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Longitudinal Diagnostics of Electron Bunches Using Coherent Transition Radiation

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

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  1. Longitudinal Diagnostics of Electron Bunches Using Coherent Transition Radiation Daniel Mihalcea Northern Illinois University Department of Physics Fermilab, Jan. 16, 2007

  2. Outline: • Fermilab/NICADD overview • Michelson interferometer • Bunch shape determination • Experimental results • Conclusions

  3. 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

  4. FNPL layout Michelson interferometer for longitudinal diagnostics

  5. Michelson Interferometer (University of Georgia & NIU) Autocorrelation = I1/I2 - Molectron pyro-electric - Golay cell (opto-acoustic) Detectors:

  6. 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.

  7. Basic Principle (1) Backward transition radiation Ginsburg-Franck: Detector aperture  1 cm

  8. 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

  9. Coherence condition Due to detector sensitivity: Acceptable resolution: Need bunch compression !

  10. Bunch Compression RF field in booster cavity Energy-Position correlation Electron bunch before compression Tail After compression Head

  11. Kramers-Kröning method: Measurement Steps FT Ideal apparatus K-K

  12. Molectron pyro-electric detectors Path difference (mm) Frequency (THz) Interference effect Missing frequencies Experimental results (1)

  13. 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 !

  14. Beam conditions: • Q = 0.5 nC • maximum compression Experimental results (3) Interference Apparatus response function: Absorption Diffraction Low detector sensitivity

  15. 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

  16. Experimental results (5) • Molectron pyroelectric detectors • Kramers-Kroning method Head-Tail ambiguity Parmela simulation Head Tail

  17. 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

  18. Complicated bunch shapes Stack 4 laser pulses Select 1st and 4th pulses (t 15ps) After compression Before compression (Parmela simulations)

  19. 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 !

  20. 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 !

  21. 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).

  22. 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.

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