bingxin yang large aperture spectrum [email protected] beam-based undulator measurement...
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Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Undulator Effective-K MeasurementsUsing Angle-Integrated Spontaneous Radiation
Bingxin Yang and Roger Dejus
Advanced Photon Source
Argonne National Lab
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Some History of the Conceptual Development1998 - 2002: APS Diagnostics Undulator e-beam energy measurement
– Using angle-integrated undulator radiation measure stored e-beam energy changeJan. 20, 2004: UCLA Commissioning workshop
– Galayda wish list for spontaneous radiation measurementsFeb. 10, 2004: X-ray diagnostics planning meeting (John Arthur)
– Roman: Not possible to measure Keff with required accuracy K/K~1.5×10-4
Sep. 22, 2004: SLAC Commissioning workshop– Bingxin Yang: Keff can be measured with required accuracy
• Large aperture improves accuracy• Electron energy jitter is the main experimental problem• Two undulator differential measurement improves speed and accuracy over single undulator
measurements.
Oct., 2004: LCLS– Jim Welch: Keff can be measured with required accuracy
• Small aperture is better• Spectrometer allows fast data taking
Apr. 18, 2005: Zeuthen FEL Commissioning workshop– Bingxin Yang: Undulator mid-plane can be located within 10 m
• Regular observation can monitor systematic changes in undulators– Jim Welch:
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Hope for this workshopForm a consensus – Spontaneous spectral measurements can be used to measure Keff
with required accuracy (K/K~1.5×10-4)
Aperture size should not be an issue – Operational experience will decide it naturally
Make decisions on the monochromator / spectrometer issues– Monochromator (simple, low cost, robust)– Differential measurements (ultra-high resolution, dependable, other
uses: vertical alignment, monitor field change / damage quickly– Spectrometer (scientific experiments)
• Need to evaluate specs / cost / schedule / R & D / risk factors / operational availability / maintenance effort– Decisions may depend on other functions– My personal bias: machine diagnostics
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Features of the spontaneous spectrum and effect of beam quality: numerical calculations
Average properties: e-beam divergence (x’, y’), x-ray beam divergence (), and energy spread ()Aperture geometry: width and height, center offset, and undulator distancesMagnetic field errors
Effects of e-beam jitter: simulated experimentsBeamline Option 1: crystal monochromator with charge, energy and trajectory angle readoutBeamline Option 2: crystal monochromator with differential undulator setupHigh-resolution experiment: locating magnetic mid-plane of the undulator. Dependence on beam centroid position (x, y)
Summary
Outline
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Spontaneous Radiation Spectrum
+
PHOTON ENERGY (eV)
7800 8000 8200 8400 8600
FL
UX
+ ... ... =
FL
UX
ANGLE-INTEGRATED PHTON FLUX
PHOTON ENERGY (eV)7800 8000 8200 8400 8600
FL
UX
10 rad
20 rad
30 rad
100 rad
0
10 rad
RADIATION SPECTRUM IN CM FRAME
PHOTON ENERGY
FL
UX
0/N
0
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Angle-integrated? How large is the aperture!
Pinhole (sinc) < << Angle-integrated (numeric)BXY: Large enough for the edge feature to be stable
UNDULATOR SPECTRA THRU SQUARE WINDOW
PHOTON ENERGY (eV)
8000 8050 8100 8150 8200 8250 8300 8350 8400
FL
UX
(10
6 PH
OT
ON
S/n
C/0
.01%
BW
)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6APERTURE = 140 rad
K = 3.5000E = 13.64 GeV
30 rad
25 rad
20 rad
15 rad 10 rad
C A B
160 rad
(5mm@167m)
(5mm@35m)
1 N
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Momentum compaction measurementsB.X. Yang, L. Emery, and M. Borland, “High Accuracy Momentum Compaction Measurement for the APS Storage Ring with Undulator Radiation,” BIW’00, Boston, May 2000, AIP Proc. 546, p. 234.
Energy spread measurementsB.X. Yang, and J. Xu, “Measurement of the APS Storage Ring Electron Beam Energy Spread Using Undulator Spectra,” PAC’01, Chicago, June 2001, p. 2338
RF frequency / damping partition fraction manipulations
B. X. Yang, A. H. Lumpkin, ‘Visualizing Electron Beam Dynamics and Instabilities with Synchrotron Radiation at the APS,” PAC’05
K/K simulationsB. X. Yang, “High-resolution undulator measurements Using angle-integrated spontaneous radiation,” PAC’05
1Resolution
N
Related publications 1 1
2 200
F F
N F F
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
How large is the aperture! FEL-relevant
37 mRMS cone-radius 7.4 rad
5 mx
GL
Capture the radiation cone: 2.35 – 5 rms radius 17 – 37 radMeasured radiation spectrum is more important that calculated from field data!
UNDULATOR SPECTRA THRU SQUARE WINDOW
PHOTON ENERGY (eV)8000 8050 8100 8150 8200 8250 8300 8350 8400
FL
UX
(106 P
HO
TO
NS
/nC
/0.01%B
W)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6APERTURE = 140 rad
K = 3.5000E = 13.64 GeV
30 rad
25 rad
20 rad
15 rad 10 rad
C A B
160 rad
(5mm@167m)
(5mm@35m)
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Marking the location of a spectral edge
We will watch how the following property changes:
HALF PEAK PHOTON ENERGY
FEATURES OF LCLS UNDULATOR SPECTRUM (n = 1)
PHOTON ENERGY (eV)
8000 8100 8200 8300 8400 8500
FL
UX
(10
6 P
HO
TO
NS
/nC
/0.0
1%B
W)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Peak Flux
HALF PEAK ENERGY
Peak Energy
(8267.2 eV)
= 3.5000
= 13.64 GeV
1 = 8265.7 eV
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Effects of Aperture Change (Size and Center)Plot the half-peak photon energy vs. aperture sizeEdge position stable for 25 – 140 rad 100 rad best operation pointIndependent of aperture size Independent of aperture center position
X-RAY SPECTRAL FEATURE OBSERVED (OBSERVED THROUGH A SQUARE APERTURE)
APERTURE (rad)
0 50 100 150 200
HA
LF
-PE
AK
EN
ER
GY
(eV
)
8264
8266
8268
8270
8272
K/K = 2.4 x 10-4
K/K = 2.4 x 10-5
UNDULATOR SPECTRA THRU SQUARE WINDOW
PHOTON ENERGY (eV)8000 8050 8100 8150 8200 8250 8300 8350 8400
FL
UX
(106 P
HO
TO
NS
/nC
/0.01%B
W)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6APERTURE = 140 rad
K = 3.5000E = 13.64 GeV
30 rad
25 rad
20 rad
15 rad 10 rad
C A B
160 rad
(5mm@167m)
(5mm@35m)
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Effects of Aperture Change (Source distance)
Calculate flux through an aperture satisfying:
≤ 100 rad ≤ allowed by chamber ID
Plot half-peak photon energyRectangular aperture reduces variation
X-RAY SPECTRAL FEATURE OBSERVED THROUGH A RECTANGULAR APERTURE
UNDULATOR TO APERTURE DISTANCE (M)
40 60 80 100 120 140 160
HA
LF
-PE
AK
EN
ER
GY
(eV
)
8266.6
8266.8
8267.0
8267.2
8267.4
K = 3.5000, E = 13.64 GeV, 1 = 8265.7 eV
Maximum vertical aperture = 4.8 mmMaximum horizontal aperture = 8 mmMaximum angle aperture = 100 rad SQ
K/K ~ 2.4 x 10-5
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Effects of Finite Energy ResolutionFour factors contribute to photon energy resolution
Electron beam energy spread (0.03% RMS X-ray energy width = 11.7 eV FWHM)
Monochromator resolution ( ~ 0.1% or 8 eV)
Photon beam divergence rad
Electron beam divergence y’ rad
222
22 2'
2.352 cot 2.35Total M
B y
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Effect of Finite Energy Resolution
Edge position moves with increasing energy spreadX-RAY SPECTRAL FEATURE OBSERVED (THROUGH 100 rad SQUARE APERTURE)
PHOTON ENERGY BOXCAR WIDTH (eV)0 10 20 30 40 50
HA
LF
-PE
AK
EN
ER
GY
(eV
)
8264
8266
8268
8270
8272
K/K = 2.4 x 10-4
K/K = 2.4 x 10-5
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Effects of Undulator Field Errors
Electron beam parameters
E = 13.640 GeV
x = 37 m
x’ = 1.2 rad
= 0.03%
Detector
Aperture
80 rad (H)
48 rad (V)
Monte Carlo integration for 10 K particle histories.
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Comparison of Perfect and Real Undulator SpectraFilename: LCL02272.ver; scaled by 0.968441 to make Keff = 3.4996
First harmonic spectrum changes little at the edge.
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Comparison of Perfect and Real Undulator Spectra
Changes in the third harmonic spectrum is more pronounced. But the edge region appears to be usable.Changes in the fifth harmonic spectrum is significant. Not sure whether we can use even the edge region.
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
The following beam qualities are not problems for measuring spectrum edge:
e-beam divergence (x’, y’),
x-ray beam divergence (), energy spread () and monochromator resolution,aperture width and height, center offset, and undulator distances
Magnetic field errorsPreliminary results show that the first harmonic edge is usable. Third harmonic edge may also be usable.How to define effective K in the presence of error is not a trivial issue. I need to learn more to understand it (BXY).
Next we move on jitter simulations.
Summary of calculations so far
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Bunch charge jitterX-ray intensity is proportional to electron bunch charge (0.05% fluctuation).
Electron energy jitterLocation of the spectrum edge is very sensitive to e-beam energy change (10-5 noise): = 2·
Electron trajectory angle jitterTrajectory angle (0.24 rad jitter) directly changes grazing incidence angle of the crystal monochromator
Jitters and Fluctuations
Damaging effect! Use simulation to assess impact.
2
1 u22 2
2( , ) ,
12
u
u
hc
K
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Beamline Option 1: Poor man’s solution
One reference undulator One flat crystal monochromator (asymmetrically cut preferred)One flux intensity detectorOne hard x-ray imaging detectorBeamline slits (get close to 100 rad)
Operation procedure for setting Keff
Pick one reference undulator (U33) and measure a full spectrum by scanning the crystal angle (angle aperture ~ 100 rad)Position the crystal angle at the mid-edge and record n-shot (n = 10 –100) data of the x-ray flux intensity (FREF) with electron energy, trajectory angle, and charge Roll out reference undulator and roll in other undulator one at a time.
Set slits to 100 rad or best available
Adjust x-position until the n-shot x-ray flux intensity data matches FREF. Use the measured electron bunch data in real-time to correct for jitters
UNDULATOR SPECTRA THRU SQUARE WINDOW
PHOTON ENERGY (eV)8000 8050 8100 8150 8200 8250 8300 8350 8400
FL
UX
(106
PH
OT
ON
S/n
C/0.01%
BW
)0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6APERTURE = 140 rad
K = 3.5000E = 13.64 GeV
30 rad
25 rad
20 rad
15 rad 10 rad
C A B
160 rad
(5mm@167m)
(5mm@35m)
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Measure fluctuating variables
Charge monitor: bunch charge
OTR screen / BPM at dispersive point: energy centroid
Hard x-ray imaging detector: electron trajectory angle (new proposal)
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
One Segment Simulation: ApproachELECTRON BUNCH CHARGE BY SHOT
BUNCH NUMBER
100 200 300 400 500
BU
NC
H C
HA
RG
E (
nC
)
0.0
0.5
1.0
1.5
2.0ELECTRON BUNCH CHARGE HISTOGRAM
BUNCH CHARGE (nC)0.0 0.5 1.0 1.5 2.0
FR
EQ
UE
NC
Y
0
100
200
300
400
500MEAN = 1.001 nCSTDEV = 0.201 nC
ELECTRON BUNCH ENERGY CENTROID
BUNCH NUMBER100 200 300 400 500
BU
NC
H E
NE
RG
Y (
nC
)
13.58
13.60
13.62
13.64
13.66
13.68
13.70
ELECTRON BUNCH ENERGY HISTOGRAM
BUNCH ENERGY (GeV)13.58 13.60 13.62 13.64 13.66 13.68 13.70
FR
EQ
UE
NC
Y
0
100
200
300
400
500 MEAN = 13.640 GeVSTDEV = 0.0137 GeV
NOMINAL PHOTON ENERGY HISTOGRAM
NOMINAL PHOTON ENERGY (eV)8200 8220 8240 8260 8280 8300 8320
FR
EQ
UE
NC
Y
0
100
200
300
400
500MEAN = 8265.3 eVSTDEV = 16.6 eV
MODEL UNDULATOR SPECTRA
PHOTON ENERGY (eV)8000 8100 8200 8300 8400
FL
UX
(10
6 PH
OT
ON
S/n
C/0
.01%
BW
)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
K = 3.5000E = 13.64 GeVWINDOW > 50 rad
A BC
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Effect of electron energy “correlation”
Define “Correlated Electron-Photon Energy”
NORMALIZE & CORRECTED COUNTS (K = 3.5000)
CORRECTED PHOTON ENERGY (eV)8100 8200 8300 8400
PH
OT
ON
CO
UN
TS
PE
R S
HO
T
0
2e+5
4e+5
6e+5
8e+5
1e+6
CHARGE NORMALIZE COUNTS (K = 3.5000)
PHOTON ENERGY (eV)8100 8200 8300 8400
PH
OT
ON
CO
UN
TS
PE
R S
HO
T
0
2e+5
4e+5
6e+5
8e+5
1e+6
01
2cotCORR
y y
D
RMS error from simulation
RAW COUNTS (K = 3.5000)
PHOTON ENERGY (eV)
8100 8200 8300 8400
PH
OT
ON
CO
UN
TS
PE
R S
HO
T
0.0
2.0e+5
4.0e+5
6.0e+5
8.0e+5
1.0e+6
1.2e+6
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Summary of 1-undulator simulations(charge normalized and energy-corrected)
Applying correction with electron charge, energy and trajectory angle data shot-by-shot greatly improves the quality of data analysis at the spectral edge.
Full spectrum measurement for one undulator segment (reference)
The minimum integration time to resolve effective-K changes is 10 – 100 shots with other undulator segment (data processing required)
As a bonus, the dispersion at the flag / BPM can be measured fairly accurately.
Not fully satisfied:Rely heavily on correction calibration of the instrumentNo buffer for “unknown-unknowns”Non-Gaussian beam energy distribution ???
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Beamline Option 2: Ultra-high ResolutionReference Undulator (U33)
Period length and B-field same as other segments
Zero cant angle
Field characterized with high accuracy
Upstream corrector capable of 200 rad steering (may be reduced if needed).
Broadband monochromator (E/E ~ 0.03%)Improves photon statistics
Suppress coherent intensity fluctuations
Big area, large dynamic range, uniform, linear detector
Hard x-ray imaging detector (trajectory angle)
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Operation Procedures for setting Keff (BL2)Steer the beam to be away from the axis in the reference undulator (U33) and measure a full spectrum by scanning the crystal angle (angle aperture ~ 100 rad)Position the crystal angle at the mid-edge Roll in other undulator one at a time (test undulator).
Adjust the x-position of the test undulator until the x-ray intensities of the two undulator matches (difference < threshold).Use the measured electron beam angle data in real-time to correct for angle jitters if necessary
UNDULATOR SPECTRA THRU SQUARE WINDOW
PHOTON ENERGY (eV)8000 8050 8100 8150 8200 8250 8300 8350 8400
FL
UX
(106
PH
OT
ON
S/n
C/0.01%
BW
)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6APERTURE = 140 rad
K = 3.5000E = 13.64 GeV
30 rad
25 rad
20 rad
15 rad 10 rad
C A B
160 rad
(5mm@167m)
(5mm@35m)
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Differential Measurements of Two Undulators
Insert only two segments in for the entire undulator.
Steer the e-beam to separate the x-raysUse one mono to pick the same x-ray energy
Use two detectors to detect the x-ray flux separatelyUse differential electronics to get the difference in flux
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Signal of Differential Measurements
Select x-ray energy at the edge (Point A).Record difference in flux from two undulators.Make histogram to analyze signal qualitySignals are statistically significant when peaks are distinctly resolved
MODEL UNDULATOR SPECTRA
PHOTON ENERGY (eV)8000 8100 8200 8300 8400
FL
UX
(10
6 PH
OT
ON
S/n
C/0
.01%
BW
)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
K = 3.5000E = 13.64 GeVWINDOW > 50 rad
A BC
DIFFERENCE COUNTS (K = 3.5005)
BUNCH NUMBER
100 200 300 400 500
CO
UN
TS
(10
3 P
ER
BU
NC
H)
-80
-60
-40
-20
0
K = 3.5005E = 13.64 GeVQ = 1.0 nC
HISTOGRAM OF DIFFERENCE COUNTS
DIFFERENCE COUNTS (103 PER BUNCH)
-100 -50 0 50 100
FR
EQ
UE
NC
Y
0
500
1000
1500
PHOTON ENERGY = 8265.7 eVTOAL COUNTS = 0.644 106
N_avg = 1 (bunch)
K = 3.5005
HISTOGRAM OF DIFFERENCE COUNTS
DIFFERENCE COUNTS (103 PER BUNCH)
-100 -50 0 50 100
FR
EQ
UE
NC
Y
0
500
1000
1500
K = 3.4995
PHOTON ENERGY = 8265.7 eVTOAL COUNTS = 0.644 106
N_avg = 1 (bunch)
K = 3.5005
K/K = 1.5 10-
4
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Summing multi-shots improves resolution
Summing difference signals over 64 bunches
Distinct peaks make it possible to calculate the difference K at the level of 10-5.
HISTOGRAM OF DIFFERENCE COUNTS
DIFFERENCE COUNTS (103 PER BUNCH)
-8 -6 -4 -2 0 2 4 6 8
FR
EQ
UE
NC
Y
0
500
1000
1500
K = 3.499965
PHOTON ENERGY = 8265.7 eVTOAL COUNTS = 0.644 106
N_avg = 1 (bunch)
K = 3.500035
HISTOGRAM OF DIFFERENCE COUNTS
DIFFERENCE COUNTS (103 PER BUNCH)
-8 -6 -4 -2 0 2 4 6 8
FR
EQ
UE
NC
Y
0
500
1000
1500
K = 3.499965
PHOTON ENERGY = 8265.7 eVTOAL COUNTS = 0.644 106
N_avg = 64 (bunches)
K = 3.500035
Example: Average improves resolution for K/K = 10-5
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Differential Measurement Recap Use one reference undulator to test another undulator simulataneouslySet monochromator energy at the spectral edgeMeasure the difference of the two undulator intensity
HISTOGRAM OF DIFFERENCE COUNTS
DIFFERENCE COUNTS (103 PER BUNCH)
-4 -2 0 2 4
FR
EQ
UE
NC
Y
0
500
1000
1500K = 3.49999
PHOTON ENERGY = 8265.7 eVTOAL COUNTS = 0.644 106
N_avg = 64 (bunches)
K = 3.50001
Simulation gives approximately:
• To get RMS error K/K < 0.710-4, we need only a single shot (0.2 nC)!
• We can use it to periodically to log minor magnetic field changes, for radiation damage.
• Any other uses?
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Other application of the techniques:
Search for the neutral magnetic plane
Set the monochromator at mid-edge (Point A).Insert only one test segment in.Move the undulator segment up and down, or move electron beam up and down with a local bump.When going through the plane of minimum field (neutral plane), the spectrum edge is highest in energy. Hence the photon flux peaks.After the undulator is roughly positioned, taking turns to scan one end at a time, up and down, to level it.
MODEL UNDULATOR SPECTRA
PHOTON ENERGY (eV)8000 8100 8200 8300 8400
FL
UX
(10
6 PH
OT
ON
S/n
C/0
.01%
BW
)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
K = 3.5000E = 16.34 GeVWINDOW > 50 rad
A BC
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Simulation of undulator vertical scan
Charge normalization only: ~ 20K shots / pointCharge-normalized and electron-energy corrected: ~ 512 shots / pointDifferential measurements (two undulators): ~ 16 shots /point gives us RMS error ~ 1.0 m ?!
UNDULATOR VERTICAL SCAN (20K x 0.2 nC)/PT
VETICAL POSITION
-50 -40 -30 -20 -10 0 10 20 30 40 50
FL
UX
(1
06 P
HO
TO
NS
/nC
)
0.395
0.400
0.405
0.410
0.415
UNDULATOR VERTICAL SCAN (16 x 0.2 nC)/PT
VETICAL POSITION
-50 -40 -30 -20 -10 0 10 20 30 40 50
DIF
FE
RE
NC
E S
IGN
AL
-0.03
-0.02
-0.01
0.00
0.01
2
5 60 110 10
0 2 u
K y K y
K
UNDULATOR VERTICAL SCAN (512 x 0.2 nC)/PT
VETICAL POSITION-50 -40 -30 -20 -10 0 10 20 30 40 50
EL
EC
TR
ON
EN
ER
GY
CO
RR
EC
TE
D
416000
418000
420000
422000
424000
426000
428000
430000
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Conclusion for Locating Magnetic Neutral Plane
Both techniques can be used to search the magnetic neutral plane, each has its own advantages and disadvantages:
Single undulator measurement (with charge-normalization and e-beam energy correction) can get required S/N ratio after averaging. Differential measurement has best sensitivity, need shortest time (keep up with mechanical scan), but required more hardware.
Finite beam sizes and centroid offset (in undulator) shift spontaneous spectrum: the apparent K is given by
22 20
02
( , ) (0,0)
(0,0)apparent eff y eff eff
n n xeff u
K x y K y K Ka x b
K x x
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Summary (The Main Idea)We propose to use angle-integrated spectra (through a large aperture, but radius < 1/) for high-resolution measurements of undulator field.
Expected to be robust against undulator field errors and electron beam jitters.
Simulation shows that we have sufficient resolution to obtain K/K < 10-4 using charge normalization. Correlation of undulator spectra and electron beam energy data further improves measurement quality. A Differential technique with very high resolution was proposed: It is based on comparison of flux intensities from a test undulator with that from a reference undulator.
Within a perfect undulator approximation, the resolution is extremely high, K/K = 3 10-6 or better. It is sufficient for XFEL applications.It can also be used for routinely logging magnet degradation.
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Summary (Continued)Either beamline option can be used for searching for the effective neutral magnetic plane and for positioning undulator vertically. The simulation results are encouraging (resolution ~1 m in theory for now, hope to get ~ 10 m in reality).
What’s next
Sources of error need to be further studied. Experimental tests need to be done.
More calculation and understanding of realistic fieldLongitudinal wake field effect,Experimental test in the APS 35IDMore?
Bingxin Yang
Large aperture spectrum measurements [email protected]
Beam-based undulator measurement workshop, Nov. 14, 2005
Monochromator RecommendationA dedicated monochromator for undulator measurement (low cost and robust, permanently installed).
Use it for K/K measurementsUse it for regular vertical alignment checkUse it for routine magnetic field measurements at regular intervals (after routine BBA operation).
Logging magnetic field changes to see trend of damage, identify sources / mechanism for damageLook for most damaged undulator segments for service for next shutdown
Location of the monochromatorFront end easy to service. Too crowded?In tunnel OK.
Differential measurement strongly recommendedBut steering magnet can be added later as an upgrade. Differential measurement saves time, improves accuracy.
Spectrometer will be easily justified by the science it supports.