M. Venturini, Sept. 26, 2013, SLAC 1 ─ M. Venturini, Sept. 26, 2013, SLAC Marco Venturini LBNL Sept. 26, 2013 THE LATE NGLS: OVERVIEW OF LINAC DESIGN, BEAM DYNAMICS

M. Venturini, Sept. 26, 2013, SLAC 2 ─ 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

M. Venturini, Sept. 26, 2013, SLAC 3 ─ 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

M. Venturini, Sept. 26, 2013, SLAC 4 ─ 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 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

M. Venturini, Sept. 26, 2013, SLAC 5 ─ 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 Adopted approach:  Two-stage compression with flexibility for single-stage compression (disabling second chicane).

M. Venturini, Sept. 26, 2013, SLAC 6 ─ M. Venturini, Sept. 26, 2013, SLAC Machine layout, highlights of linac settings Magnetic compressors are conventional C-shaped chicanes  215MeV (Sufficiently high to reduce CSR effects on transverse emittance)  720MeV (There may be room for optimizing beam energy) Potential harm from large angle (36 deg) between linac axis and FELs (CSR) Linearizer off Large dephasing to remove energy chirp Large dephasing to remove energy chirp

M. Venturini, Sept. 26, 2013, SLAC 7 ─ M. Venturini, Sept. 26, 2013, SLAC 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: 2 nd 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 relatively long tail is a signature of velocity compression  head flat core head curvature  slice  ┴ ≤0.6  m proj.  ┴ =0.72  m I pk ~45 A

M. Venturini, Sept. 26, 2013, SLAC 8 ─ M. Venturini, Sept. 26, 2013, SLAC Elegant tracking: Longitudinal dynamics through BCs BC2 exit (~5 compression) BC1 exit (factor ~2 compression BC1 exit (factor ~2 compression) I pk ~90 A I pk ~500 A substantial portion of bunch is in the tail  substantial portion of bunch is in the tail  Curvature of energy profile, to cause current spikes, harm radiation coherence if we compressed much more Curvature of energy profile, to cause current spikes, harm radiation coherence if we compressed much more flat current profile as desired (current not very high but adequate) flat current profile as desired (current not very high but adequate) head

M. Venturini, Sept. 26, 2013, SLAC 9 ─ M. Venturini, Sept. 26, 2013, SLAC Elegant tracking: Longitudinal dynamics through linac and Spreader Entrance to FEL beamlines Exit of linac Energy profile relatively flat within beam core Note: tracking done through fast-kicker based spreader Flat core is > 300fs long CSR long. wake in spreader helps somewhat with energy chirp removal CSR long. wake in spreader helps somewhat with energy chirp removal head

M. Venturini, Sept. 26, 2013, SLAC 10 ─ M. Venturini, Sept. 26, 2013, SLAC Careful lattice design keeps projected emittance almost unchanged by the exit of spreader (<0.8  m) (two-stage compression) 10 x/z and x’/z sections vertical horizontal Two-stage compression Projected emittances through spreader horizontal vertical Slice* x-emittance (exit of spreader) 10 *slice is 5  m head head

M. Venturini, Sept. 26, 2013, SLAC 11 ─ M. Venturini, Sept. 26, 2013, SLAC Aside on setting of linearizer wakefields (RF, CSR) generate energy chirp w/ positive quadratic term within bunch Turning on linearizer would add to positive quadratic chirp, pushing beam tail forward upon compression, and causing current spike* Turning on linearizer would add to positive quadratic chirp, pushing beam tail forward upon compression, and causing current spike* *Details depend on machine settings Elegant simulations for baseline working point; linearizer off head Exit of BC1 Exit of BC1 Exit of BC2 Exit of BC2 Exit of linac head

M. Venturini, Sept. 26, 2013, SLAC 12 ─ M. Venturini, Sept. 26, 2013, SLAC One-stage compression causes 25% growth of projected emittance BC1 at beam energy ~ 250MeV;  BC2 off Linearizer on (20MV), decelerating mode Reduced dephasing of L3S (20 deg) Projected emittances through spreader Longitudinal phase space is comparable to that of 2-stage compression Exit of spreader One-stage compression vertical horizontal head head

M. Venturini, Sept. 26, 2013, SLAC 13 ─ M. Venturini, Sept. 26, 2013, SLAC Transverse space charge effects in low-energy section of linac  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 projected emittance growth.  Possible remedy: Increase beam energy at exit of injector  2 vs 1 cryomodules? E=94 MeV emittances with space charge (dashed) w/o space charge (solid lines) y x y x with space charge (dashed) w/o space charge Entrance of L1Exit of injector

M. Venturini, Sept. 26, 2013, SLAC 14 ─ M. Venturini, Sept. 26, 2013, SLAC The microbunching instability can damage the longitudinal phase space Seeded by shot noise and perturbations at the source (e.g. non-uniformity in photo-gun laser pulse) Consequences  Slice energy spread  Slice energy spread (penalty on lasing efficiency)  Slice average energy  Slice average energy (penalty on radiation spectral purity, in particular in externally seeded FELs beamlines) IMPACT; Modeling primarily by IMPACT; simulations w/ multi- billion macroparticles to minimize numerical noise. Linear gain for 2-stage compression 2-stage compression 5keV 10keV head Current profileLongitudinal phase space head 5keV 10keV Compare various degree of heating (rms) Note: beam not fully compressed

M. Venturini, Sept. 26, 2013, SLAC 15 ─ M. Venturini, Sept. 26, 2013, SLAC shot noise Microbunching seeded by shot noise Two-stage compression:  Slice energy spread  Slice energy spread is minimum for  E =15keV heating slice energy  Variations of slice energy are on the order of the energy spread (~200keV). Too big? Slice* energy spread Slice* energy spread vs. Heater setting Two-stage compression *Slice is 1  m ~ coop length Slice energy Slice energy along bunch Lower bound One-stage compression:  Instability is effectively suppressed for  E =10keV heating IMPACT simulations  E =15keV heating  E~200keV

M. Venturini, Sept. 26, 2013, SLAC 16 ─ M. Venturini, Sept. 26, 2013, SLAC current perturbation Microbunching seeded by sinusoidal current perturbation at cathode (I) Amplification of modulation depends strongly on period of perturbation Initial current profile w/ perturbation 5% perturbation, 3.4ps period Current profiles at exit of linac (Two-stage compression) 5% perturbation, 0.8ps period z (mm) head IMPACT simulations

M. Venturini, Sept. 26, 2013, SLAC 17 ─ M. Venturini, Sept. 26, 2013, SLAC Microbunching seeded by sinusoidal current perturbation at cathode (II) 5% amplitude perturbation on current at cathode  0.8 ps period  E =15keV heating Energy profile for one-stage compression remains Relatively smooth Energy profile for two-stage compression shows ~200keV ripple (comparable to instability Seeded by shot noise) Slice energy along core of bunch (exit of Spreader) (exit of Spreader) IMPACT simulations 17

M. Venturini, Sept. 26, 2013, SLAC 18 ─ M. Venturini, Sept. 26, 2013, SLAC Specs for Heater with  E ~15keV heating power are not too demanding u 5.4 cm L  m EbEb 94 MeV ┴┴ 160  m Laser peak power* requirement for  E =12keV PM Undulator gap vs. e-beam *Neglecting diffraction effects Accurate simulation of 3D laser-beam interaction w/ collective forces (“trickle” effect) still missing.

M. Venturini, Sept. 26, 2013, SLAC 19 ─ M. Venturini, Sept. 26, 2013, SLAC How could availability of passive “dechirping” insertions affect the linac design? 1. Save on no. of cryomodules in last linac section (or allow for higher beam energy)  5m long, r=3mm corrugated pipe would do the dechirping job (L3S on crest ) 2. Allow for compression through the spreader lines (a bit far fetched…)  Different FEL lines with differently compressed bunches 3. 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 19 add 5-m long de-chirper (r = 3 mm) L3 on crest …or 35-deg off crest Longitudinal Phase Space P.Emma

M. Venturini, Sept. 26, 2013, SLAC 20 ─ M. Venturini, Sept. 26, 2013, SLAC Tracking the origin of the long bunch tail: longitudinal dynamics in the injector 20 Fig. from C. Papadopoulos Energy profile Current profile head head (kinetic E)

M. Venturini, Sept. 26, 2013, SLAC 21 ─ M. Venturini, Sept. 26, 2013, SLAC A walk down the injector (1): half-way through the gun 21 Fig. from C. Papadopoulos Energy profile Current profile head head

M. Venturini, Sept. 26, 2013, SLAC 22 ─ M. Venturini, Sept. 26, 2013, SLAC A walk down the injector (2): past the exit of the gun 22 Energy profile Current profile head head Space-charge induced energy chirp Fig. from C. Papadopoulos

M. Venturini, Sept. 26, 2013, SLAC 23 ─ M. Venturini, Sept. 26, 2013, SLAC A walk down the injector (3): right before the buncher 23 Energy profileCurrent profile head head Fig. from C. Papadopoulos

M. Venturini, Sept. 26, 2013, SLAC 24 ─ M. Venturini, Sept. 26, 2013, SLAC A walking down the injector (4): right after the buncher 24 energy chirp imparted by buncher about zero-crossing energy chirp imparted by buncher about zero-crossing) Energy profile Current profile head head Fig. from C. Papadopoulos

M. Venturini, Sept. 26, 2013, SLAC 25 ─ M. Venturini, Sept. 26, 2013, SLAC A walking down the injector (5): ballistic compression begins 25 Energy profile Current profile head head Fig. from C. Papadopoulos

M. Venturini, Sept. 26, 2013, SLAC 26 ─ M. Venturini, Sept. 26, 2013, SLAC A walking down the injector (6): a tail in current profile develops 26 Current profile head head Energy profile Long tail is associated with 2 nd order chirp Fig. from C. Papadopoulos

M. Venturini, Sept. 26, 2013, SLAC 27 ─ M. Venturini, Sept. 26, 2013, SLAC 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 (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 27 Snap-shot of NGLS baseline downstream the buncher (IMPACT simulations) Possible layout for injector, first linac Section Possible layout for injector, first linac Section. Long. phase space at exit of linac Passive insertion used for dechirping

M. Venturini, Sept. 26, 2013, SLAC 28 ─ M. Venturini, Sept. 26, 2013, SLAC 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   x =0.6  m (slice) preserved;  x =0.8  m projected (two-stage compression)   x =1  m (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  E = 10keV in one-stage compression mode  In two-stage compression, heating to  E = 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.