1. Thomas Jefferson National Accelerator Facility 1 of 35 Distribution State A “Operational” Beam Dynamics Issues D. Douglas, JLab

2. Thomas Jefferson National Accelerator Facility 2 of 35 Distribution State A A.K.A., “The JLab Dirty Dozen" 1. Cathode 2. Injector Operations 3. Merger Issues 4. Space Charge down linac (esp. LSC) 5. BBU a) Stability b) Propagating modes 6. CSR/LSC during recirculation/compression a) THz heating 7. Halo 8. Ions 9. Resistive wall/RF heating 10. Momentum acceptance 11. Magnet field quality/reproducibility 12. RF transients/stability

3. Thomas Jefferson National Accelerator Facility 3 of 35 Distribution State A 1. Cathode Cesiated GaAs Excellent performance for R&D system When 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?

4. Thomas Jefferson National Accelerator Facility 4 of 35 Distribution State A 2. Injector Operational Challenges 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; scatted 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 350 kV/2.5 MV 500 kV/2.5 MV 350 kV/5 MV 500 kV/5 MV Courtesy P. Evtushenko

5. Thomas Jefferson National Accelerator Facility 5 of 35 Distribution State A 3. 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)

6. Thomas Jefferson National Accelerator Facility 6 of 35 Distribution State A System Layout 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  p/p (~12%)  get slope, curvature, and torsion right (quads, sextupoles, octupoles)

7. Thomas Jefferson National Accelerator Facility 7 of 35 Distribution State A 4. Space Charge – Esp. LSC – Down Linac Had significant space charge issues in linac during commissioning: Why was the beam momentum spread asymmetric around crest? dp/p ahead of crest ~1.5 x smaller than after crest; average ~ PARMELA Why did the “properly tuned lattice” not fully compress the bunch? M 55 measurement showed proper injector-to-wiggler transfer function, but beam didn’t “cooperate”… minimum bunch length at “wrong” compaction Why was the bunch “too long” at the wiggler? bunch length at wiggler “too long” even when fully “optimized” could only get 300-400 fsec rms, needed 200 fsec 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

8. Thomas Jefferson National Accelerator Facility 8 of 35 Distribution State A 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 Simple estimates => imposed correlated momentum spread ~1/L b 2 and 1/r b 2 The latter observed – bunch length clearly match-dependent The former quickly checked…

9. Thomas Jefferson National Accelerator Facility 9 of 35 Distribution State A 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”)

10. Thomas Jefferson National Accelerator Facility 10 of 35 Distribution State A Happek Scan

11. Thomas Jefferson National Accelerator Facility 11 of 35 Distribution State A 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 E t E t

12. Thomas Jefferson National Accelerator Facility 12 of 35 Distribution State A Streak Camera Data from IR Upgrade -5 o -6 o 0o0o -1 o -2 o -3 o -4 o (t,E) vs. linac phase after crest (data by S. Zhang, v.g. from C. Tennant)

13. Thomas Jefferson National Accelerator Facility 13 of 35 Distribution State A +5 o +6 o 0o0o +1 o +2 o +3 o +4 o 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)

14. Thomas Jefferson National Accelerator Facility 14 of 35 Distribution State A ±4 and ±6 degrees off crest “+” on rising, “-” on falling part of waveform L bunch consistent with dp/p and M 56 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… +4 o -4 o -6 o +6 o

15. Thomas Jefferson National Accelerator Facility 15 of 35 Distribution State A 5. BBU After considerable effort, stability is usually a nonissue A bad setup can have ½ mA threshold A good setup can be absolutely stable (skew quad rotator) Threshold sometimes lasing dependent (laser on>laser off) – but with bad match… Propagating modes can be an issue (well, a nusiance) – even at our low beam powers High frequency from beam talks to cold window temp. monitors in waveguide; trips us off (CWWT) Typically run masked, monitor values & determine response to beam is prompt, not thermal… Good example of “power going to the wrong place at the wrong time” BBU video courtesy C. Tennant

16. Thomas Jefferson National Accelerator Facility 16 of 35 Distribution State A 6. CSR/LSC during recirculation/compression (with a side of THz heating…) 135 pC/0.35 psec bunch ~ 400 A peak current CSR/LSC effects evident Enhanced by parasitic compressions (Bates bend) Initial operation irradiated outcoupler – THz heating (next slide…) Use CSR enhancement at tuning cue CSR video courtesy K. Jordan

17. Thomas Jefferson National Accelerator Facility 17 of 35 Distribution State A CSR/THz (Mis)Management Parasitic compressions Very short bunch after optical cavity chicane and at 1 st dipole of return arc Sprayed THz onto outcoupler – “power where we didn’t want it… Added chicane between wiggler and arc to lengthen bunch (overcompression), move source point away from outcoupler And, yes, its putting more power where we didn’t want it…

18. Thomas Jefferson National Accelerator Facility 18 of 35 Distribution State A Learning About THz Management/Mirror Loading July ’04 10 kW run provided illumination on problem of THz loading of mirrors “THz chicane” installed during next down to move source point away from downstream optic Reduced THz power onto optic, but also modified distribution of remaining THz, directing it onto center of mirror with resulting aggravated loading/distortion “THz traps” developed to capture/block remnant; alleviate remaining loading Image courtesy G. Biallas

19. Thomas Jefferson National Accelerator Facility 19 of 35 Distribution State A Lucky 7: Halo Huge operational problem Many potential sources Ghost pulses from drive laser Cathode temporal relaxation Scattered light on cathode Cathode damage Field emission from gun surfaces Space charge/other nonlinear dynamical processes Dark current from SRF cavities… We see multiple sources (CW beamlets at various energies [even with beam off]), large-amplitude energy tails/spatial halo (beam on) all through system Much of our tune time is spent getting halo to “fit” though (can’t throw it away; get activation & heating damage; can’t collimate it, it just gets mad…) Tends to be mismatched to, out of phase with, core beam Can “tweak” it through – though this might not work a large system…. Look at activation patterns, beam loss, tune on BLMs Wafer 25 mm dia Active area 16 mm dia Drive laser 8 mm dia

20. Thomas Jefferson National Accelerator Facility 20 of 35 Distribution State A Caveats Yet another example of “putting power where you don’t want it…” Halo is not like beam loss during storage ring operation, its more like beam loss during injection into a storage ring… so unless injection efficiencies are always (cathode to stored beam) 99.999% or so (0.00001% loss), halo is a problem. ERLs are transport lines. Large acceptance systems are hoist by their own petard: stuff that might “go away” in a more conventional machine will instead fit into the, well, LARGE acceptance… and can end up going away someplace unexpected or bad “unexpected” in our system – e.g. the middle of the 1 st reverse bend (dark current) where the chamber is about 1 foot wide “bad” in our system – the small aperture wiggler chamber, where its ½ inch wide… HALO NEEDS IMMEDIATE ATTENTION! Large apertures/small beam envelopes…

21. Thomas Jefferson National Accelerator Facility 21 of 35 Distribution State A 8. Ions Not a problem. Not a problem for CEBAF-ER. Not a problem for the IR Demo. Not a problem for CEBAF. In other words, its not a problem for 4 machines (including 3 ERLs) spanning two orders of magnitude in energy (20 Mev to 6 GeV), seven orders of magnitude in current (yes, seven, … wait just a minute…) and eight orders of magnitude in bunch charge (yes, EIGHT: Hall B takes 1 nA, ~10 electrons/bunch, that’s a nano-nano Coulomb…) We have no clue why Estimates on all the machines show that they “should” have problems – and also show they “should” be problem free. Nature chose, we don’t know how. Gotta love cryopumping? IONS NEED IMMEDIATE ATTENTION! (Thanks Todd!)

22. Thomas Jefferson National Accelerator Facility 22 of 35 Distribution State A Caveat “Not a problem” implies a. We know what the signature(s) of ions will be in a recirculator or ERL (we don’t…) i. CEBAF emittance “growth”? b. That (those) signature(s) are missing from the aforementioned machines (we don’t know if they are or aren’t…) i. CEBAF emittance “growth”? We just haven’t seen anything in ~20 years of operation that screams “IONS”… and we really don’t know why, or what to look for…

23. Thomas Jefferson National Accelerator Facility 23 of 35 Distribution State A 9. Resistive Wall & RF Heating Yes, we’re STILL “putting power where we don’t want it…” Resistive wall seen when new narrower wiggler chamber installed in Fall ‘05 Observed drift in optical diagnostics traced to beam- induced heating of wiggler chamber: chamber expands, moving hardware Temperature rise depends both on current and bunch length; 5 mA CW beam/short bunch led to 50 o C rise in a few minutes Attributed to resistive wall effects after analysis by SRF, CASA collegues Managed by adding cooling Images courtesy T. Powers courtesy T. Powers

24. Thomas Jefferson National Accelerator Facility 24 of 35 Distribution State A Beam Current-Driven Effects RF heating of OCMMS/x-ray cube OCCMS, x-ray cube also heated up over 40 o C when running ~5 mA CW Heating depended on current but not on bunch length K. Beard analysis with Microwave Studio showed OCMMS resonates at ~1500 MHz; X- ray cube is ~10 cm x 10 cm x 10 cm – or roughly a pi-mode cavity at 1500 MHz Suggests heating due to deposition of RF power into the devices X-ray cube removed, RF control/damping added to downstream OCMMS Images courtesy T. Powers courtesy T. Powers

25. Thomas Jefferson National Accelerator Facility 25 of 35 Distribution State A Beam Current-Driven Effects Momentum spread enhancement by OCCMS/x-ray cube Over the summer, a large blow-up of momentum spread evolved at short bunches ~10% exhaust energy spread observed for short bunch – even without lasing Compressing beam at various locations localized this effect to region between wiggler and THz chicane Lasing remained okay, suggesting effect due to beam interaction with downstream OCMMS (known to be resonant at RF frequencies) Change of match, installation of shorting clips, RF dampers in OCCMs, removal of x-ray cube mitigated effect Images courtesy G. Biallas

26. Thomas Jefferson National Accelerator Facility 26 of 35 Distribution State A 10. Momentum Acceptance FEL exhaust energy spread ~12-13% full Need Large acceptance beam transport Energy compression during energy recovery Decelerating 14 MeV spread to 10 MeV… Requires ~30 o phase acceptance in linac Use incomplete energy recovery, control of path length (aberrations) Tune momentum compactions through 3 rd order Harmonic RF difficult to implement

27. Thomas Jefferson National Accelerator Facility 27 of 35 Distribution State A Cautionary Tale (Tail?) Serving as a Warning to Others: Demo Dump – core of beam off center, even though BLMs showed edges were centered

28. Thomas Jefferson National Accelerator Facility 28 of 35 Distribution State A 11. Magnet Field Quality/Reproducibility Magnet field quality excellent e.g. GX at 145 MeV/c Top: measured field Bottom: design calculation (contours @ 1/2x10 -4 ) (Thanks to George Biallas, Tom Hiatt & the magnet measurement facility staff, Chris Tennant, and Tom Schultheiss) Reproducibility: Large magnets – great Small magnets – bad (consumes a lot of tune time)

29. Thomas Jefferson National Accelerator Facility 29 of 35 Distribution State A ERL Field Quality Requirement  B   x’ =  Bl/B  B/B)  (dipole)  x’   l = M 52  x’  l   E dump = E linac sin  0 (2  l/ RF ) = E linac sin  0 (2    B/B  / RF ) “Field quality”  B/B needed to meet budgeted  E dump must improve (get smaller) for longer linac (higher E linac ), shorter RF, larger dispersion (M 52 =M 16 ) must make better magnets use lower energy linac reduce M 52 (dispersion) provide means of compensation (diagnostics & correction knobs)

30. Thomas Jefferson National Accelerator Facility 30 of 35 Distribution State A Put ANOTHER Way…  B   x’=  Bl/B  Bl/(33.3564 kg-m/GeV * E linac ) (error integral)  l   E dump = sin  0 (2    Bl/33.3564 kg-m  / RF ) (GeV) “Error field integral”  Bl is independent of linac length/energy gain tolerable relative field error falls as energy (required field) goes up Numbers for upgrade:  E dump ~ 3400 MeV *  B/B   which we see: we have 10 -4 and see few 100 keV)  E dump ~ 0.16 keV/g-cm *  Bl 

31. Thomas Jefferson National Accelerator Facility 31 of 35 Distribution State A 12. RF Transients & Stability If you energy compress during recovery, M 56 is nonzero (wiggler to linac) FEL turn off/on => phase shift => transient beam loading Similar for beam off/on… See Powers & Tennant, ERL2007 Big driver of RF power requirements…

32. Thomas Jefferson National Accelerator Facility 32 of 35 Distribution State A An Appeal… This is a challenge – not operational from IR Demo/Upgrade, but a concern given CEBAF & CEBAF-ER experience PLEASE CONSIDER RECIRCULATION as cost savings/performance enhancement for x-fel drivers (and multipass ERLs) Quantum excitation becomes problem for emittance preservation, but Addressed in SLC, managed in a generation of storage rings, being attacked for CEBAF 12 GeV upgrade

33. Thomas Jefferson National Accelerator Facility 33 of 35 Distribution State A

34. Thomas Jefferson National Accelerator Facility 34 of 35 Distribution State A Acknowledgements: Funding by ONR, JTO, DOE

35. Thomas Jefferson National Accelerator Facility 35 of 35 Distribution State A Details…

36. Thomas Jefferson National Accelerator Facility 36 of 35 Distribution State A Another Surface of Section… 1. Cathode 2. Injector Operation a. Space charge in front end (solenoid settings b. Deceleration of low energy beam in multicell cavity c. How to phase (observables) d. Matching across merger into “long” linac 3. Merger Issues 4. Beam quality preservation a. Space charge down linac, esp. LSC b. CSR/LSC during recirculation/compression 5. BBU 6. Putting power where you don’t want it… a. Propagating HOMs b. Halo c. Resistive wall d. RF heating e. THz heating (mirrors) 7. Momentum acceptance 8. Magnetic field quality/reproducibility 9. Ions 10. RF Transients/stability 11. Synchrotron radiation excitation (larger machines, e.g. CEBAF-ER) 12. A dare… (ERLers – GO MULTIPASS; X-FELers – RECIRCULATE!)

37. Thomas Jefferson National Accelerator Facility 37 of 35 Distribution State A 2. 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)

38. Thomas Jefferson National Accelerator Facility 38 of 35 Distribution State A Injector Operational Challenges 1 st cavity decelerates beam to ~175 keV, aggravates space charge; E(  ) nearly constant for ±20 o 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) 350 kV/1.5 MV 500 kV/1.5 MV 350 kV/2.5 MV 500 kV/2.5 MV

39. Thomas Jefferson National Accelerator Facility 39 of 35 Distribution State A 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)

40. Thomas Jefferson National Accelerator Facility 40 of 35 Distribution State A “Miniphase” System is underconstrained, 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 1 st SRF cavity by duplicating focusing (beam profile at 1 st view downstream of cryounit) 3. Set phase of 2 nd SRF cavity by recovering energy at spectrometer BPM this avoids necessity of fighting with 1 st SRF cavity… (V w/ PM, MSE) (V w/ PM, BPM) (BPM) (V)