1
Operational Beam Dynamics Issues

• D. Douglas, JLab

2
A.K.A., The JLab Dirty Dozen"

• Cathode
• Injector Operations
• Merger Issues
• Space Charge down linac (esp. LSC)
• BBU
• Stability
• Propagating modes
• CSR/LSC during recirculation/compression
• THz heating
• Halo
• Ions
• Resistive wall/RF heating
• Momentum acceptance
• Magnet field quality/reproducibility
• RF transients/stability

3
1. Cathode

• Cesiated GaAs
• Excellent performance for RD 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, cant 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
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

5
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

6
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 dp/p (12)
• get slope, curvature, and torsion right
• (quads, sextupoles, octupoles)

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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?
• M55 measurement showed proper injector-to-wiggler
  transfer function, but beam didnt 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

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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 gt imposed correlated momentum
  spread 1/Lb2 and 1/rb2
• The latter observed bunch length clearly
  match-dependent
• The former quickly checked

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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 800900 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)

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Happek Scan
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Key Points

• Lengthen thy bunch at injection, lest space
  charge rise up to smite thee (Pv. 321, or
  Hernandez-Garcia et al., Proc. FEL 04)
• best injected emittance DOES NOT NECESSARILY
  produce best DELIVERED emittance!
• LSC effects visible with streak camera

12
Streak Camera Data from IR Upgrade
(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)
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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(-)gtdp/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)
• gt LSC a concern

-4o
-6o
4o
6o
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5. BBU
BBU video courtesy C. Tennant

• 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
  ongtlaser 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

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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
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CSR/THz (Mis)Management

• Parasitic compressions
• Very short bunch after optical cavity chicane and
  at 1st dipole of return arc
• Sprayed THz onto outcoupler power where we
  didnt 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 didnt
  want it

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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
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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 (cant throw it away get activation
  heating damage cant 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

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Caveats

• Yet another example of putting power where you
  dont 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 1st 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

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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, thats 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 dont
  know how.
• Gotta love cryopumping?
• IONS NEED IMMEDIATE ATTENTION! (Thanks Todd!)

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Caveat

• Not a problem implies
• We know what the signature(s) of ions will be in
  a recirculator or ERL (we dont)
• CEBAF emittance growth?
• That (those) signature(s) are missing from the
  aforementioned machines (we dont know if they
  are or arent)
• CEBAF emittance growth?
• We just havent seen anything in 20 years of
  operation that screams IONS and we really
  dont know why, or what to look for

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9. Resistive Wall RF Heating

• Yes, were STILL putting power where we dont
  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 50o
  C rise in a few minutes
• Attributed to resistive wall effects after
  analysis by SRF, CASA collegues
• Managed by adding cooling

courtesy T. Powers
Images courtesy T. Powers
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Beam Current-Driven Effects

• RF heating of OCMMS/x-ray cube
• OCCMS, x-ray cube also heated up over 40o 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

courtesy T. Powers
Images courtesy T. Powers
25
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
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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 30o phase acceptance in linac
• Use incomplete energy recovery, control of path
  length (aberrations)
• Tune momentum compactions through 3rd order
• Harmonic RF difficult to implement

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Cautionary Tale (Tail?) Serving as a Warning to
OthersDemo Dump core of beam off center, even
though BLMs showed edges were centered
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11. Magnet Field Quality/Reproducibility

• Magnet field quality excellent
• e.g. GX at 145 MeV/c
• Top measured field
• Bottom design calculation
• (contours _at_ 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)

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ERL Field Quality Requirement

• DB ? dx DBl/Br (DB/B) q (dipole)
• dx ? dl M52 dx
• dl ? DEdump Elinacsin f0 (2p dl/lRF)
• Elinacsin f0 (2p M52(DB/B)q/lRF)
• Field quality DB/B needed to meet budgeted
  DEdump must improve (get smaller) for longer
  linac (higher Elinac), shorter lRF, larger
  dispersion (M52M16)
• must
• make better magnets
• use lower energy linac
• reduce M52 (dispersion)
• provide means of compensation (diagnostics
  correction knobs)

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Put ANOTHER Way

• DB ? dxDBl/Br DBl/(33.3564 kg-m/GeV
  Elinac) (error integral)
• dl ? DEdump sin f0 (2p M52(DBl/33.3564
  kg-m)/lRF) (GeV)
• Error field integral DBl is independent of
  linac length/energy gain
• tolerable relative field error falls as energy
  (required field) goes up
• Numbers for upgrade
• DEdump 3400 MeV (DB/B)
• (which we see we have 10-4 and see few 100
  keV)
• DEdump 0.16 keV/g-cm (DBl)

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12. RF Transients Stability

• If you energy compress during recovery, M56 is
  nonzero (wiggler to linac)
• FEL turn off/on gt phase shift gt transient beam
  loading
• Similar for beam off/on
• See Powers Tennant, ERL2007
• Big driver of RF power requirements

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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

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(No Transcript)
34
Acknowledgements Funding by ONR, JTO, DOE
35
Details
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Another Surface of Section

• Putting power where you dont want it
• Propagating HOMs
• Halo
• Resistive wall
• RF heating
• THz heating (mirrors)
• Momentum acceptance
• Magnetic field quality/reproducibility
• Ions
• RF Transients/stability
• Synchrotron radiation excitation (larger
  machines, e.g. CEBAF-ER)
• A dare (ERLers GO MULTIPASS X-FELers
  RECIRCULATE!)

• Cathode
• Injector Operation
• Space charge in front end (solenoid settings
• Deceleration of low energy beam in multicell
  cavity
• How to phase (observables)
• Matching across merger into long linac
• Merger Issues
• Beam quality preservation
• Space charge down linac, esp. LSC
• CSR/LSC during recirculation/compression
• BBU

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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

38
Injector Operational Challenges

• 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

39
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

40
Miniphase

• 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