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DC Gun

DC Gun. The Jlab UV FEL Driver ERL. IR Wiggler. SRF Linac. Bunching Chicane. UV FEL Transport Line. Dump. Design Requirements. Power recovery from exhaust beam Transverse, longitudinal matching Collective effect/instability control space charge, BBU, FEL/RF interaction

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DC Gun

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  1. 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)

  4. Relevant Phenomena • Space charge (transverse, longitudinal) • Inject long (2.5 psec/1.33orms), 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. gun Design Solution injector Reinjection/recovery transverse match merger Energy recovery through linac Transverse match: linac to arc recovery Bates bend linac Betatron match to wiggler Bates bend Return bypass transport to energy recovery arc wiggler Extraction line to dump Betatron match from wiggler to recovery transport 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. “oscillator” E “amplifier” f E E E f E f f E f f Example 1: Longitudinal Matching in an ERLSchematic Longitudinal Matching for ERL-Driven FEL injector linac dump wiggler

  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 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

  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. E E E t t t Energy Compression 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 E • 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. E E E E f f f f E E f f 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)

  19. 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 g Figure 4: quads at -215, but sextupoles 3000 g below design, at 10726 g-cm Figure 5: where we left it: trim quads -215 g sextupoles at 12726 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 • 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. Wafer 25 mm dia 500 kV/5 MV 350 kV/5 MV Active area 16 mm dia 500 kV/2.5 MV 350 kV/2.5 MV Drive laser 8 mm dia 2. Injector Operational Challenges (V) (V w/ PM, MSE) (V w/ PM, BPM) (BPM) • 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; … Courtesy P. Evtushenko

  26. 3. 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

  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 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

  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. E E t t 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

  32. Streak Camera Data from IR Upgrade -4o -3o -2o -5o -1o -6o 0o (t,E) vs. linac phase after crest (data by S. Zhang, v.g. from C. Tennant)

  33. Streak Camera Data from IR Upgrade +4o +3o +2o +5o +1o +6o 0o (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) (V w/ PM, MSE) (V w/ PM, BPM) (BPM) • 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. 500 kV/2.5 MV 350 kV/2.5 MV 500 kV/1.5 MV 350 kV/1.5 MV Injector Operational Challenges (V) (V w/ PM, MSE) (V w/ PM, BPM) (BPM) • 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

  37. 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

  38. “Miniphase” (V) (V w/ PM, MSE) (V w/ PM, BPM) (BPM) • 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…

  39. Module Design: Linac

  40. Module Design: Transverse Match to Recirculator

  41. Module Design: Bates Bend

  42. Module Design: Bypass to UV FEL

  43. Module Design: Match to Wiggler

  44. Module Design: Match from Wiggler

  45. Module Design: Return transport to recovery arc

  46. Module Design: Bates Bend

  47. Module Design: Reinjection Match

  48. Module Design: recovery pass through linac

  49. Module Design: Extraction line to recovery dump7

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