DC Gun
The Jlab UV FEL
Driver ERL
Dump
Design Requirements
•Delivery
of appropriately
configuredbeam
beam to FEL
•Power recovery
from exhaust
•Transverse,
longitudinal
phasematching
space management
•Transverse,
longitudinal
•beam size/divergence: optical mode overlap
•Collective effect/instability control
•Bunch compression: high peak current at FEL
•space charge
•Preservation of beam quality
•BBU,
•space charge, wake/collective effects, CSR
•FEL/RF interaction
•Loss (halo) management
Parameters (Achieved)
Parameter
IR
UV
88-165
135
Iave (mA)
9.1
2
Qbunch (pC)
135
60
eN transverse/longitudinal
(mm-mrad/keV-psec)
8/75
5/50
0.4%, 160
0.4%, 100
400
250
0.586-75
1.172-18.75
hFEL
2.5%
0.8%
DEfull after FEL
~15%
~7%
Energy (MeV)
sdp/p, sl (fsec)
Ipeak (A)
FEL repetition rate (MHz)
(cavity fundamental 4.6875)
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
Design Concept:
add-on to IR Upgrade
DC Gun
• 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
Dump
gun
Design Solution
injector
Reinjection/recovery
merger
transverse
match
Energy
recovery
through linac
Transverse match: linac to arc
linac
Bates bend
recovery
Bates bend
Betatron match
Return bypass transport
to wiggler
to energy recovery arc
wiggler
Betatron
match from
wiggler to recovery
UV bypass
transport
transport
Extraction line to dump
grafted onto Bates bend
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 providing 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 ~±lRF/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 Scenario
Requirements on phase space:
• high peak current (short bunch) at FEL
–
•
E
bunch length compression at wiggler
using quads and sextupoles to adjust compactions
f
“small” energy spread at dump
–
–
energy compress while energy recovering
“short” RF wavelength/long bunch,
large exhaust dp/p (~10%)
 get slope, curvature, and torsion right
(quads, sextupoles, octupoles)
E
f
E
E
f
E
f
f
E
f
JLab IR Demo Dump
core of beam off center,
even though BLMs showed
edges were centered
(high energy tail)
Module Design: Injector
• 42 psec (FWHM) 75 (1497/20) MHz green drive laser
pulse train with flexibly gated time structure
– It’s always the drive laser
• 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?
– New results from Cornell!
Injector Design and Operation
(V)
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
• At highest level…
System is moderately bright & operates at moderate6000
power
Halo & tails are significant issue
Wafer 25
500mm
kV/5dia
MV
Must produce very specific beam properties to match
downstream acceptance;
have
very
5000
350 kV/5 MV
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
kinetic energy (keV)
–
–
–
4000
Active area 16 mm dia
3000
2000
500 kV/2.5 MV
350 kV/2.5 MV
Drive laser 8 mm dia
1000
0
Halo/tails
0.0E+00 5.0E-10
1.0E-09
1.5E-09
• Divots in cathode; scattered drive laser light; cathode relaxation; … time (sec)
2.0E-09
2.5E-09
Courtesy P. Evtushenko
3.0E-09
Injector Operational Challenges
(V)
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
• 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
– precludes use of 1st solenoid for emittance
compensation
– Must set to provide transmission through buncher, not
balance space charge
Injector Operational Challenges
(V)
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
• 1st cavity
•
500 kV/2.5 MV
5000
kinetic energy (keV)
•
– decelerates beam to ~175 keV,
aggravates space charge;
– E(f) 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
6000
350 kV/2.5 MV
500 kV/1.5 MV
4000
3000
350 kV/1.5 MV
2000
1000
0
0.0E+00
5.0E-10
1.0E-09
1.5E-09
time (sec)
2.0E-09
2.5E-09
3.0E-09
Behavior Can Be Counterintuitive…
•
Example: Proper phasing of JLab injector produces excellent beam quality,
stable operation, “completely incorrect” phasing also produces a beam...
– one expects that by reversing the source phase (~180o out of phase with
the SRF cavities), the beam would be accelerated to a halt
– in fact, the beam is captured and accelerated, albeit at poor quality
• phase slip retards low energy beam by additional half-RF-period,
leads to capture
Units: energy: MeV, position: lRF; time: tRF
Injector Operational Challenges
(V)
•
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
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
“Miniphase”
(V)
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
• 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…
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
r=0.6 m
q= 20o
normal entry/exit
bend plane focal length ~ r/sin q ~ 1.75 m
bend-to-bend drift ~ 1.5 m
dispersion in center dipole ~0.5 m
M56 ~ -0.18 m
Merger Issues
(V)
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
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
Module Design: Linac
~30 m
• 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
• Relies on RF focusing in front (back) end on 1st (2nd)
pass
• Wear and tear limits full energy to ~135 MeV or so
Module Design: Transverse Match to
Recirculator
• 6 quad telescope
9 MeV beam to dump
– Match bx,y, ax,y, yx,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 S56 & 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 b, h, l, so CSR, space charge
not aggravated (parasitic crossings not
an issue)
Module Design: Bypass to UV FEL
• Building design – including location for UV wiggler,
predates machine design by over 5 years
– IR system funded/built before the UV (for which the
building was originally designed)
– Bates bend directs beam to IR FEL
• UV FEL “must” live in wiggler pit
– Wiggler stand height >> beam elevation
• Need bypass solution
– must provide appropriate matching
• Transverse spot size, bunch compression, dispersion suppression
– Constrained by pit location
– Add 1 betatron wavelength in bend plane to image/suppress
dispersion
– Select out-of-bend-plane phase advance to manage envelopes,
Reduce
bend
angle
in finalbetween
dipole ofbends
Bates to
bend
to direct
• Use
FODO
quad
transport
manage
aberrations (1/2 betatron wavelength works well)
beam
toward
UV wiggler
pit envelopes
dispersion,
control
betatron
– Use 7 quad FODO transport
– “short out” half of coil pack to reduce bend angle by half
– Sextupoles at high-dispersion points (at horizontally focusing quads)
• Complete
when
transverse offset
providebend
correction
of at
2ndproper
-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
• 2nd Bates bend geometrically identical to 1st
one
• Momentum compactions different (trim quad,
sextupole settings) to complete longitudinal
match giving energy compression during
energy recovery
Details to follow…
Module Design: Reinjection Match
• More or less a mirror image of linac-to-arc match
• Particular care on choice of phase advance needed to
ensure “chromatic balance” & adequate momentum
aperture
– Phase advance between/across telescopes used to control
chromatic aberrations
– Use “extra” quads in the triplet pairs
– Select Twiss parameters in arc with care
9 MeV injected beam
Module Design: Recovery Pass Through Linac
• Two issues:
– Match (beam loss, halo control, tails, scraping…)
– Steering (beam loss, halo control, blah blah blah)
• Multipass steering tricky
– Best to steer 1st pass “locally” (insofar as possible) and
thread 2nd pass through using choice of reinjection orbit
– Situation rendered complex by absence of 2-pass BPMs, RF
skew quad in 5-cell cavity HOM couplers
Multipass Orbit Correction
• Energy of each pass differs at any point of
linac
– Orbit response to steering differs
• Need localized observation & correction
– Multipass BPMs - for high-frequency CW bunch
trains separated by ½ RF period...
• Example: JLab IR Upgrade: orbit bump on 1st
pass of linac
Two Example Orbit Bumps… or NOT
Multipass Orbit Correction –
Common Transport
• “multiple beams” not limited to linac –
some ERL geometries use single
beamlines for multiple beams
– E.g. CEBAF-ER
injector
1L22
l/2 (l/4) path length chicane
2L24
extracted, energy -recovered beam
2L23 2L22
Module Design: Extraction Line to Recovery Dump
• Mirror image of injection merger
• Quad triplet after extraction => betatron spot control at dump
face
• Dispersion ~ 1 m: useful diagnostic for phasing beam during
energy recovery & energy compression