BeamBased Alignment of the LCLS FEL Undulator

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Beam-Based Alignment of the LCLS FEL Undulator
Paul Emma SLAC Dec. 4, 2001
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Beam-Based Alignment of the LCLS FEL Undulator

• LCLS introduction
• Undulator design
• Alignment motivation
• Alignment method
• Steps used in procedure
• Errors used in simulations
• Simulation results
• Summary

BBA used at TTF-FEL DESY (P. Castro, et. al.)
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Linac Coherent Light Source (LCLS)
4th-Generation X-ray SASE FEL Based on SLAC Linac
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Peak brightness increased 108 over existing
sources
?105 by FEL gain
?103 by improved beam quality, long undulators
courtesy J. Roßbach
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Undulator (ANL/APS)
1-mm BPM ? cavity and button-type in compact
package (G. Decker, ANL)
Quad-magnets and undulator sections on
independent movers
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LCLS 9-Pole Prototype Undulator (ANL)
3.4-m section is under construction at ANL
Courtesy E. Gluskin, ANL
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FODO Lattice with ??? ? 18 m

• 32 undulator sections, L 3.4 m
• gt4000 undulator periods (3 cm)
• Permanent magnet quadrupoles
• 15º b-phase adv./cell, ????18 m
• Magnet movers on each quad (x,y)
• Movers on each undulator section
• 1-mm BPM at each quad (x,y)
• Quads aligned w.r.t. adjacent undulator sections
  to lt50 mm

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Alignment Motivation

• Need good overlap of e-/photon beam (30 mm rms)
• With 1.5-Å radiation, phase slippage between
  photons and e- is very sensitive to trajectory
  errors
• The e- trajectory through the undulator needs to
  be straight to lt5 mm over a 10-m FEL gain length
• This level is difficult to achieve with survey
  methods
• LCLS will, therefore, rely on beam-based alignment

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The Basic Strategy

• Take beam position monitor (BPM) readings as a
  function of large, deliberate changes in e-
  energy
• Calculate and correct quadrupole BPM
  misalignments and adjust initial trajectory
  launch
• Repeat 3 times with first application
• Re-apply one iteration per month (?)

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The Method

• BPM readings, mi, written as sum of upstream
  kicks offset, bi

• Kicks (? BPM reading) are sensitive to momentum,
  pk, while offsets , bi, are not

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mi
linear only if Cij independent of p
offset -bi
1/p
p??
(15 GeV/c)-1
(10 GeV/c)-1
(5 GeV/c)-1

• Reference line defined by incoming x0, x?0 launch
  conditions

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• For thick lens quadrupole, define

where Q j is the transfer matrix across the jth
quad, and R ji is the transfer matrix from end of
jth quad to ith BPM, all evaluated at kth
momentum setting, pk.

• Then solve

BPM readings at p1
BPM offsets
BPM readings at p2
quad offsets
known optical functions at each pk
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• Must solve this system with soft-constraints
  placed on the resulting BPM and quadrupole offsets

1 mm

• Without this reasonability weighting, the
  resulting BPM and quad offset can stray out to
  large values at low frequencies

• Scanning energy gives sensitivity to (and
  correction of) all field errors, including
  undulator poles, and Earths field, etc

C. Adolphsen, 1989 PAC
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Schematic Layout
permanent magnet quadrupoles and undulator poles
suggested by C. Adolphsen
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Beam-Based Alignment Steps
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Undulator Parameters
3.42 m
undulator section
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Input Errors for the Simulation
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Initial BPM and Quad Misalignments (w.r.t. linac
axis)
Now launch beam through undulator?
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Initial trajectory before any correction is
applied
Note, all trajectory plots are w.r.t. linac axis
(except last 3)
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Trajectory after initial rough steering
Save as 1st set of BPM readings
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Energy now reduced to 10 GeV
Save as 2nd set of BPM readings
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Energy now reduced to 5 GeV
Save as 3rd set of BPM readings
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Fitted quadrupole offsets
results differ by straight line
similar plot for BPM offsets (not shown)
use linear component of fitted offsets to
re-adjust launch
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Absolute trajectory after 1st pass of BBA
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Possible Absolute Trajectory
Beam is launched straight down undulator, with
possible inconsequential kink at boundary
LINAC
dispersion generated is insignificant
Now look at trajectory w.r.t. undulator axis ?
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After 1st pass of BBA (now w.r.t. undulator line)
Now repeat procedure of energy changes
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After 2nd pass of BBA
and repeat again
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After 3rd pass of BBA
Dj ? 100
RON (FEL-code) simulation shows Lsat increased by
lt1 gain-length R. Dejus, N.Vinokurov
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Electron/photon accumulated phase errors 3rd pass
of BBA
?x?y
?x
?y
Imperfect trajectory generates phase error 100º
(0.4-Å slippage)
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Final quadrupole magnet mover settings after 3rd
pass BBA
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Summary and Comments
Alignment can be achieved at adequate level using
beam-based technique, given that

• BPMs resolve trajectory at level of 1 mm rms

• BPM readings drift lt1 mm over a 1-2 hours
  (temperature)

• Magnet movers are settable to within ?1 mm (or
  use coils)

• BPM readings are not sensitive to variable beam
  size, etc.

• Machine is stable enough in energy, charge,
  trajectory,