thomas jefferson national accelerator facility 1 of 35 distribution state a “operational” beam...

Thomas Jefferson National Accelerator Facility

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Distribution State A

“Operational” Beam Dynamics Issues

D. Douglas, JLab


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A.K.A., “The JLab Dirty Dozen"

1. Cathode

2. Injector Operations

3. Merger Issues

4. Space Charge down linac (esp. LSC)

5. BBUa) Stability

b) Propagating modes

6. CSR/LSC during recirculation/compressiona) THz heating

7. Halo

8. Ions

9. Resistive wall/RF heating

10. Momentum acceptance

11. Magnet field quality/reproducibility

12. RF transients/stability


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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?


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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

0

1000

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3000

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0.0E+00 5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09

time (sec)

kin

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c e

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keV

)

350 kV/2.5 MV

500 kV/2.5 MV

350 kV/5 MV500 kV/5 MV

Courtesy P. Evtushenko


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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



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System LayoutRequirements on phase space:• high peak current (short bunch) at FEL

• bunch length compression at wigglerusing 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)


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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 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


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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 => 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 ~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”)


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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. 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


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Streak Camera Data from IR Upgrade

-5o

-6o

0o

-1o

-2o-3o-4o

(t,E) vs. linac phase after crest

(data by S. Zhang, v.g. from C. Tennant)


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+5o

+6o

0o

+1o

+2o+3o+4o

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(-)>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

-4o -6o

+6o


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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


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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 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…


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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 (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 diaActive area 16 mm dia

Drive laser 8 mm dia


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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 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, 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!)


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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…


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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 50o 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

courtesyT. 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

Images courtesy T. Powers

courtesyT. Powers


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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 Others:Demo 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 @ 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

• B x’ = Bl/BB/B) (dipole)

• x’ l = M52 x’

• l Edump = Elinacsin 0 (2l/RF)

= Elinacsin 0 (2B/B/RF)

• “Field quality” B/B needed to meet budgeted Edump must

improve (get smaller) for longer linac (higher Elinac), shorter

RF, larger dispersion (M52=M16)

• 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…

• B x’=Bl/BBl/(33.3564 kg-m/GeV * Elinac) (error

integral)

• l Edump = sin 0 (2Bl/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:

• Edump ~ 3400 MeV * B/B

which we see: we have 10-4 and see few 100 keV)

• Edump ~ 0.16 keV/g-cm * Bl


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12. RF Transients & Stability

• If you energy compress during recovery, M56 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…


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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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Acknowledgements: Funding by ONR, JTO, DOE


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Details…


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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!)


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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



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Injector Operational Challenges

• 1st cavity • decelerates beam to ~175 keV,

aggravates space charge; • E() 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


0

1000

2000

3000

4000

5000

6000

0.0E+00 5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09

time (sec)

kin

eti

c e

nerg

y (

keV

)

350 kV/1.5 MV

500 kV/1.5 MV

350 kV/2.5 MV500 kV/2.5 MV


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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



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“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 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…


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