patrick krejcik [email protected] may 3-6, 2004 patrick krejcik r. akre, p. emma, m. hogan,...
TRANSCRIPT
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Patrick KrejcikR. Akre, P. Emma, M. Hogan, (SLAC),
H. Schlarb, R. Ischebeck (DESY), P. Muggli (USC Los Angeles),
A. Cavalieri (University of Michigan)
Sub-Picosecond Electron Bunch Length Measurements at SLAC
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Motivation behind ultra-short electron bunches
Compressing electron bunches from a linac to reach very high peak currents (kAmps)
Enables them to lase in a long undulator
4th generation light sources: LCLS, TESLA …
Ultra-short pulse for probing experiments down to femtosecond level
Short pulse x-rays SubPicosecond Pulsed Souce, SPPS
Advanced accelerator studiesPlasma wakefield acceleration
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Diagnostics Challenge for Measuring Sub-Picosecond Bunch Length
The SPPS e- bunch is 80 fwhm (12 m rms)Conventional streak camera technology ~1/2 ps
Ideally look for resolution <100 fs
Single pulse measurement important in linacs
Reconstruction of bunch length charge profile
Fast, relative measurements for feedback control
timing measurement relative to fs laser
Diagnose new instabilities – microbunching instability
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Bunch length diagnostic comparisonDevice Type Invasive
measurementSingle shot measurement
Abs. or rel. measurement
Timing measurement
Detect bunchin
g
RF Transverse Deflecting Cavity
Yes: Steal 3 pulses
No: 3 pulses Absolute No No
Coherent Radiation Spectral power
No for CSR Yes for CTR
Yes Relative No Yes
Coherent RadiationAutocorrelation
No for CSR Yes for CTR
No Absolute(2nd moment
only)
No No
Electro Optic Sampling
No Yes Absolute Yes No
Energy Wake-loss
Yes No Relative No No
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Relative bunch length measurement at SPPSbased on wakefield energy loss scan
Relative bunch length measurement at SPPSbased on wakefield energy loss scan
Energy change measured at the end of the linac
as a function of the linac phase (chirp) upstream of the compressor chicane
Shortest bunch has greatest energy loss
Predicted wakeloss___(P. Emma)
For bunch length z __
Predicted shape due to wakeloss plus RF curvature
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Bunch Length Measurements with the RF Transverse Deflecting CavityBunch Length Measurements with the RF Transverse Deflecting Cavity
yy
Asymmetric parabola indicates incoming tilt to beam
Y = A * (X - B)**2 + CA = 1.6696E-02 STD DEV = 1.3536E-03B = 28.23 STD DEV = 3.084C = 1328. STD DEV = 8.235RMS FIT ERROR = 23.63
-80 -40 0 40 80
SBST LI29 1 PDES (S-29-1)
1.7
1.6
1.5
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1.3
X103
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**
*
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********
-80 -40 0 40 80
SBST LI29 1 PDES (S-29-1)
1.7
1.6
1.5
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1.3
X103
E
-80 -40 0 40 80
SBST LI29 1 PDES (S-29-1)
1.7
1.6
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X103
-80 -40 0 40 80
SBST LI29 1 PDES (S-29-1)
1.7
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X103
-80 -40 0 40 80
SBST LI29 1 PDES (S-29-1)
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X103
-80 -40 0 40 80
SBST LI29 1 PDES (S-29-1)
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X103
E
MANUAL STEPPING. STEPS = 30
1-APR-03 20:21:16
Cavity on
Cavity off
Cavity on- 180°
Bunch length reconstructionMeasure streak at 3 different phases
z = 90 m
(Str
eak
size
)2
0 180
2.4 m 30 MW
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Calibration scan for RF transverse deflecting cavity
Beam centroid[pixels]
Cavity phase [deg. S-Band]
• Bunch lenght calibrated in units of the wavelength of the S-band RF
Further requirements for LCLS:
•High resolution OTR screen•Wide angle, linear view optics
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
OTR Profile Monitor in combination withRF Transverse Deflecting Cavity
Simulated digitized video image
Injector DL1 beam line is shown
Best resolution for slice energy spread measurement would be in adjacent spectrometer beam line.
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
4 THz
LCLS BC2 Bunch length monitor spectrum
BC2 bunch length feedback requires THz CSR detector
Demonstrated with CTR at SPPS
Bunch profile
200 fs
Bunch spectrum
>>z
THz spectromete
r
THz power detector
B4 Bend
Bunch Compressor Chicane
CSR
Vacuum port with reflecting foil
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Far-Infrared Detection of Wakefields from Ultra-Short Bunches
Wakefield diffraction radiation wavelength comparable
to bunch length
Pyroelectric detector
foilLINAC
FFTB
Comparison of bunch length minimized according to
wakefield loss and THz power
GADC
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100
200
300
400
500
600P
yrom
eter
sig
nal [
arb.
uni
ts]
linac phase offset from crest [deg. S-Band]
FFTB Pyrometer Signal
-26 -24 -22 -20 -18 -16 -14 -12200
250
300
350
400
450
500
ener
gy lo
ss [
MeV
]
Linac Wake Loss
Linac phase
Wake energy loss
THz power
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
400 GHz1.2 mm
BC1 Bunch Length Monitor
CSR Power spectral density signal for bunch length feedback
CSR Power spectral density signal for bunch length feedback
Spectral lines accompanying micro-bunching instability
– Z. Huang.
Spectral lines accompanying micro-bunching instability
– Z. Huang.
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Dither feedback control of bunch length minimization - L. Hendrickson
Dither time steps of 10 seconds
Bunch length monitor response Feedback correction
signal
Linac phase
“ping”
optimum
Jitter in bunch length signal over 10 seconds ~10% rms
Jitter in bunch length signal over 10 seconds ~10% rms
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Transition radiation is coherent at wavelengths longer than the bunch length,>(2)1/2 z
P. Muggli, M. Hogan
Limited by long wavelength cutoff
Interferometer for autocorrelation of CTR
12 m rms
e-
VariablePositionMirror
Interferometer Pyro Detector
12.5 µm MylarBeam Splitters
RT≈0.17
12.5 µm MylarVacuum Window
(3/4” dia)
Ref. Pyro Detector
Alignment Laser
1 µm Titanium Foil
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
0
10
20
30
40
0
0.2
0.4
0.6
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1
1.2
-300 -200 -100 0 100 200 300
CTRautocSigmaz12.7_3
Au
toco
rre
latio
n w
/o F
ilte
r (a
.u.)
Au
toco
rrela
tion
with
Filte
r (a.u
.)
Delay (µm)
z=14 µm
z=9 µm10-17
10-15
10-13
10-11
10-9
10-7
10-5
0.001
0.1
10
10-4
10-3
10-2
10-1
100
10 100 1000CTRFSpecSigmaz20Mylar12.5_3
Po
we
r S
pe
ctru
m (
a.u
.) F
ilter A
mp
litue
(a.u
.) Wavelength (µm)
Mylarresonances
Gaussian, z=20 µm, d=12.7 µm, n=3 Mylar window+splitter
Effect of Mylar Window and Beam Splitter
• Smaller measured width:
Autocorrelation < bunch !
• Modulation/dips in the interferogram
Simple model:
• Fabry-Perot resonance: =2d/m, m=1,2,…
• Signal attenuated by Mylar: (RT)2 per sheet
• Fabry-Perot resonance: =2d/m, m=1,2,…
• Signal attenuated by Mylar: (RT)2 per sheet
P. Muggli, M. Hogan
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Electro Optic Bunch Length Measurement
Probe laser
Defining aperture
Beam axis
M1 M2EO xtal
Geometry chosen to measure direct
electric field from bunch, not wakefieldModelled by H. Schlarb
electrons
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Resolution limit in temporal-to-spectral translation
0res CT T TBW limited pulse Short chirp
Long chirp
Temporal profile
Spectral profiles
However, recent work shows this limit can be overcome with noncollinear cross correlation of the light before and after the EO crystal
S.P. Jamison, Optics Letters, 28, 1710, 2003
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Er
ErP
Elevation view End view
Plan view
electrons
EO Xtal
Temporal to spatial geometry under test at SPPS
Er
Principal oftemporal-spatial
correlation
Line image camera
polarizer
analyzer
xtal
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Jitter determination from Electro Optic sampling
Er
Principal oftemporal-spatial correlation
Line image camera
polarizer
analyzer
EO xtal
seconds, 300 pulses: z = 530 fs ± 56 fs rms t = 300 fs rmsseconds, 300 pulses: z = 530 fs ± 56 fs rms t = 300 fs rms
single pulse
A. Cavalieri
centroidwidth
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
EO resolution limit due to wakefields – H. Schlarb
r
e-
•Apparent change in z when measured at
increasing radii relative to the aperture from the edge of the laser mirror
• negligible perturbation if EO crystal is closer to beam than mirror edge.
•Apparent change in z when measured at
increasing radii relative to the aperture from the edge of the laser mirror
• negligible perturbation if EO crystal is closer to beam than mirror edge.
Patrick Krejcik
BIW04 [email protected]
May 3-6, 2004
Timing system requirements
Synchronization of fiducials in low-level RF with distribution of triggers in the control system
1/360 sLinac 476 MHzMain Drive Line Sector feed
Fiducialdetector
MasterPattern
Generator
SLCControlSystem Event
Generator360 Hz Triggers8.4 ns±10 ps
128-bit wordbeam codes
119 MHz
360 Hz fiducials phase locked to low level RF