LCLS Undulator Alignment and Motion Review Beam-Based Alignment (BBA) Paul Emma, SLAC Oct. 21, 2005

1
LCLS Undulator Alignment and Motion Review
Beam-Based Alignment (BBA)Paul Emma, SLACOct.
21, 2005

• Brief Review of Method
• Simulations
• Drift Tolerances

2
Motivation

• For SASE FEL at 1.5 Å
• Electron trajectory through undulator needs to be
  straight to lt5 mm over 10 m,
• Traditional survey methods inadequate here,
• BPM data acquired for several beam energies (14,
  7.0, 4.5 GeV) can resolve this level,
• Beam-based, energy-variation method is sensitive
  to all fields, not just misaligned quadrupole
  magnets

3
The Method

• BPM readings, mi, written as sum of upstream
  kicks offset, bi
• Kicks are sensitive to momentum, pk, while
  offsets, bi, are not

bi gt 0
s
4
...The Method

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

mi
linear only if Cij independent of p
offset -bi
1/p
p??
(15 GeV/c)-1
(7.0 GeV/c)-1
(4.5 GeV/c)-1
5
Schematic layout before BBA (cartoon example)
Undulator misaligned w.r.t. linac axis with
uncorrelated and correlated (random walk)
component
initial incoming launch error
x?0
x0
suggested by C. Adolphsen
6
Final trajectory after BBA (cartoon example)
Beam is launched straight down undulator, with
possible inconsequential kink at boundary
LINAC
dispersion generated is insignificant
Quadrupole magnets moved onto straight line and
BPM offsets subtracted in software, while
undulators track quadrupoles
7
Input errors used for simulation
8
Initial BPM and quad misalignments (w.r.t. linac
axis)
quad positions
BPM offsets
Now launch beam through undulator?
9
Initial trajectory before any correction applied
real trajectory
quad positions
BPM readings
10
After weighted steering prior to BBA procedure
11
After 1st pass of BBA (13.6 GeV)
sx ? 44 mm
Dj ? 3322
sy ? 33 mm
12
Steering coils used for small, final corrections
Use steering coils for final iterations (quad
move equivalent down to 0.5 mm)
BPM
BPM
?7 mm
13
After 3rd pass of BBA (13.6 GeV)
sx ? 3.2 mm
Dj ? 98
RON (FEL-code) simulation shows Lsat increased by
lt1 gain-length R. Dejus, N.Vinokurov
sy ? 2.5 mm
14
Run BBA on 25 Different Random Seeds
1-mm BPM resolution 100-mm initial BPM quad
offsets
x y mover distrib.
Djx,y
15
FEL code evaluation of BBA simulation results...
FEL Saturation Power at 1.5 Å
FEL Saturation Length at 1.5 Å
B. Fawley, H.-D. Nuhn, S. Reiche, PE
16
Alignment drift during BBA procedure

• Quadrupole and BPM alignment may change during
  BBA procedure.
• One iteration of BBA procedure will require lt1 hr
  after full development and experience.
• Simulate alignment drift during BBA, using
  ?2.5-mm quad and BPM (separately) changes, which
  are uncorrelated ?

17
?2.5-mm uniform quad and BPM changes during BBA
procedure
Dj ? 167
REQUIRE lt ?2 mm quad/BPM stability over 1 hr
18
Allow ?5-mm uniform quad and BPM drift over long
term (24 hrs)
After beam-based alignment
?10 mm quad and BPM drift
MICADO steering applied
Tolerance set at ?5 mm over 24 hrs
19
Summary
Alignment can be achieved at adequate level using
beam-based technique, given that

• BPMs resolve trajectory to 1-2 mm rms
• Quad positions and BPM readings drift lt2 mm
  over 1-hr procedure
• Trajectory is stable to lt20 of beam size
  (already demonstrated in FFTB)
• BBA procedure repeated no more than once per week
  given ?5 mm drift tolerance