Transcript IV_final-2.ppt

LCLS First Undulator
Prototype: Magnetic
Measurements
I. Vasserman
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
• Magnetic measurements: key measurements and issues
• Mechanical stability
• Temperature effects
• End phasing
• Summary
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Critical Tolerances (per undulator section ;
undulator and quadrupole)/Achieved
Trajectory walk-off
x, y
2m
0.5m
|A|/|A0| –1
2%
0.1%
-2n
10 deg
y
50m
Complex Amplitude of Radiation
z
L
A   I1 y ( z ) e
z


0
0
ik / 2 2 [ z  I ix 2 ( z ' ) dz'  I iy 2 ( z ' ) dz' ]
dz
0
Phase slippage over the length of one undulator section
k
 2
2
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L
 L 2

2
L

I

z

dz

I

z

dz



1y

 0 1 x

0
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Magnetic Measurements (key measurements
and issues)
• Coil measurements
•
•
•
•
- Moving coil used as reference (especially for horizontal field)
Field integrals (no multipole components)
Hall probe measurements
- Both vertical and horizontal field
Fixed gap
- Easier to tune field integrals and phase errors (no gap
dependence)
Small gap (~ 6.4 mm)
- Shimming more complicated
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Magnetic Measurements (key measurements
and issues), continued
• High magnetic field stability
- Very precise measurements needed
- Reproducibility of measurements must be << required precision of
the field; ΔBeff/Beff ~ 1.5 x 10-4 (~ 2 Gauss; ~ 1 μm gap change)
- Earth field 0.15 Gauss --> 2.0 μm trajectory offset (requirement)
- Obtaining the exact field is a challenge
- To obtain the real trajectory: environmental field in the tunnel has
to be taken into account
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Hall Probe: Horizontal Field Measurements
• Test of Hall probe horizontal field measurements using Sentron
Hall probe done for Undulator A in 1998 showed good
agreement with moving coil reference measurements. It means
that planar Hall probe effect and cross talk between two
sensors is not so big
• LCLS undulator is longer by 1 m and has stronger vertical field
that exaggerates the errors of measurements. A test was done
to check the horizontal field readings of a Sentron probe in
presence of vertical field
• Results show that for small perturbations and small horizontal
field this probe could be used for tuning but final trajectory
measurements should be compared with reference done by
moving coil
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Horizontal Field Nonlinearity vs. Vertical Field
• Measurements done
•
•
•
•
•
with 2-axis probe in
calibration magnet
The angle  ~1 degree
was introduced to
have X- component of
magnetic field
Bx=By* 
Hall probe sensitivity:
5V/T
Cross talk is evident
Bx=By* 
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Table II. Magnetic Measurement Parameters achieved
• Absolute Hall probe
•
calibration accuracy
0.5 Gauss
Reproducibility:
- Particle beam angle at exit
and entrance
2.5 G-cm/0.001 mrad
- Displacement at exit and
entrance
3400 G-cm/ 0.0004 mm
- Beff RMS error
- Phase error
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0.15 Gauss
0.02 degree
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Shimming
• Novel trajectory shims
• Phase shims
• Mechanical shims
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Shims ….phase (flat) and trajectory (side)
Both types of shims
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Trajectory shims
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Temperature Dependence
• Accurate measurements of temperature dependence of Beff need
•
•
to take into account temperature dependence of Hall probe
Temperature dependence of recent Hall probes typically <10-4 / C
- Two of three Sentron probes (S/N 157 and 367 at APS) have
temperature dependence close to specified
- Result of calibration for third Hall probe (S/N 409 using the APS
calibration magnet) shows large deviation from vendor data
- (ΔBeff/Beff)/ΔT appears to be 3.0 x 10-5 /C if Hall probe temperature
dependence is neglected (one calibration file is used);
- (ΔBeff/Beff)/ΔT = -5.5 x 10-4 /C when Hall temperature dependence
is taken into account
- ΔT ~ ± 0.3 C will result in ΔBeff/Beff requirement of 1.5 x 10-4)
Undulator end-phase corrections will relax the temperature
stability requirement
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Hall Probe Temperature Calibration
Coefficient 1.0021 was
applied to the curve of
26.85 deg to coincide with
23.2 degree
The shape is close and one
coefficient could be used
for each temperature
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Time for undulator to reach thermal
equilibrium
•
Temperature response at downstream (D/S) and upstream
(U/S) end of prototype core and nearby air in the magnetic
measurement laboratory
It takes ~one hour to
stabilize the room
temperature
More than 24 hrs is
needed for titanium core
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Mechanical Stability
•
The prototype was removed from the bench and moved twice
around APS storage ring.
•
Prototype was aligned and measured at the bench before and
after being moved
Before move
(23.5 C)
After move
(23.6 C)
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Beff (Gauss)
rms (Gauss) of
Beff
13724.7
0.1
13724.3
0.22
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Undulator End Phasing
• Full range is ±0.1 mm
• Implemented into the design
• Sensitivity to field deviations from
•
one undulator section to the next
can be made lower by using the
end-gap correction system
Remotely controlled at the submicron level
PZT translator located at the end of the
undulator to adjust the magnetic gap of the
end section. This adjusts the phasing between
undulators
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Undulator End Phasing (cont’d)
•
•
End-phase corrections effect on the FEL performance
- Calculations of complex amplitude of radiation amplitude
- Simulations of beam bunching using code RON
Measured phase versus end-gap change
- Full range for one end ±0.100 mm is 0.16 period = ± 29°
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Undulator End Phasing (cont’d): Complex
Amplitude of Radiation
• Complex amplitude of radiation, |A(L)| defines the intensity of
•
•
•
radiation and is almost 100% compared to ideal case
Ideal case
- Regular part of the device: cosine-type field distribution versus z
- Ends: from measured data
Measured slippage length for 113 periods of phase was 3.668 m at
6.35 mm gap (K value of 3.729)
With two devices in a row, an error of 7x10-4 in ΔBeff/Beff of the
second device could be corrected by applying an end-phase
correction of 24.5° from both ends of the device
- Complemented by detailed RON simulations (R. Dejus) using random
uniform distribution of Keff; an error of up to ~ 10x10-4 could be
compensated by end phase correction
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Undulator End Phasing (cont’d): Complex
Amplitude of Radiation
• Absolute value of complex amplitude of radiation versus z for
two devices
- Second device field changed by 7x10-4
- Phase correction of 24.5° applied
90
9.04963
8
6
4
2
ER1kd
180
0
0
8
1.6102810
Correcting phase of
upstream end of 2nd
undulator is important
for maximizing length
of vector (absolute
value of radiation
amplitude)
270
ER kd
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Undulator End Phasing (cont’d): RON
Simulations
• ΔBeff/Beff variation from undulator to undulator for 33
undulators: with and without end-phase corrections @ 1.2 mmmrad and 1.5 Ǻ
ΔBeff/Beff = {3.5, 7.0, 10.5} x 10-4
With end-phase
corrections applied
Curves from top to bottom: 1. Ideal case; 2-4: with corrections; 5-7: no corrections
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Conclusions
• Measurements and tuning were done
• The prototype met all stringent mechanical and magnetic
•
•
•
•
tolerances after a few design changes and magnetic tuning
Tuning time is about two days after the exact effective field is set
Setting of exact effective field with accuracy better than 1.5x10-4
is a challenge
End-phase corrections of ±29° total range allows compensation
of Beff/ Beff of ~ 8.2x10-4 or ±1.5°C
Lessons learned to simplify production
- The biggest source of errors is variation of pole heights on
assembled device. The tolerances for 1st prototype were ±0.05
mm
- Do not need to measure individual magnet blocks in the halfperiod fixture with Hall probe with such pole height errors
- Mechanical tolerances of ±0.025mm for gap uniformity will
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