DC Gun

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The Jlab UV FEL Driver ERL

•Delivery of appropriately configured beam to FEL •Transverse, longitudinal phase space management•Preservation of beam quality

•space charge, wake/collective effects, CSR

•Power recovery from exhaust beam•Transverse, longitudinal matching•Collective effect/instability control

•space charge, •BBU, •FEL/RF interaction

•Loss (halo) management

Design Requirements

Parameters (Achieved)Parameter IR UV

Energy (MeV) 88-165 135

Iave (mA) 9.1 2

Qbunch (pC) 135 60

N transverse/longitudinal

(mm-mrad/keV-psec)8/75 5/50

p/p, l (fsec) 0.4%, 160 0.4%, 100

Ipeak (A) 400 250

FEL repetition rate (MHz)

(cavity fundamental 4.6875)0.586-75 1.172-18.75

FEL 2.5% 0.8%

Efull after FEL ~15% ~7%

Relevant Phenomena

• Space charge (transverse, longitudinal)• Inject long (2.5 psec/1.33o rms), low momentum spread

(~¼%) bunch (LSC)

• BBU• CSR

• Compress high charge bunch => potentially degrade beam quality (and get clear signature of short bunch) & put power where you don’t want it…

• Other wake, impedance effects• RF heating, resistive wall

DC Gun

SRF Lin

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UV FEL

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Design Concept: add-on to IR Upgrade• retain beam dynamics

solution from IR– Use same modular approach

(and same optics modules) as in IR side of machine

• divert beam to UV FEL with minimal operational modification

Design Solutiongun

injector

merger

linac

Transverse match: linac to arc

Bates bend

recovery Bates bend

UV bypass transport grafted onto Bates bend

Betatron match to wiggler

wigglerBetatron match from wiggler to recovery transport

Return bypass transport to energy recovery arc

Reinjection/recovery transverse matchEnergy recovery

through linac

Extraction line to dump

Phase Space Management• Transport system is “functionally modular”: design

embeds specific functions (e.g. beam formation, acceleration, transverse matching, bending, dispersion suppression, etc) within localized regions– largely avoids need for S2E analysis

• allows use of potentially ill-defined/poorly controlled components– demands design provide operational flexibility– requires use of beam-based methods

• needs extensive suite of diagnostics & controls

Its a cost-performance optimization (i.e. religious) issue: pay up front for a sufficient understanding of physics,

component/hardware quality, or provide operational flexibility and adequate diagnostic capability?

“Modules” for Phase Space Control• Transverse matching – quad telescopes in nondispersed regions;

decoupled from longitudinal match– Injector to linac– Linac to recirculator– Match to wiggler– Match out of wiggler to recovery transport– Reinjection match

• Longitudinal matching – handled in Bates bends– Path length variable over ~±RF/2 (for control of 2nd pass RF phase)– Independent control of momentum compaction through third order

(M56, T566, W5666) and dispersion through 2nd order (T166, T266)– Relatively decoupled from transverse match

• Phase space exchange (IR side only)– H/V exchange using 5 quad rotator for BBU control– Decoupled from transverse, longitudinal matching

Transverse Matching (Linear)• Multiple quad telescopes along transport system massage H/V phase

space to match to lattice acceptance– Injector to linac (4 quads)– Linac to recirculator (6 quads)– Match to wiggler (6 quads)– Match wiggler to recovery transport (6 quads)– Reinjection match (6 quads)

• Key points– ERLs do not have closed orbits nor do they need to be betatron stable– ERLs may not have uniquely defined “matched” Twiss envelopes– Deliberate “mismatch” (to locally stable transport) may be beneficial

• e.g. to manage chromatic aberrations, halo, avoid aperture constraints

– Design optimization must explore parameter space to determine “best” choice

– Beam envelopes and lattice Twiss parameters are **not** in general the same!

Operationally…• Measure beam envelopes

– multislit, quad scan, and/or multi-monitor emittance measurement

• Back-propagate results to reference point upstream of matching region

• Adjust quads to “match” envelopes to design values and/or acceptance of specific sections of transport system lattice

• Caveat: RF focusing is VERY important– dominates behavior in injector

• is the “observable” used to set phase on a daily basis

– defines injector-to-linac match• Limits tolerable gradient in first (last) cavity

Longitudinal Matching• Space charge forces injection of a long bunch (SRF

gradients are too low to preserve beam quality if bunch is short) – Must compress bunch length during/after acceleration to

produce high peak current needed by FEL• After lasing, beam energy spread is too large (15-20

MeV) to recover without unacceptable loss– Must energy compress (during energy recovery) to “fit”

beam into dump line acceptance

The manipulations needed to meet these requirements constitute the longitudinal match

Longitudinal Matching

Schematic Longitudinal Matching for ERL-Driven FEL

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“oscillator”

“amplifier”

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Energy Recovery: DetailsLongitudinal Match to Wiggler• Inject long, low-energy-spread bunch to avoid LSC problems

– need 1-1.5o rms with 1497 MHz RF @ 135 pC in our machine• Chirp on the rising part of the RF waveform

– counteracts LSC– phase set-point then determined by required momentum spread at

wiggler• Compress (to required order, including curvature/torsion

compensation) using recirculator compactions M56, T566, W5666,…

• Entire process generates a parallel-to-point longitudinal image from injector to wiggler

Longitudinal Match to Dump• FEL exhaust bunch is short & has very large energy spread (10-15%)• => Must energy compress during energy recovery to avoid beam loss linac

during energy recovery; this defines the longitudinal match to dump– Highest energy must be phase-synchronous with (or precede) trough of RF wave-form– Transport momentum compactions must match the slope (M56), curvature (T566), torsion

(W5666),… of the RF waveform

• Recovered bunch centroid usually not 180o out of phase with accelerated centroid

– Not all RF power recovered, but get as close as possible (recover ahead of trough), because…

– Additional forward RF power required for field control, acceleration, FEL operation; more power needed for larger phase misalignments

• For specific longitudinal match, energy & energy spread at dump does not depend on lasing efficiency, exhaust energy, or exhaust energy spread

– Only temporal centroid and bunch length change as lasing conditions change

• The match constitutes a point-to-parallel image from wiggler to dump

Energy Compression

• Beam central energy drops, beam energy spread grows• Recirculator energy must be matched to beam central energy to maximize acceptance• Beam rotated, curved, torqued to match shape of RF waveform• Maximum energy can’t exceed peak deceleration available from linac

– Corollary: entire bunch must preced trough of RF waveform

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All e- after trough go into high-energy tail at dump

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Higher Order Corrections• Without nonlinear corrections, phase space

becomes distorted during deceleration• Curvature, torsion,… can be compensated by

nonlinear adjustments – differentially move phase space regions to match

gradient required for energy compression

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• Required phase bite is cos-1(1-EFEL/E); this is >25o at the RF fundamental for 10% exhaust energy spread, >30o for 15%

– typically need 3rd order corrections (octupoles)– also need a few extra degrees for tails, phase

errors & drifts, irreproducible & varying path lengths, etc, so that system operates reliably

• In this context, harmonic RF very hard to use…

JLab IR Demo Dump

core of beam off center, even though BLMs showed edges were centered

(high energy tail)

Longitudinal Matching ScenarioRequirements on phase space:• high peak current (short bunch) at FEL

– bunch length compression at wigglerusing quads and sextupoles to adjust compactions

• “small” energy spread at dump– energy compress while energy recovering– “short” RF wavelength/long bunch,

large exhaust p/p (~10%) get slope, curvature, and torsion right

(quads, sextupoles, octupoles)

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Nonlinearity Control Validated By Measurement

Figure 1: Inner sextupoles to 12726 g-cm and trim quads to -215 g Figure 2: trim quads at -185 g with same sextupoles Figure 3: trim quads at -245 gFigure 4: quads at -215, but sextupoles 3000 g below design, at 10726 g-cmFigure 5: where we left it: trim quads -215 g sextupoles at 12726 g-cm

arrival

launch

Injector to Wiggler Transport

If you do it right linac produces stable ultrashort pulses

Can regularly achieve 300 fs FWHM electron pulses

~150 fsec rms

Injector to Reinjection Transport

Module Design: Injector

• 42 psec (FWHM) 75 (1497/20) MHz green drive laser pulse train with flexibly gated time structure

• DC photocathode (GaAs) gun at 325 kV• Room-temp buncher providing initial longitudinal

control• Pair of 5-cell SRF (CEBAF) cavities at 1497 MHz

accelerate to ~10 MeV• Quad telescope to match to linac

Cathode• Cesiated GaAs

– Excellent performance for R&D system• When charge lifetime limited, get 500 C between

cesiations (50k sec, ~14 hrs at 10 mA, many days at modest current), O(10 kC) on wafer

– Typically replace because we destroy wafer in an arc event, can’t get QE

• When (arc, emitter, vacuum,…) limited, ~few hours running

– Not entirely adequate for prolonged user operations

• Other cathodes?– Need proof of principle for required combination

of beam quality, lifetime?

Injector Design and Operation

• At highest level…– System is moderately bright & operates at moderate power– Halo & tails are significant issue– Must produce very specific beam properties to match downstream acceptance; have very

limited number of free parameters to do so

• Issues:– Space charge & steering in front end– Deceleration by first cavity – Severe RF focusing (with coupling)– FPC/alignment steering – phasing a challenge

• Miniphase– Halo/tails

• Divots in cathode; scattered drive laser light; cathode relaxation; …

(V w/ PM, MSE) (V w/ PM, BPM) (BPM)(V)

Wafer 25 mm dia

Active area 16 mm dia

Drive laser 8 mm dia

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350 kV/5 MV500 kV/5 MV

Courtesy P. Evtushenko

Injector Operational Challenges

• At highest level…– System is moderately bright & operates at moderate power– Halo & tails are issue– Must produce very specific beam properties for rest of system,

and have very limited number of free parameters to do so• Space charge: have to get adequate transmission through

buncher – steering complicated by running drive laser off cathode axis

(avoid ion back-bombardment)– solenoid must be reoptimized for each drive laser pulse length– Vacuum levels used as diagnostic

(V w/ PM, MSE) (V w/ PM, BPM) (BPM)(V)

Injector Operational Challenges

• 1st cavity – decelerates beam to ~175 keV,

aggravates space charge; – E() nearly constant for ±20o around

crest (phase slip)• Normal & skew quad RF modes in

couplers violate axial symmetry & add coupling

• Dipole RF mode in FPC– Steer beam in “spectrometer”, make

phasing difficult– Drive head-tail emittance dilution

(V w/ PM, MSE) (V w/ PM, BPM) (BPM)(V)

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Injector Operational Challenges

• FPC/cavity misalignment steering ~ as big as dispersive changes in position

– Phasing takes considerable care and some time– Have to back out steering using orbit measurement in linac

• RF focusing very severe – can make beam large/strongly divergent/convergent at end of cryounit – constrains ranges of tolerable operating phases

• Phasing– 4 knobs available: drive laser phase, buncher phase, 2 SRF cavity phases– Constrained by tolerable gradiants, limited number of observables (1 position at

dispersed location), downstream acceptance– Typically spectrometer phase with care every few weeks; “miniphase” every few hours

(V w/ PM, MSE) (V w/ PM, BPM) (BPM)(V)

“Miniphase”

• System is under-constrained, difficult to spectrometer phase with adequate resolution

• Phases drift out of tolerance over few hours• Recover setup by

1. Set drive laser phase to put buncher at “zero crossing”(therein lies numerous tales, … or sometimes tails...)

2. Set drive laser/buncher gang phase to phase of 1st SRF cavity by duplicating focusing (beam profile at 1st view downstream of cryounit)

3. Set phase of 2nd SRF cavity by recovering energy at spectrometer BPM

this avoids necessity of fighting with 1st SRF cavity…

(V w/ PM, MSE) (V w/ PM, BPM) (BPM)(V)

Module Design: Merger• Injection line: 3 dipole “Penner bend”

– Achromatic (M16, M26 = 0)

– Modest M56<0 providing some bunch length control

• Very strongly focusing in bend plane; drift-like in non-bend plane– Focusing => space charge management– drift => transverse asymmetry => bad for space charge

• Adequate performance at ~100 pC x several mm-mrad level

• hard to match across• somewhat archaic design• Reinjection line: achromatic chicane with geometry matched

to final dipole of Penner bend– mechanically convenient (fair amount of space)

Numbers

=0.6 m= 20o

normal entry/exitbend plane focal length ~ /sin ~ 1.75 mbend-to-bend drift ~ 1.5 mdispersion in center dipole ~0.5 mM56 ~ -0.18 m

Merger Issues

Low charge (135 pC), low current (10 mA); beam quality preservation notionally not a problem; however…

• Can have dramatic variation in transverse beam properties after cryounit

• 4 quad telescope has extremely limited dynamic range• Must match into “long” linac with limited acceptance

– Matched envelopes ~10 m, upright ellipse– Have to get fairly close (halo, scraping, BBU,…)

• Beam quality is match sensitive (space charge)Have to iterate injector setup & match to linac until adequate

performance achieved

(V w/ PM, MSE) (V w/ PM, BPM) (BPM)(V)

Module Design: Linac

• Three JLab-standard (more or less) cryomodules – Outboard pair: 8 five-cell 1497 MHz cavities– Middle module: 8 seven-cell 1497 MHz cavities

• Triplet focusing in warm regions• Wear and tear limits full energy to ~135 MeV

or so

~30 m

Space Charge – Esp. LSC – Down Linac• Had a number of issues in linac during commissioning:

– Why was the bunch “too long” at the wiggler?• bunch length at wiggler “too long” even when fully “optimized” (with good

longitudinal emittance out of injector)– could only get 300-400 fsec rms, needed 200 fsec

– Why did the “properly tuned lattice” not fully compress the bunch?• M55 measurement showed proper injector-to-wiggler transfer function, but

beam didn’t “cooperate”… minimum bunch length at “wrong” compaction– Why was the beam momentum spread asymmetric around crest?

• dp/p ahead of crest ~1.5 x smaller than after crest; average ~ PARMELA

We blamed wakes, mis-phased cavities, fundamental design flaws, but in reality it was LSC…

• PARMELA simulation (C. Hernandez-Garcia) showed LSC-driven growth in correlated & uncorrelated dp/p; magnitudes consistent with observation

• Simulation showed uncorrelated momentum spread (which dictates compressed bunch length) tracks correlated (observable) momentum spread

Space-Charge Induced Degradation of Longitudinal Emittance

• Mechanism: self-fields cause bunch to “spread out”– Head of bunch accelerated, tail of bunch decelerated, causing

correlated energy slew• Ahead of crest (head at low energy,

tail at high) observed momentum spread reduced

• After crest (head at high energy, tail at low) observed energy spread increased

– “Intrinsic” momentum spread similarly aggravated (driving longer bunch)

• Simple estimates => imposed correlated momentum spread ~1/Lb2 and 1/rb

2

– The latter observed – bunch length clearly match-dependent– The former quickly checked…

Solution

• Additional PARMELA sims (C. Hernandez-Garcia) showed injected bunch length could be controlled by varying phase of the final injector cavity. – bunch length increased, uncorrelated momentum spread fell (but

emittance increased) – reduced space charge driven effects – both correlated asymmetry

across crest and uncorrelated induced momentum spread• When implemented in accelerator:

– final momentum spread increased from ~1% (full, ahead of crest) to ~2%;

– bunch length of ~800–900 fsec FWHM reduced to ~500 fsec FWHM (now typically 350 fsec)

– bunch compressed when “decorrelated” injector-to-wiggler transfer function used (“beam matched to lattice”)

Happek Scan

Key Points

• “Lengthen thy bunch at injection, lest space charge rise up to smite thee” (Pv. 32:1, or Hernandez-Garcia et al., Proc. FEL ’04)

• “best” injected emittance DOES NOT NECESSARILY produce best DELIVERED emittance!

• LSC effects visible with streak camera

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Streak Camera Data from IR Upgrade

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-6o

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(t,E) vs. linac phase after crest

(data by S. Zhang, v.g. from C. Tennant)

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Streak Camera Data from IR Upgrade

(t,E) vs. linac phase, before crest

asymmetry between + and - show effect of longitudinal space charge after 10 MeV

(data by S. Zhang, v.g. from C. Tennant)

±4 and ±6 degrees off crest• “+” on rising, “-” on falling

part of waveform• Lbunch consistent with dp/p

and M56 from linac to observation point

• dp/p(-)>dp/p(+)• on “-” side there are

electrons at energy higher than max out of linac

• distribution evolves “hot spot” on “-” side (kinematic debunching, beam slides up toward crest…)

=> LSC a concern…

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Module Design: Transverse Match to Recirculator

• 6 quad telescope– Match x,y, x,y, x,y from linac to Bates bend

– Full transverse envelope match + phase advance allows

• management of chromatic aberrations – Choice of phase advance = aberrations destructively interfere

• control of turn-to-turn phase advance– BBU suppression

Module Design: Bates Bend Requirements• “delivery of appropriately configured beam to wiggler” =>

– betatron matched to wiggler acceptance– Compressed bunch length– Nondispersed after recirculation transport

• “power recovery from exhaust beam” (without loss…) =>– Betatron matched to linac acceptance– Decompressed bunch length with 56 & phasing set to insure energy

compression during energy recovery– Nondispersed at reinjection

=> Need (nonlinearly) achromatic recirculation arc with large acceptance, good betatron behavior, and variable path length/momentum compaction (through nonlinear order)

• Bates Bend– Originally used as the basis of an energy doubling

recirculator (and a demonstration of current doubling and energy recovery in the early ’80s), the MIT implementation had >8.5% momentum acceptance

– JLab FEL drivers (IR Demo, IR upgrade) have both used Bates bends

Bates Recirculator• We used the Sargent/Flanz design from MIT because it is really robust,

really easy to operate (if you instrument it) and really simple (if you think about it the right way).– Good acceptance (10+%, over 30o RF phase, good focusing properties (esp. if

matching in/out uses chromatically balanced telescopes)

Ancillary benefit: No simultaneously small , , l, so CSR, space charge not aggravated (parasitic crossings not an issue)

Module Design: Bypass to UV FEL

• Bates bend directs beam to IR FEL • UV FEL “must” live in wiggler pit

– Wiggler stand height >> beam elevation

• Need by-pass solution – must provide appropriate matching

• Transverse spot size• Bunch compression• Dispersion suppression

– Constrained by pit location

• Reduce bend angle in final dipole of Bates bend to direct beam toward UV wiggler pit– “short out” half of coil pack to reduce bend angle by half

• Complete bend when at proper transverse offset

Bypass Concept• Use FODO quad transport between bends to manage

dispersion, control betatron envelopes

– Add 1 betatron wavelength in bend plane to image/suppress dispersion

– Select out-of-bend-plane phase advance to manage envelopes, aberrations (1/2 betatron wavelength works well)

– Use 7 quad FODO transport– Sextupoles at high-dispersion points (at horizontally focusing quads)

provide correction of 2nd-order dispersion

• Reduce bend angle in final dipole of Bates bend to direct beam toward UV wiggler pit– “short out” half of coil pack to reduce bend angle by half

• Complete bend when at proper transverse offset• Use FODO quad transport between bends to manage

dispersion, control betatron envelopes– Add 1 betatron wavelength in bend plane to image/suppress

dispersion– Select out-of-bend-plane phase advance to manage envelopes,

aberrations (1/2 betatron wavelength works well)– Use 7 quad FODO transport– Sextupoles at high-dispersion points (at horizontally focusing quads)

provide correction of 2nd-order dispersion

Bypass Concept

Module Design: Match to Wiggler

• Similar to linac-to-arc match– 6 quad telescope (two triplets)

• Set transverse envelopes, phase advances

Module Design: Match from Wiggler

• Just like previous match: 6 quad telescope, set envelopes, phase advances…

Module Design: Return transport to recovery arc

• Reflection of translation from Bates bend to UV backleg

Module Design: Bates Bend

Module Design: Reinjection Match

Module Design: recovery pass through linac

Module Design: Extraction line to recovery dump7