the Late NGLS: Overview of LinAC Design, Beam dynamics

1. the Late NGLS: Overview of LinAC Design, Beam dynamics Marco Venturini LBNL Sept. 26, 2013

2. 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 emittancegrowth • The microbunchinginstability • 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

3. Requirements informing choice of linacdesign • All bunches exiting the linachave 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

4. 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

5. 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).

6. Machine layout, highlights of linacsettings Large dephasing to remove energy chirp 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 linacaxis and FELs (CSR)

7. Baseline beam out of injector (used in Elegant simulations of linac) Out of injector beam (ASTRA simulations) • 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) • CSR • Not included: • LSC, RW wakes, transverse RF wakes flat core  curvature  head Ipk~45 A head relatively long tail is a signature of velocity compression slice e┴ ≤0.6 mm proj. e┴ =0.72 mm

8. Elegant tracking: Longitudinal dynamics through BCs BC1 exit (factor ~2 compression) BC2 exit (~5 compression) head Curvature of energy profile, to cause current spikes, harm radiation coherence if we compressed much more Ipk~500 A Ipk~90 A head flat current profile as desired  (current not very high but adequate) substantial portion of bunch is in the tail 

9. Elegant tracking: Longitudinal dynamics through linacand Spreader Exit of linac Entrance to FEL beamlines head CSR long. wake in spreader helps somewhat with energy chirp removal head Energy profile relatively flat within beam core  Flat core is >300fs long Note: tracking done through fast-kicker based spreader

10. 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 Two-stage compression x/z and x’/z sections Slice* x-emittance (exit of spreader) head head *slice is 5mm 10

11. Aside on setting of linearizer Turning on linearizer would add to positive quadratic chirp, pushing beam tail forward upon compression, and causing current spike* wakefields(RF, CSR) generate energy chirp w/ positive quadratic term within bunch Exit of BC1 Exit of BC2 Exit of linac head head Elegant simulations for baseline working point; linearizer off *Details depend on machine settings

12. One-stage compressioncauses 25% growth of projected emittance Longitudinal phase space is comparable to that of 2-stage compression Exit of spreader head Projected emittances through spreader horizontal head vertical One-stage compression • BC1 at beam energy ~ 250MeV; • BC2 off • Linearizer on (20MV), decelerating mode • Reduced dephasing of L3S (20 deg)

13. Space charge affects: matching Transverse space charge effects in low-energy section of linac with space charge (dashed) y x w/o space charge (solid lines) • some effects in section between Laser Heater (~95MeV) and BC1 (~210MeV) • IMPACT simulations (JiQiang) • Emittance growth not large (~10%) • but a portion of it is slice rather than projected emittance growth. • Possible remedy: Increase beam energy at exitof injector • 2 vs 1 cryomodules? Exit of injector Entrance of L1 emittances with space charge (dashed) y x E=94 MeV w/o space charge

14. The microbunching instability can damage the longitudinal phase space Linear gain for 2-stage compression Current profile Longitudinal phase space 2-stage compression head head 10keV 10keV Compare various degree of heating (rms) 5keV 5keV • 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/ multi-billion macroparticles to minimize numerical noise. Note: beam not fully compressed

15. Microbunching seededby shot noise Slice energy along bunch • One-stage compression: • Instability is effectively suppressed for sE=10keV heating sE=15keV heating IMPACT simulations Slice* energy spread vs. Heater setting DE~200keV Two-stage compression Lower bound • Two-stage compression: • Slice energy spread is minimum for sE=15keV heating • Variations of slice energy are on the order of the energy spread (~200keV). Too big? *Slice is 1mm ~ coop length

16. Microbunching seeded by sinusoidalcurrent perturbation at cathode (I) Initial current profile w/ perturbation Current profiles at exit of linac(Two-stage compression) 5% perturbation, 3.4ps period 5% perturbation, 0.8ps period head head Amplification of modulation depends strongly on period of perturbation z (mm) z (mm) IMPACT simulations

17. Microbunching seeded by sinusoidal current perturbation at cathode (II) Slice energy along core of bunch (exit of Spreader) 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 • 5% amplitude perturbation on current at cathode • 0.8 ps period • sE=15keV heating

18. Specs for Heater with sE~15keV heating power are not too demanding PM Undulator gap vs. e-beam energy @LH Laser peak power* requirement for sE=12keV Accurate simulation of 3D laser-beam interaction w/ collective forces (“trickle” effect) still missing. • ~0.16 MW laser peak power • for ~15keV rms energy spread • /laser pulse • ~2.2 W laser average power @1MHz (at LH undulator) • Dedicated laser system • Commercially existing, high-repetition rate, short-pulse, high-power laser *Neglecting diffraction effects

19. 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 cryomodulesin last linacsection (or allow for higher beam energy) • 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…) • Different FEL lines with differently compressed bunches • Increase amount of magnetic compression relative to RF compression as a way to increase beam quality • Deliver beams with more compact current profile and possibly higher peak current

20. Tracking the origin of the long bunch tail: longitudinal dynamics in the injector (kinetic E) Fig. from C. Papadopoulos Current profile Energy profile head head

21. A walk down the injector (1): half-way through the gun Fig. from C. Papadopoulos Current profile Energy profile head head

22. A walk down the injector (2): past the exit of the gun Fig. from C. Papadopoulos Current profile Energy profile head head Space-chargeinduced energy chirp

23. A walk down the injector (3): right before the buncher Fig. from C. Papadopoulos Energy profile Current profile head head

24. A walking down the injector (4): right after the buncher Fig. from C. Papadopoulos Current profile Energy profile energy chirp imparted by buncher(@ about zero-crossing) head head

25. A walking down the injector (5): ballistic compression begins Fig. from C. Papadopoulos Current profile Energy profile head head

26. A walking down the injector (6): a tail in current profile develops Long tail is associated with 2nd order chirp Fig. from C. Papadopoulos Energy profile Current profile head head

27. With moderate (RF compression, beam is close to parabolic. A 650MHz booster for the APEX injector? 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. Passive insertion used for dechirping • 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 • (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

28. 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 harmfulat 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.