Linac Commissioning P. Emma LCLS Commissioning Workshop, SLAC Sep. 22-23, 2004

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Linac CommissioningP. Emma LCLS Commissioning
Workshop, SLAC Sep. 22-23, 2004
LCLS
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Linac Commissioning Document Available

• gt23 page DRAFT document describing
• initial checkout
• setup of linac
• how/where to measure beam
• what device to scan and by how much
• expected resolution
• how/where to correct beam
• etc

http//www-ssrl.slac.stanford.edu/lcls/linac/notes
/linac_commissioning.ps
(not complete yet, but close)
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Linac Commissioning (Physics)

• Initial checkout (tunnel and beam-based)
• How to setup Ipk 3.4 kA, ge ? 1.2 mm, etc.
• What to measure and what to adjust (simulations)
• Longitudinal phase space
• Bunch Length
• Energy Spread (proj. slice)
• Transverse phase space
• Projected Emittance
• Slice Emittance

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Initial Machine Conditions

• Magnets warmed, standardized, and set properly
• Reasonable quality e- beam available
• Rough beam steering established
• Rough RF phasing of each klystron (1 or 2 deg)
• Any beam obstruction and loss issues solved

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Initial Checkout - TUNNEL

• Power-on polarity checks of all new magnets
• Validate proper cabling connections (magnets,
  BPMs, diagnostics)
• Rough alignment inspection (tape measure, eye)
• Verify all insertable devices move in and out,
  and are finally removed

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Initial Checkout - BEAM-BASED

• Take full set of x and y oscillation data
  (checks optics and BPM scaling)
• Verify all profile monitor scaling with known
  centroid shifts (wires, OTR, YAG, screens)
• Repeat above at high dispersion points (DL1, BC1,
  BC2, DL2) using energy changes
• Calibrate all BPM-charge, and toroid readings
  based on one well established reference
• Document all the above for future scrutiny

CHECKOUT MUST BE COMPLETE BEFORE MEASUREMENTS AND
TUNING BEGIN!
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Location of Main Linac Diagnostics

• 5 energy spread meas. stations (optimized with
  small b)

• 5 emittance meas. stations designed into optics
  (Dyx,y)

• BPMs at or near most quadrupoles and in each bend
  syst.
• RF deflectors for slice e and sE measurements (L0
  L3)

T-cav.
rf gun
T-cav.
gex,y
gex,y
gex,y
gex,y
L0
gex,y
L3
L1
X
L2
...existing linac
sE
sE
sE
sE
?E?
?E?
?E?
sE
?E?
?E?
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Setup of Linac-0 RF
sy bunch length

• Establish initial bunch length
• Phase linac-0

RF-deflector at 1 MV
minimize energy spread with L0 RF phase
RF-deflector
Phasing L0-Linac
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Setup of Linac-1 RF
20-mm res. BC1 BPM
20-mm res. BC1 BPM
LX RF phase
L1 RF phase
Linac-1 RF phase scan (X-band off, BC1 bends at
295 MeV)
Linac-X RF phase scan (BC1 bends at 250 MeV)
set phase to -25?0.5
set phase to -160?0.5

• Turn on BC1 energy feedback
• Switch off BC2 chicane
• Use sec-25 RF-deflector to meas. bunch length
  (sz1)
• Adjust S-band RF phase (?0.5) to set sz1 ? 195
  ?10 mm

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Setup of Linac-2 RF
40-mm res. BC2 BPM
5-mm res. LTU BPM
(NO CSR)
L2 RF phase
L2 phase
Scan Linac-2 phase use LTU BPM to find max.
wake-induced energy loss across Linac-3
Linac-2 RF phase scan (BC2 bends at 5.9 GeV, or)
set phase to -41?0.5

• Turn on BC2 energy feedback
• Use sec-25 RF-deflector to measure length (sz2)
• Adjust L2 RF phase (?0.5) to set sz2 ? 20 ? 2 mm
• Linac-3 phasing is trivial (?5)

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Emittance Meas. Simulation in LCLS

• Multiple OTR screens
• Multiple wire-scanners

• Quad-scans on single OTR
• Quad-scans on OTR with RF-deflector

OTR1,2,3
WS11,12,13
BC1
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135-MeV OTR x-profiles from tracking
sx ? 118 mm
sx ? 49 mm
sx ? 120 mm
non-Gaussian
gex 0.75 0.04 mm
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x-Slice-Emittance Measurement
sy ? bunch length
RF-deflector at 1 MV
slice OTR 10 times
QE03 quad scanned
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x-Slice-Emittance Measurement Simulation
QE03 scanned ?20 (centered on 106 of its
nominal gradient)
Injector at 135 MeV with S-band RF-deflector
at 1 MV
(same SLAC slice-e code used at BNL/SDL)
DL1 slice-emit on WS02
slice-5
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y-slice-Emittance Measurement in BC1
Q21201 scanned 20
0.1 mm
3.9 mm
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y-slice-Emittance Measurement Simulation
Q21201 scanned ?20 (centered on 92 of its
nominal gradient)
slice-y emittance in BC1 using x-chirp
slice-5
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Empirical BC1/BC2 Dispersion Correction
actual data from SPPS chicane
Residual x-dispersion (and its angle) is
precision minimized using tweaker quads in the
chicane
SPPS chicane with quads
Correct h and (ah bh?), orthogonally with 2
quads
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Transverse Wakefield Compensation
All quadrupoles, RF structures, and BPMs
misaligned by 300 mm rms
trajectory after steering
gex ? 2.1 mm gey ? 5.2 mm
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Repeat for 100 Different Seeds
M. Borland
De/e ? 20 (projected)
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Energy Jitter Measurement (Feedback)
x1
x2
Two 5-mm BPMs
relative energy centroid resolution 0.003
undulator
(hx ? 125 mm)
Difference of BPM x-readings is proportional to
energy change, and insensitive to incoming
trajectory jitter.
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Slice-Emittance Configuration for LTU
nominal optics (proj. emit.)
WS31-34
undulator
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Slice-Emittance Measurements in LTU
QEM3 (or tweaker) scanned ?3 (centered on its
nominal gradient)
LTU at 14 GeV with S-band RF-deflector at 24 MV
x
z
x
y
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Slice Energy Spread Measurements in LTU
LTU at 14 GeV with S-band RF-deflector at 24 MV
sE/E ? 10-4
sx ? 12 mm
FEL goal
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Summary

• Most diagnostics incorporated into design and
  simulated (1st pass)
• Two RF-deflectors allow time resolved
  measurements at low and high energy
• Tune-up algorithms considered, but more
  refinement needed
• Feedback systems must maintain setup while tuning
  progresses (bunch length monitors?)