the Late NGLS: Overview of LinAC Design, Beam dynamics
description
Transcript of the Late NGLS: Overview of LinAC Design, Beam dynamics
1 ─ M. Venturini, Sept. 26, 2013, SLAC
Marco Venturini
LBNL
Sept. 26, 2013
THE LATE NGLS: OVERVIEW OF LINAC DESIGN,
BEAM DYNAMICS
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
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 energyQ=300 pC/ bunch
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: 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 ─ 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).
6 ─ M. Venturini, Sept. 26, 2013, SLAC
Machine layout, highlights of linac settings
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)
Linearizer off
Large dephasingto remove energy chirp
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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
relatively long tailis a signature of velocitycompression
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flat core
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curvature
slice e┴ ≤0.6 mm proj. e┴ =0.72 mm
Ipk~45 A
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Elegant tracking: Longitudinal dynamics through BCs
BC2 exit (~5 compression)BC1 exit (factor ~2 compression)
Ipk~90 A Ipk~500 A
substantial portion of bunch is in the tail
Curvature of energy profile, to cause current spikes, harm radiation coherenceif we compressed much more
flat current profile as desired (current not very high but adequate)
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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 inspreader helps somewhat with
energy chirp removal
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10 ─ M. Venturini, Sept. 26, 2013, SLAC
Careful lattice design keeps projected emittance almost unchanged by the exit of spreader (<0.8mm)(two-stage compression)
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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 5mm
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𝜸 𝜺𝒙=𝟎 .𝟔𝝁𝒎
11 ─ M. Venturini, Sept. 26, 2013, SLAC
Aside on setting of linearizerwakefields (RF, CSR) generate energy chirpw/ positive quadratic term within bunch
Turning on linearizer would add to positive
quadratic chirp, pushing beamtail forward upon compression,
and causing current spike*
*Details depend on machine settingsElegant simulations for baseline working point; linearizer off
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Exit of BC1 Exit of BC2 Exit of linac
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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)
angle rad R56 mBC1 0.0955 0.0856392BC2 0. IndeterminateVoltage MV phase deg E MV Acc. Grad MVm no. modlsL1 201.6 30. 174.591 13.8763 2L2 549.998 0. 549.998 12.619 6HL 20. 180 20. 8.25971 1L3 1706.67 20. 1603.75 13.0524 18
Projected emittances through spreader
Longitudinal phase space is comparable to that of 2-stage compression
Exit of spreader
One-stage compression vertical
horizontal
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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
Space charge affects: matching
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
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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 (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.
Linear gain for 2-stage compression
2-stagecompression
5keV
10keV
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Current profile Longitudinal phase space
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5keV
10keV
Compare various degree of heating (rms)
Note: beam not fully compressed
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Microbunching seeded by shot noiseTwo-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?
5 1 0 1 5 2 01 0 01 5 02 0 02 5 03 0 03 5 04 0 04 5 0
E Lase r H ea te r keV
final
EkeV
Slice* energy spread vs. Heater setting
Two-stagecompression
*Slice is 1mm ~ coop length
Slice energy along bunch
Lower bound
One-stage compression: Instability is effectively suppressed for sE=10keV
heating
IMPACT simulations
sE =15keVheating
DE~200keV
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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) z (mm)
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IMPACT simulations
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Microbunching seeded by sinusoidal current perturbation at cathode (II)
5% amplitude perturbation on current at cathode 0.8 ps periodsE=15keV heating
Energy profile for one-stage compression remainsRelatively smooth
Energy profile for two-stage compression shows~200keV ripple(comparable to instabilitySeeded by shot noise)
Slice energy along core of bunch (exit of Spreader)
IMPACT simulations
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18 ─ M. Venturini, Sept. 26, 2013, SLAC
Specs for Heater with sE~15keV heating power are not too demanding
~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 systemCommercially existing, high-repetition rate, short-pulse, high-power laser
lu 5.4 cmlL 1.064 mm
Eb 94 MeV
s┴ 160 mm
Laser peak power* requirement for sE=12keV
PM Undulator gap vs. e-beam energy @LH
*Neglecting diffraction effects
Accurate simulationof 3D laser-beam interaction w/ collectiveforces (“trickle” effect)still missing.
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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
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add 5-m long de-chirper(r = 3 mm)
L3 on crest
…or 35-deg off crest
Longitudinal Phase Space
P.Emma
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Tracking the origin of the long bunch tail: longitudinal dynamics in the injector
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0 1 2 3 4
0 .52
0 .54
0 .56
0 .58
z mm
EMeV
s 1 .2mm
Fig. from C. Papadopoulos
Energy profile Current profile
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(kinetic E)
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A walk down the injector (1): half-way through the gun
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Fig. from C. Papadopoulos
14 16 18 20 22 24 260 .750 .800 .850 .900 .951 .00
z mmEMeV
s 2cmEnergy profile Current profile
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A walk down the injector (2): past the exit of the gun
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65 70 751 .2351 .2401 .2451 .2501 .255
z mmEMeV
s 7cmEnergy profile Current profile
headheadSpace-charge inducedenergy chirp
Fig. from C. Papadopoulos
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A walk down the injector (3): right before the buncher
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695 700 7051 .23
1 .24
1 .25
1 .26
z mmEMeV
s 70cmEnergy profile Current profile
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Fig. from C. Papadopoulos
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A walking down the injector (4): right after the buncher
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995 1000 1005
1 .30
1 .32
1 .34
1 .36
z mmEMeV
s 1m
energy chirp imparted by buncher (@ about zero-crossing)
Energy profile Current profile
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Fig. from C. Papadopoulos
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A walking down the injector (5): ballistic compression begins
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1392 1394 1396 1398 1400 1402 1404 1406
1 .301 .311 .321 .331 .341 .351 .361 .37
z mm
EMeV
s 1 .4mEnergy profile Current profile
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Fig. from C. Papadopoulos
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A walking down the injector (6): a tail in current profile develops
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2186 2188 2190 21921 .3151 .3201 .3251 .3301 .3351 .3401 .3451 .350
z mmEMeV
s 2 .19m Current profile
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Energy profile
Long tail is associated with 2nd order chirp
Fig. from C. Papadopoulos
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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
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With moderate ( RF compression, beam
is close to parabolic.
Snap-shot of NGLS baseline beam @0.4m downstream the buncher (IMPACT
simulations)
Possible layout for injector, first linac Section.
Long. phase space at exit of linac
Pas
sive
inse
rtion
use
d fo
r dec
hirp
ing
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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.