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


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


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


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


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).


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


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


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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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


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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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


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


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


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


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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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.


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


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)


A walk down the injector (1): half-way through the gun

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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


headheadSpace-charge inducedenergy chirp



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


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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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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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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



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


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.

the Late NGLS: Overview of LinAC Design, Beam dynamics

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