E MeV Energy profile s 2cm 1.00 0.95 0.90 0.85 0.80 0.75 Current profile head head 14 16 18 20 22 24 26 z mm 21 ─ M. Venturini, Sept. 26, 2013, SLAC 21 A walk down the injector (2): past the exit of the gun Fig. from C. Papadopoulos E MeV s 7cm Energy profile 22 ─ Current profile 1.255 1.250 1.245 1.240 1.235 head Space-charge 65 70 75induced z mm M. Venturini, Sept. 26, 2013, SLAC energy chirp head 22 A walk down the injector (3): right before the buncher Fig. from C. Papadopoulos s 70cm Energy profile Current profile E MeV 1.26 1.25 1.24 head head 1.23 695 700 z mm 705 23 ─ M. Venturini, Sept. 26, 2013, SLAC 23 A walking down the injector (4): right after the buncher Fig. from C. Papadopoulos Energy profile s 1m Current profile E MeV 1.36 24 ─ 1.34 1.32 energy chirp 1.30 imparted by buncher (@ 995 about zero-crossing) head head 1000 1005 M. Venturini, Sept. 26, z 2013, mm SLAC 24 A walking down the injector (5): ballistic compression begins Fig. from C. Papadopoulos Energy profile s 1.4m Current profile 1.37 1.36 E MeV 1.35 1.34 1.33 1.32 1.31 head head 1.30 1392 1394 1396 1398 1400 1402 1404 1406 z mm 25 ─ M. Venturini, Sept. 26, 2013, SLAC 25 A walking down the injector (6): a tail in current profile develops Long tail is associated with 2nd order chirp Fig. from C. Papadopoulos E MeV Energy profile s 2.19m 26 ─ 1.350 1.345 1.340 1.335 1.330 1.325 1.320 1.315 Current profile head head 2186 2188 2190 2192 M. Venturini, Sept. 26, z2013, mmSLAC 26 With moderate ( ≃ 2) RF compression, beam is close to parabolic. A 650MHz booster for the APEX injector? Option of very low RF compression enabled by availability of passive dechirpers (we could afford making more magnetic compression) ~10A peak current, ~1.2cm FW bunch length (300pC) Bunches are too long for a 3.9GHz linearizer choose 1.3GHz rf frequency for the linearizer (same as in Linac structures) injector booster at 650MHz Snap-shot of NGLS baseline beam @0.4m downstream the buncher (IMPACT simulations) Long. phase space at exit of linac Possible layout for injector, first linac Section. 27 ─ M. Venturini, Sept. 26, 2013, SLAC 27 Passive insertion used for dechirping (Very) preliminary study using LiTrack and parabolic model of beam layout with three magnetic BCs (BC1 functionally replacing most of the RF compression in the injector) simulations show improvement in longitudinal phase space transverse emittance could suffer from low-energy compression Conclusions Delivered beam meets FEL design requirements I=500A flat current profile over about 300fs core Relatively long tail is harmless but wastes a good fraction of charge Relatively flat energy profile in core Nonlinear energy chirp in the beam tail ex=0.6 mm (slice) preserved; ex=0.8 mm projected (two-stage compression) ex=1 mm (projected) for 1-stage compression CSR in spreader not harmful at this current CSR longitudinal wake helps with energy chirp removal from beam core (but adds some nonlinearity on energy chirp) The microbunching instability seeded by shot noise is effectively suppressed by heating to sE = 10keV in one-stage compression mode In two-stage compression, heating to sE = 15keV yields ~150 keV final slice rms energy spread (acceptable) but also slice average energy variations of the same magnitude. Beam current at cathode should be smooth within a few %’s, or much less depending on spectral content of noise Availability of reliable dechirper-insertion would open up interesting possibilities Reduce RF compression for better beam quality. 28 ─ M. Venturini, Sept. 26, 2013, SLAC