longitudinal diagnostics of electron bunches using coherent transition radiation
DESCRIPTION
Longitudinal Diagnostics of Electron Bunches Using Coherent Transition Radiation. Daniel Mihalcea. Northern Illinois University Department of Physics. Outline:. Fermilab/NICADD overview Michelson interferometer Bunch shape determination Experimental results Conclusions. - PowerPoint PPT PresentationTRANSCRIPT
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Fermilab, Jan. 16, 2007
Longitudinal Diagnostics of Electron Bunches Using
Coherent Transition Radiation
Daniel Mihalcea
Northern Illinois University
Department of Physics
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•Fermilab/NICADD overview
• Michelson interferometer
• Bunch shape determination
• Experimental results
• Conclusions
Outline:
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Fermilab NICADD Photo-injector Laboratory
FNPL is a collaborative effort of several institutes and universities to operate a high-brightness photo-injector dedicated to accelerator and beam physics research.
Collaborators include:
U. of Chicago, U. of Rochester, UCLA, U. of Indiana, U. of Michigan, LBNL, NIU, U. of Georgia, Jlab, Cornell University
DESY, INFN-Milano, IPN-Orsay, CEA-Saclay
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FNPL layout
Michelson interferometer for longitudinal diagnostics
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Michelson Interferometer
(University of Georgia & NIU)
Autocorrelation = I1/I2 Detectors: - Molectron pyro-electric
- Golay cell (opto-acoustic)
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Data Flow
Interferometer
ICT Detectors Stepping motor
Scope
Controller
Get Q Get I1 and I2
LabView code: Advances stepping motor between x1 and x2 with adjustable step size (50 m)
At each position there are N readings (5)
A reading is valid if bunch charge is within some narrow window (Ex: 1nC 0.1 nC)
Position, average values of I1 and I2 and their ’s are recorded.
Autocorrelation function is displayed.
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Basic Principle (1)
222
22
03
22
)cos1(
sin
4
c
e
dd
UdIe
Ginsburg-Franck:
)(fIe
Detector aperture 1 cm
Backward transition radiation
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Basic Principle (2)
2)()1()( fINNI eIntensity of Optical Transition Radiation:
Coherent part N2
Form factor )(f related to longitudinal charge distribution: dzczizf )/exp()()(
To determine (z) need to know I() and the phase of f()
dttEctEI2
1 )()/(
dttE
dtctEtE
I
IS 2
2
1
)(
)/()(Re)(
dE
deES
ci
2
/2
)(
)(Re)( deSEI ci
/2)()()(
Kramers-Kröning technique
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Coherence condition
z mm3max
Due to detector sensitivity:
Acceptable resolution: mmz 6.0 Need bunch compression !
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Bunch Compression RF field in booster cavity
Electron bunch before compression
Energy-Position correlation
Head
Tail
After compression
mm1
mm15.0
2566 c
Tt f
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Measurement Steps
Ideal apparatus
mmz 19.0
2/
FT
K-K
mmz 18.0
Kramers-Kröning method:
dczIc
z
x
IxIdx
/)(cos)(1
)(
)(/)(ln)(
0
022
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Experimental results (1)
Path difference (mm)
Frequency (THz)c
ndE
22 sin||
Molectron pyro-electric detectors
Interference effect Missing frequencies
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Experimental results (2)
Golay detectors: no problem with interference !
Still need to account for:
low detector sensitivity at low frequencies
diffraction at low frequencies
absorption at large frequencies
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Experimental results (3)
Low detector sensitivity
DiffractionAbsorption
Interference
Apparatus response function:
)(
)()(
simulation
measured
I
IR
Beam conditions:
Q = 0.5 nC
maximum compression
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Experimental results (4)
Auto-correlation function:
Q = 3nC
9-cell phase was 3 degrees from maximum compression
Power spectrum:
Asymptotic behavior
low frequencies:
high frequencies:
Least square fit.
21)( aI
)(I
Molectron pyroelectric detectors
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Experimental results (5)
• Molectron pyroelectric detectors
• Kramers-Kroning method
Head
Tail
Parmela simulation
Head-Tail ambiguity
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Experimental results (6)
FT
Spectrum correction with R()
Spectrum completion for:
andzc /0 0
Beam conditions:
Q = 3.0 nC
moderate compression
K-K
Golay cell
Start point
z 1ps
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Complicated bunch shapes
Stack 4 laser pulses
Select 1st and 4th pulses (t 15ps)
Before compression
After compression
(Parmela simulations)
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Experimental results (7)
Double-peaked bunch shapes
Beam conditions:
Q = 0.5 nC each pulse
15 ps initial separation between the two pulses
both pulses moderately compressed
K-K method may not be accurate for complicate bunch shapes !
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K-K method accuracy
R. Lai and A. J. Sievers, Physical Review E, 52, 4576, (1995)
Generated
Reconstructed
K-K method accurate if:
• Simple bunch structure
• Stronger component comes first
Calculated widths are still correct !
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Other approaches
Major problem: the response function is not flat.
1. Complete I() based on some assumptions at low and high frequencies.
R. Lai, et al. Physical Review E, 50, R4294, (1994).
S. Zhang, et al. JLAB-TN-04-024, (2004).
2. Avoid K-K method by assuming that bunches have a predefined shape and make some assumptions about I() at low frequencies.
A. Murokh, et al. NIM A410, 452-460, (1998).
M. Geitz, et al. Proceedings PAC99, p2172, (1999).
This work:
D. Mihalcea, C. L. Bohn, U. Happek and P. Piot, Phys. Rev. ST Accel. Beams 9, 082801 (2006).
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Conclusions:
Longitudinal profiles with bunch lengths less than 0.6 mm can be measured.
Systematic uncertainties dominated by approximate knowledge of response function and completion procedure.
Golay cells are better because the response function is more uniform.
Some complicate shapes (like double-peaked bunches) can be measured.