Transcript New Measurements

Recent Measurements of the
LCLS Prototype Undulator:
Canted Pole Gap Design and
Hall Probe Center Calibration
I. Vasserman
LCLS UNDULATOR SYSTEM REVIEW
March 3 - 4, 2004
Argonne National Laboratory
Office of Science
U.S. Department of Energy
A U.S. Department of Energy
Office of Science Laboratory
Operated by The University of Chicago
Outline
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Magnetic measurements vs. X (horizontal motion) of
canted pole gap design (3 mrad cant)
- Effective magnetic field
- RMS phase error
- Trajectory
Fringe Fields
Fine adjustment of effective magnetic field
End gap correction (is it needed)?
Hall probe center calibration
Summary
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Effective magnetic field
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Measured slope of 6.6 Gauss/mm agree with calculations (~ 5.7
Gauss/mm for 3 mrad cant)
Alignment accuracy needed for DB/B ~ 1.5x10-4 ~ 2 Gauss -> 0.3 mm
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RMS phase error
• No significant dependency on X
• An RMS phase error of ~ 6.5 degree is an upper limit for nearperfect (~100%) performance
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Horizontal trajectory (averaged over period length) at
14.1 GeV
• Trajectory vs. X well behaved and well within the tolerance
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requirement of 2 mm maximum walk-off from a straight line
Operational range is ±1.2 mm for ±1.0°C temperature
compensation
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Fringe fields at X=65 and 100 mm
• Fringe fields with new shims are close to earth field for X=100
mm. (Earth field contribution to trajectory shift has to be
corrected.)
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Fine adjustment of effective magnetic field
(Procedure to tune the field)
1. Select spacers with thickness step ~ 15 µm to set the effective
field in the range of ±30 Gauss (1 µm in gap corresponds to ~ 2
Gauss in field).
2. Set horizontal position of spacers to adjust the effective field to
~ ±6 Gauss (spacers are wedged with 3 mm/mm cant)
3. Set horizontal position of the undulator as a whole so that the
effective field is in the range ±2 Gauss (DB/B ~ ±1.5x10-4)
(This step is required to save time and to provide better
accuracy)
4. The undulator horizontal position could be remotely controlled
during operation to compensate for in-tunnel temperature
variations (motion of ±1.2 mm for ±1°C needed). Such option is
available, if quadrupoles are separated from undulator sections
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End gap correction (is it needed)?
• Slippage length L = u (1+K2eff /2) -> change in slippage period in
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free space is the main contributor to break length errors (if Keff is
adjusted to compensate for particle energy loss)
Tolerance criterion for phase error between undulator segments is
~ 10. It corresponds to DL/ L~ u * Keff * DKeff /L= 10/360, from where
DKeff/Keff ~ 1.6% for minimum break length.
Real break length error depends on actual particle energy loss
and break length chosen. This error will accumulate over
distance.
Computer simulations are required to examine this case.
Remotely-controlled end gap correction for final part of undulator
line may be useful. (Or adjustment of break lengths can be done in
advance, if reliable energy loss data will be available.)
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Magnetic needles for alignment
• Only one needle is required for alignment in the X direction
• One more needle has to be added at Y=0 for alignment in the Y
direction
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Hall probe center calibration in X
one needle used
• Accuracy of calibration < 50 µm (limited by encoder resolution)
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Hall probe center calibration in Y
one needle used
• Accuracy of calibration < 5 µm. This procedure will be used to
define distance of needle (By=0) from device magnetic center
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Summary
• A 3 mrad pole gap cant was chosen and tested, using wedged
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spacers between the aluminum base plates and the titanium
core
Magnetic measurements show good agreement with calculated
change of the effective magnetic field versus X (horizontal
motion).
No significant change of the RMS phase error versus X was
measured, hence no degradation of FEL performance is
expected.
Additional flexibility of gap correction is provided by shifting
the spacers in the X direction so that the effective magnetic
field can be set to the required tolerance (DB/B ~ 1.5x10-4). This
can be achieved even at X=0 by moving the spacers.
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Summary, cont’d
• Additional advantage of the canted pole gap design is the easy of
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introducing a tapered field by moving the ends of the device in
opposite directions in X during tuning.
(This is impossible for other fixed-gap devices.)
A disadvantage of this option is the necessity to provide a
separate support for vacuum chamber
Estimate of particle energy loss shows that remotely-controlled
end corrections are not needed for the regular part of the
undulator line. Computer simulations will be required to
investigate the necessity to apply this correction to the final part
of the undulator line.
Magnetic needle tests confirmed that this feature is an effective
alignment tool.
The canted pole gap design looks very promising and appears to
be the best choice to proceed for fine adjustment of the effective
magnetic field
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