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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 overview • Michelson interferometer • Bunch shape determination • Experimental results • Conclusions
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
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: • 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.
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: • low detector sensitivity at low frequencies • diffraction at low frequencies • absorption at large frequencies Golay detectors: no problem with interference !
Beam conditions: • Q = 0.5 nC • maximum compression Experimental results (3) Interference Apparatus response function: Absorption Diffraction Low detector sensitivity
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. Experimental results (4) Molectron pyroelectric detectors
Experimental results (5) • Molectron pyroelectric detectors • Kramers-Kroning method Head-Tail ambiguity Parmela simulation Head Tail
Beam conditions: • Q = 3.0 nC • moderate compression Experimental results (6) Golay cell • FT • Spectrum correction with R() • Spectrum completion for: 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: • Q = 0.5 nC each pulse • 15 ps initial separation between the two pulses • both pulses moderately compressed 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: • Simple bunch structure • Stronger component comes first 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: • 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.