Electron beam diagnostic methods
description
Transcript of Electron beam diagnostic methods
August, 2003 W.S. Graves 1
Electron beam diagnostic methodsElectron beam diagnostic methods
William S. Graves
MIT-Bates Laboratory
Presented at 2003 ICFA S2E Workshop
DESY-Zeuthen
August, 2003
August, 2003 W.S. Graves 2
Experimental MethodsExperimental Methods
•Hardware/software controls.
•Thermal emittance measurement using solenoid scan.
•Cross-correlation of UV photoinjector drive laser with 100 fs IR oscillator.
•Streak camera time resolution
•Electron beam longitudinal distribution measured using RF zero phasing
method.
•Slice transverse parameters are measured by combining RF zero phase with
quadrupole scan.
•Will not address undulator diagnostics. See Shaftan, Loos, Doyuran.
Enables injector optimization and code benchmarking.
August, 2003 W.S. Graves 3
DUVFEL Facility at BNLDUVFEL Facility at BNL
1.6 cell gun with copper cathode
70 MeVBend
50 m
5 MeV
Bend
DumpDump
Coherent IRdiagnostics
RF zero phase screen
Undulators Linac tanks
Bunch compressorwith post accel.
30 mJ, 100 fsTi:Sapphire laser
NISUS 10mundulator
Photoinjector: 1.6 cell BNL/SLAC/UCLA with copper cathode
4 SLAC s-band 3 m linac sections
Bunch compressor between L2 and L3
Approximately 60 CCD cameras on YAG screens.
Triplet Triplet70-200 MeV
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Control systemControl system
Automated measurements very important for gathering large amounts of data and repeating studies.
All sophisticated control is done in the MATLAB environment on a PC. Physicists quickly integrate hardware control with data analysis.
Low level control is EPICS on a SUN and VME crates.
August, 2003 W.S. Graves 5
Thermal EmittanceThermal Emittance
xxxN xx 22
Use Lawson’s expression for the RM S width of them om entum distribution of a therm alized beam
factor.t enhancemen field theis and phase, andamplitude RF theare sin and 95MV/m
eV, 59.4eV, 67.4 ,4
,sin
0
rf
rfrf
Cu
rfrfrfCuk
E
he
EhE
2N
2
mc
Emc
E
kx
kx
The norm alized em ittance is
(1)
(2)
(3)
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Projected emittance vs charge and FWHMProjected emittance vs charge and FWHM
0 2 4 6 8 100.58
0.6
0.62
0.64
0.66
0.68
0.7
0.72
0.74
FWHM (ps)
Em
ittan
ce (
mm
-mra
d)
0 5 10 15 200.61
0.62
0.63
0.64
0.65
0.66
0.67
0.68
Charge (pC)
Em
ittan
ce (
mm
-mra
d)Charge 2 pC
Energy 3.7 MeV
Laser spot 0.5 mm RMS
FWHM 3 ps
Energy 3.7 MeV
Laser spot 0.5 mm RMS
HOMDYN simulations estimate limits on maximum bunch length and charge. Choose working parameters of 2 pC, 2 ps FWHM.
Simulation Simulation
YAG:Ce screen very useful for low charge, high resolution profiles. Screen thickness, surface quality, multiple reflections, and camera lens depth-of-focus and resolution are all important issues.
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Solenoid Scan LayoutSolenoid Scan Layout
CCD Camera
YAG screen
Mirror
Solenoid
1.6 cell photoinjectorDipole trim
1cm
65 cm
12 cm33 cm
Can measure•charge•energy•x and y centroid•x and y beamsize•px and py
Telecentric lens magnif. = 1
•YAG:Ce screen very useful for low charge, high resolution profiles.
• Screen thickness, surface quality, multiple reflections, and camera lens depth-of-focus and resolution are all important issues. See SLAC GTF data.
August, 2003 W.S. Graves 8
Low Energy Beam MeasurementsLow Energy Beam Measurements
102 103 104 105 106 107 1080
1
2
3
4
5
6
7
8x 10-9
Solenoid Current (Amp)
11
File: a10pc.txt = -30.8 1.7 = 8.11 0.44 mN
= 0.605 0.025 mm-mrad
= 0.773 0.0177 mm' = 2.94 0.0658 mradE = 3.69 MeV
pop02a.bmp
5 10 15 20 25 30
5
10
15
20
25
30
0 50 100 150 200 250 3000
500
1000
1500
2000Hor. RMS width = 39.5 um
Beam size (um)
Inte
nsity
(A
.U.)
pixels
Video processing
•3x3 median filter applied.
•Dark current image subtracted.
•Pixels < few % of peak are zeroed.
Error estimates
Monte Carlo method using measured beam size jitter.
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Emittance vs laser sizeEmittance vs laser size
0 0.2 0.4 0.6 0.8 1 1.20
0.2
0.4
0.6
0.8
1
1.2
Horizontal RMS laser size (mm)
N (m
m-m
rad)
Horizontal
0 0.2 0.4 0.6 0.8 1 1.20
0.2
0.4
0.6
0.8
1
1.2
N (m
m-m
rad)
Vertical
Horizontal RMS laser size (mm)
FWHM 2.6 ps
Charge 2.0 pC
Gradient 85 MV/m
RF phase 30 degrees
Emittance shows expected linear dependence on spot size.
Small asymmetry is always present.
][93.16.0][ mmmradmm xx ][92.11.0][ mmmradmm yy
eV 0.43
2
N2energy kinetic Averagex
k d
dmcE
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Beam size and divergence vs laser spot sizeBeam size and divergence vs laser spot size
Error bars are measured data.
Blue lines are from HOMDYN simulation using RF fields from SUPERFISH model and measured solenoid B-field.
Upper blue line has 1/2 cell field 10% higher than full cell.
Lower blue line has 1/2 cell field 10% lower than full cell.
0 0.2 0.4 0.6 0.8 1 1.20
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
RMS laser hor. size (mm)
RM
S e
-bea
m h
or. s
ize
(mm
)
0 0.2 0.4 0.6 0.8 1 1.20
1
2
3
4
5
6
7
8
9
RMS laser hor. size (mm)
RM
S e
beam
hor
. div
erge
nce
(mra
d)
High
High
Low
LowSize Divergence
][1.112.0][ 01 mmmm xx ][5.39.0][' 0 mmmrad xx
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Error bars are measured data points.
Curve is nonlinear least squares fit with βrf and Φcu as parameters: βrf = 3.10 +/- 0.49 and Φcu = 4.73 +/- 0.04 eV.
The fit provides a second estimate of the electron kinetic energy Ek = 0.40 eV, in close agreement with the estimate from the radial dependence of emittance.
0 10 20 30 40 50 60 70 800.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
RF phase (degrees)
N (
mm
-mra
d)
Emittance vs RF phaseEmittance vs RF phase
2N mc
Ekx
04
,sin
e
EhE rfrfrfCuk
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-10-8
-6-4
-20 2
-40-30
-20-10
010
2030
400
0.5
1
1.5
2
Time (ps)
Phase matching
angle (mrad)
Sig
nal (V
)
Time profile of UV laser pulseTime profile of UV laser pulse
Phase matching angle of harmonic generation crystals used to produce UV affects time and spatial modulations.
Cross-correlaton difference frequency generation – experiment by B. Sheehy and H. Loos
100 fs IR
5 ps UV
BBO crystal
200 fs blue
Power meter
Note: “Sub-ps” streak camera is
inadequate for this measurement
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Above: Laser cathode image of air force mask in laser room.
Below: Resulting electron beam at pop 2.
Above: Laser cathode image with mask removed showing smooth profile.
Below: Resulting electron beam showing hot spot of emission.
Laser masking of cathode image
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Streak CameraStreak Camera
100 200 300 400 500
50
100
150
200
250
300
350
400
450
500
Streak image
Time
•Hammamatsu FESCA 500
•765 fs FWHM measured resolution
•Reflective input optics (200-1600 nm)
•Wide response cathode (200-900 nm)
•Optical trigger (<500 fs jitter)
•Designed for synchroscan use. Also good single-shot resolution.
•6 time ranges: 50 ps - 6 ns
50 ps window
Very helpful for commissioning activies and for timing several optical signals.
Limited time resolution below 1 ps.
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Streak camera time profiles of laser pulsesStreak camera time profiles of laser pulses
14 16 18 20 22 24 26160
180
200
220
240
260
280
300
320
340
360
sign
al (
arb
units
)
streak delay (picoseconds)32 34 36 38
180
185
190
195
200
205
210
215
220
sign
al (
arb
units
)
streak delay (picoseconds)
6 8 10196198200202204206208210212214216218220222224226228230232234
sign
al (
arb
units
)
streak delay (picoseconds)
Oscillator 796 nm
FWHM 765 fs
Amplified IR 796 nm
FWHM 1.01 ps single shot
UV 266 nm
FWHM 2.40 ps singleshot
Time resolution depends on photon energy: energetic UV photons create photoelectrons with energy spread that degrades time resolution
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L1
L2
L3
L4
RF zero-phase time profile
L3 phase = +90, amp. set to
remove chirp
L4 phase = +/-90, amp. set to
add known chirp
L2 phase varies, amp.
fixed
L1 phase = 0, amp. fixed
Chicane varies from 0 cm < R56 <
10.5 cm
100 200 300 400 500 600
50
100
150
200
250
300
350
400
450
100 200 300 400 500 600
50
100
150
200
250
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400
450
100 200 300 400 500 600
50
100
150
200
250
300
350
400
450
L4 phase = -90 degrees L4 phase = +90 degreesL3 corrects residual chirp,
L4 is off
Pop 14 YAG screen
YAG images at pop 14
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Quad scan during RF zero phaseQuad scan during RF zero phase
Movie of slice emittance measurement.
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Right side
Circle is matched, normalized phase space area at upstream location.
Ellipse is phase space area of slice at same location.
Straight lines are error bars of data points projected to same location.
Left side
Beam size squared vs quadrupole strength.
Each plot is a different time slice of beam.
Software is used to time-slice beam.
Collaboration with Dowell, Emma, Limborg, Piot
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Slice emittance and Twiss parametersSlice emittance and Twiss parameters
Note strongly divergent beam due to solenoid overfocusing at tail, where current is low.
Space charge forces near cathode caused very different betatron phase advances for different parts of beam.
is parameter that characterizes mismatch between target and each slice.
= ½ (0 – 2 0 + 0)
0, 0, 0 are target Twiss param.
, , are slice Twiss param.
Beam Parameters: 200 pC, 75 MeV, 400 fs slice width
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Different slices require different solenoid strengthDifferent slices require different solenoid strength
HeadTail HeadTail HeadTail
HeadTail HeadTail HeadTail
Increasing solenoid current
Vertical dynamicsLattice is set to image end of Tank 2 to RF-zero phasing YAG.
Particles in tail of beam are diverging, and in head converging.
Head has higher current and so reaches waist at higher solenoid setting.
Current projection
Time
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Solenoid = 98 A
Solenoid = 108 A
Solenoid = 104 A
Slice emittance vs solenoid strength. Charge = 200 pC.
Solenoid
Eyn
Alpha
Beta
98 A
3.7 um (3.2)
0.4 (1.0)
1.3 m (1.3)
104 A
2.1 um (2.8)
-6.9 (-3.6)
9.8 m (6.8)
108 A
2.7 um (2.7)
-9.0 (-9.6)
45 m (36)
Projected Values
(parmela in parentheses)
Data
Parmela
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Slice parameters vs chargeSlice parameters vs charge10 pC
100 pC 200 pC
50 pC
Low charge cases show low slice emittance and little phase space twist.
High charge cases demonstrate both slice emittance growth and phase space distortion.
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Longitudinal structureLongitudinal structure
100 200 300 400 500 600
100
200
300
400
50 100 150 200 250 300
50
100
150
0 2 4 60
50
100
150
200
Time (ps)
Cur
rent
(A
)
File: csr01, FWHM = 2.2 ps
Analysis of RF zero phasing data can be complicated by modulations in energy plane.
See contribution from T. Shaftan for detailed description.
August, 2003 W.S. Graves 24
RF zero phasing
•Method uses accelerating mode to “streak” the beam by increasing the energy spread (chirping).
•Uncorrelated energy spread is ~5 keV and coherent modulations can be ~20 keV. Streak chirp must be much larger than this.
•Time and energy axes are difficult to disentangle.
RF deflector cavity
•Transverse momentum is ~ 1 eV/c. Relatively small deflecting field gives excellent time resolution.
•Less sensitive to coherent energy modulations.
•Can obtain simultaneous time/energy/transverse beam properties when combined with dipole in other plane.
RF zero phasing vs RF deflectorsRF zero phasing vs RF deflectors
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Concluding RemarksConcluding Remarks
•Experience seems to indicate that most differences between experiment and simulation are due to experimental inaccuracies.
•Beam can be used to diagnose many hardware/applied field difficulties.
•“Easy to use” control system integrating realtime analysis and hardware/beam control very important.
•With adequate diagnostics, meeting beam quality and FEL performance goals is straightforward.
•See work by Shaftan, Loos, Doyuran of BNL on undulator diagnostics and trajectory analysis.