Transcript userweb.jlab.org

“Operational” Beam Dynamics Issues
D. Douglas, JLab
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Thomas Jefferson National Accelerator Facility
Distribution State A
A.K.A., “The JLab Dirty Dozen"
1.
2.
3.
4.
5.
Cathode
Injector Operations
Merger Issues
Space Charge down linac (esp. LSC)
BBU
a)
b)
6.
CSR/LSC during recirculation/compression
a)
7.
8.
9.
10.
11.
12.
Stability
Propagating modes
THz heating
Halo
Ions
Resistive wall/RF heating
Momentum acceptance
Magnet field quality/reproducibility
RF transients/stability
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Thomas Jefferson National Accelerator Facility
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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
(V)
At highest level…
•
•
•
•
System is moderately bright & operates at moderate
power
6000
Wafer500
25kV/5
mmMV
dia
Halo & tails are significant issue
5000
350 kV/5 MV
Must produce very specific beam properties to match downstream acceptance; have
4000
Active area 16 mm dia 500 kV/2.5 MV
very limited number of free parameters to do so
kinetic energy (keV)
•
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
Issues:
•
•
•
•
2000
Space charge & steering in front end
Deceleration by first cavity
1000
Severe RF focusing (with coupling)
0
FPC/alignment steering – phasing a challenge 0.0E+00
•
•
3000
350 kV/2.5 MV
Drive laser 8 mm dia
5.0E-10
1.0E-09
1.5E-09
2.0E-09
2.5E-09
3.0E-09
time (sec)
Miniphase
Halo/tails
• Divots in cathode; scatted drive laser light; cathode relaxation; …
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Courtesy P. Evtushenko
Thomas Jefferson National Accelerator Facility
Distribution State A
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
Thomas Jefferson National Accelerator Facility
Distribution State A
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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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Thomas Jefferson National Accelerator Facility
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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?
•
•
Why did the “properly tuned lattice” not fully compress the bunch?
•
•
dp/p ahead of crest ~1.5 x smaller than after crest; average ~ PARMELA
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
Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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Happek Scan
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Key Points
E
E
•
•
•
“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
t
t
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Thomas Jefferson National Accelerator Facility
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Streak Camera Data from IR Upgrade
-4o
-5o
-6o
-3o
(t,E) vs. linac phase after
crest
-2o
-1o
(data by S. Zhang, v.g. from C. Tennant)
Thomas Jefferson National Accelerator Facility
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0o
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Streak Camera Data from IR Upgrade
+4o
+5o
+3o
+2o
(t,E) vs. linac phase,
before crest
+1o
asymmetry between +
and - show effect of
longitudinal space charge
after 10 MeV
+6o
(data by S. Zhang, v.g. from C. Tennant)
Thomas Jefferson National Accelerator Facility
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0o
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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…)
-4o
-6o
+4o
+6o
=> LSC a concern…
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Thomas Jefferson National Accelerator Facility
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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 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…
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Good example of “power going to the wrong place at the wrong time”
Thomas Jefferson National Accelerator Facility
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
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Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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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
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Thomas Jefferson National Accelerator Facility
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Image courtesy G. Biallas
Lucky 7: Halo
Wafer 25 mm dia
Active area 16 mm dia
•
•
Huge operational problem
Many potential sources
•
•
•
•
•
•
•
•
•
Drive laser 8 mm dia
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
Thomas Jefferson National Accelerator Facility
Distribution State A
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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…
Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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…
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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 beaminduced 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
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Thomas Jefferson National Accelerator Facility
Distribution State A
Images courtesy T. Powers
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; Xray 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
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courtesy
T. Powers
Thomas Jefferson National Accelerator Facility
Distribution State A
Images courtesy T. Powers
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
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Thomas Jefferson National Accelerator Facility
Distribution State A
Images courtesy G. Biallas
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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Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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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 (M52=M16)
•
must
•
make better magnets
•
use lower energy linac
•
reduce M52 (dispersion)
•
provide means of compensation (diagnostics & correction knobs)
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Thomas Jefferson National Accelerator Facility
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Put ANOTHER Way…
•
DB  dx’=DBl/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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Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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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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Thomas Jefferson National Accelerator Facility
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Another Surface of Section…
1.
2.
3.
4.
5.
Cathode
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
Merger Issues
Beam quality preservation
a.
Space charge down linac, esp. LSC
b.
CSR/LSC during recirculation/compression
BBU
6.
7.
8.
9.
10.
11.
12.
Putting power where you don’t want it…
a.
Propagating HOMs
b.
Halo
c.
Resistive wall
d.
RF heating
e.
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!)
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Thomas Jefferson National Accelerator Facility
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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 backbombardment)
solenoid must be reoptimized for each drive laser pulse length
Vacuum levels used as diagnostic
Thomas Jefferson National Accelerator Facility
Distribution State A
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Injector Operational Challenges
(V)
•
(V w/ PM, MSE) (V w/ PM, BPM) (BPM)
1st cavity
6000
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
•
•
kinetic energy (keV)
•
500 kV/2.5 MV
5000
4000
350 kV/2.5 MV
500 kV/1.5 MV
3000
350 kV/1.5 MV
2000
1000
0
0.0E+00
5.0E-10
1.0E-09
1.5E-09
2.0E-09
2.5E-09
3.0E-09
time (sec)
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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
•
•
•
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“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
1.
2.
3.
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…
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