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.
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Longitudinal Diagnostics of Electron Bunches Using Coherent Transition Radiation
Daniel Mihalcea
Northern Illinois University
Department of Physics
Fermilab, Jan. 16, 2007
Outline:
Fermilab NICADD Photo-injector Laboratory
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
FNPL layout
Michelson interferometer for longitudinal diagnostics
Michelson Interferometer
(University of Georgia & NIU)
Autocorrelation = I1/I2
- Molectron pyro-electric
- Golay cell (opto-acoustic)
Detectors:
Interferometer
Stepping motor
Scope
Detectors
ICT
Controller
Data Flow
Get Q
Get I1 and I2
LabView code:
Basic Principle (1)
Backward transition radiation
Ginsburg-Franck:
Detector aperture 1 cm
Form factor
related to longitudinal charge distribution:
Basic Principle (2)
Intensity of Optical Transition Radiation:
Coherent part N2
To determine (z) need to know I() and the phase of f()
Kramers-Kröning technique
Coherence condition
Due to detector sensitivity:
Acceptable resolution:
Need bunch compression !
Bunch Compression
RF field in booster cavity
Energy-Position correlation
Electron bunch before compression
Tail
After compression
Head
Kramers-Kröning method:
Measurement Steps
FT
Ideal apparatus
K-K
Molectron pyro-electric detectors
Path difference (mm)
Frequency (THz)
Interference effect
Missing frequencies
Experimental results (1)
Experimental results (2)
Still need to account for:
Golay detectors: no problem with interference !
Beam conditions:
Experimental results (3)
Interference
Apparatus response function:
Absorption
Diffraction
Low detector sensitivity
Auto-correlation function:
Power spectrum:
low frequencies:
high frequencies:
Experimental results (4)
Molectron pyroelectric detectors
Experimental results (5)
Head-Tail ambiguity
Parmela simulation
Head
Tail
Beam conditions:
Experimental results (6)
Golay cell
and
Start point
K-K
z 1ps
Complicated bunch shapes
Stack 4 laser pulses
Select 1st and 4th pulses (t 15ps)
After compression
Before compression
(Parmela simulations)
Beam conditions:
Experimental results (7)
Double-peaked bunch shapes
K-K method may not be accurate for complicate bunch shapes !
K-K method accuracy
R. Lai and A. J. Sievers, Physical Review E, 52, 4576, (1995)
Generated
Reconstructed
K-K method accurate if:
Calculated widths are still correct !
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).
Conclusions:
be measured.