1. The Jlab UV FEL Driver ERL
DC Gun
The Jlab UV FEL Driver ERL
IR Wiggler
SRF Linac
Bunching Chicane
UV FEL Transport Line
Dump

2. Design Requirements Power recovery from exhaust beam
Transverse, longitudinal matching
Collective effect/instability control
space charge,
BBU,
FEL/RF interaction
Loss (halo) management
Delivery of appropriately configured beam to FEL
Transverse, longitudinal phase space management
Preservation of beam quality
space charge, wake/collective effects, CSR

3. Parameters (Achieved)IR UV
Energy (MeV)
88-165
135
Iave (mA)
9.1 2
Qbunch (pC) 60
eN transverse/longitudinal
(mm-mrad/keV-psec)
8/75
5/50
sdp/p, sl (fsec)
0.4%, 160
0.4%, 100
Ipeak (A)
400
250
FEL repetition rate (MHz)
(cavity fundamental )
hFEL
2.5%
0.8%
DEfull after FEL
~15%
~7%

4. Relevant Phenomena Space charge (transverse, longitudinal) BBU CSR
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

5. 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
IR Wiggler
SRF Linac
Bunching Chicane
UV FEL Transport Line
Dump

6. Design Solution gun injector Reinjection/recovery transverse match
recovery Bates bend
injector
Return bypass transport to energy recovery arc
Reinjection/recovery transverse match
merger
Energy recovery through linac
Transverse match: linac to arc
linac
Betatron match from wiggler to recovery transport
Betatron match to wiggler
Bates bend
wiggler
Extraction line to dump
UV bypass transport grafted onto Bates bend

7. Phase Space Management
Transport system is “functionally modular”: design embeds specific functions (e.g. transverse matching, dispersion suppression, etc) within localized regions
precludes need for S2E analysis
allows use of potentially ill-defined/poorly controlled components
demands design with 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?

8. “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

9. 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!

10. 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

11. 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

12. Example 1: Longitudinal Matching in an ERL Schematic Longitudinal Matching for ERL-Driven FEL
“oscillator” E f E f
injector
“amplifier”
linac
dump E f E f
wiggler E f E f

13. Energy Recovery: Details
ERL operational experience has shown how to successfully energy recover; this has implications on system efficiency
Longitudinal Match to Wiggler
Inject long, low-energy-spread bunch to avoid LSC problems
need 1-1.5o rms with 1497 MHz 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

14. 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

15. Energy Compression E E t t
All e- after trough go into high-energy tail at dump
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

16. 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 gradiant required for energy compression t
Required phase bite is cos-1(1-DEFEL/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…

17. JLab IR Demo Dump core of beam off center, even though BLMs showed
edges were centered
(high energy tail)

18. Longitudinal Matching Scenario
Requirements on phase space:
high peak current (short bunch) at FEL
bunch length compression at wiggler
using quads and sextupoles to adjust compactions
“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 f E f E f E f E f

19. Nonlinearity Control Validated By Measurement
Figure 1: Inner sextupoles to g-cm and trim quads to -215 g Figure 2: trim quads at -185 g with same sextupoles Figure 3: trim quads at -245 g Figure 4: quads at -215, but sextupoles 3000 g below design, at g-cm Figure 5: where we left it: trim quads -215 g sextupoles at g-cm
launch f
arrival f

20. Injector to Wiggler Transport

21. If you do it right linac produces stable ultrashort pulses
Can regularly achieve 300 fs FWHM electron pulses
~150 fsec rms

22. Injector to Reinjection Transport

23. Module Design: Injector

24. 1. Cathode Cesiated GaAs Other cathodes?
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?

25. 2. Injector Operational Challenges
Courtesy P. Evtushenko
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
(V)
Wafer 25 mm dia
Active area 16 mm dia
Drive laser 8 mm dia
350 kV/2.5 MV
500 kV/2.5 MV
350 kV/5 MV
500 kV/5 MV
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; …

26. 3. Merger Issues
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
(V)
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

27. 4. 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 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

28. 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/rb2
The latter observed – bunch length clearly match-dependent
The former quickly checked…

29. 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”)

30. Happek Scan

31. Key Points E t E t
“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

32. Streak Camera Data from IR Upgrade
(t,E) vs. linac phase after crest
(data by S. Zhang, v.g. from C. Tennant)

33. 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)

34. ±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…
-4o
-6o
+4o
+6o

35. 2. Injector Operational Challenges
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
(V)
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

36. Injector Operational Challenges
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
(V)
1st cavity
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
350 kV/1.5 MV
500 kV/1.5 MV
350 kV/2.5 MV
500 kV/2.5 MV

37. Injector Operational Challenges
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
(V)
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

38. “Miniphase”
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
(V)
System is underconstrained, difficult to spectrometer phase with adequate resolution
Phases drift out of tolerance over few hours
Recover setup by
Set drive laser phase to put buncher at “zero crossing”
(therein lies numerous tales, … or sometimes tails...)
Set drive laser/buncher gang phase to phase of 1st SRF cavity by duplicating focusing (beam profile at 1st view downstream of cryounit)
Set phase of 2nd SRF cavity by recovering energy at spectrometer BPM
this avoids necessity of fighting with 1st SRF cavity…

40. Module Design: Linac

41. Module Design: Transverse Match to Recirculator

42. Module Design: Bates Bend

43. Module Design: Bypass to UV FEL

44. Module Design: Match to Wiggler

45. Module Design: Match from Wiggler

46. Module Design: Return transport to recovery arc

47. Module Design: Bates Bend

48. Module Design: Reinjection Match

49. Module Design: recovery pass through linac

50. Module Design: Extraction line to recovery dump7