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Bunch Length Measurements in the E167 Experiment

Bunch Length Measurements in the E167 Experiment. Ian Blumenfeld E167 Collaboration SLAC/UCLA/USC. Contents. Introduction Theory CTR and Autocorrelation Practice Interferometry Simulation and Measurement Future. Introduction to Bunch Length Measurements.

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Bunch Length Measurements in the E167 Experiment

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  1. Bunch Length Measurements in the E167 Experiment Ian Blumenfeld E167 Collaboration SLAC/UCLA/USC

  2. Contents • Introduction • Theory • CTR and Autocorrelation • Practice • Interferometry • Simulation and Measurement • Future

  3. Introduction to Bunch Length Measurements • Short Bunch in past not important for Particle Physics experiments, so not measured directly • Important for plasma experiment due to need for high peak current

  4. Linear Plasma Theory • According to linear plasma theory the wake amplitude is: • This is optimized for if yielding: • In reality, we are no longer in this regime, but simulations show that this scaling still holds

  5. Previous Methods • E167 Efforts • Streak Camera • Pyroelectric Detectors • Phase space matching • Also: E/O’s, transverse deflection cavities (LOLA), etc. • Desire direct measurement

  6. CTR and Bunch Length • Radiation generated when charged particles moves from one dielectric medium to another • Longitudinal profile of CTR is the same as that of the beam • Coherent for wavelengths longer than bunch length

  7. CTR and Bunch Length (cont’) • Analytically, the radiation intensity is related to the Fourier Transform of the electron number density • Coherence due to interference of electrons in the bunch • Thus CTR spectrum yields information about the bunch

  8. CTR and Bunch Length • We need only measure the longitudinal profile of the CTR for the bunch length • Problem: • We have short bunches, ~10 microns or ~30fs

  9. CTR and Bunch Length • This means no time resolved measurement • Must use interferometry • Like in femtosecond laser pulse measurements • Despite disadvantages of symmetric measurement and averaging

  10. Autocorrelation and CTR • Autocorrelation function gives information on the pulse shape • Width of this function is correlated to the width of the original pulse

  11. CTR Properties • CTR differential energy angular distribution obeys the Ginzburg-Frank Formula

  12. CTR Properties • CTR energy peaks at 1/gamma off the axis of propagation

  13. The Michelson Interferometer • Chose Michelson Interferometer for autocorrelation due to small opening angle • Can easily adjust delay arm with micron precision

  14. sz =9µm The Michelson Interferometer: First Results • First results • Translates to bunch of ~18 microns

  15. Simulation: Understanding the Results • Simulations show that ideal trace does not contain dips apparent in measured spectrum

  16. Material Effects • Turns out materials in interferometer have large effect on trace • E.g. loss of long wavelength generates large dips

  17. Material Effects • Measurements using Bruker interferometer at LBNL in M/FIR show material transmission characteristics • Special Thanks to Michael Martin and Zhao Hao of LBNL and Walt Zacherl of Stanford University for making this happen

  18. Material Properties • Measurements done from 16 microns to ~320 microns • Mylar and TPX appear to have uneven response in this range

  19. Material Properties (cont’) • HDPE possibly good for long wavelength • Silicon has flattest response • ~50% transmission means could be used as beam splitter

  20. Simulation Results • Material effects distort our expected signal • The silicon appears to cause less distortion

  21. The New setup • Used Silicon beam splitter and Vaccum Window, as well as gold coated mirrors • As Silicon is opaque to visible light, had to align with 1.5micron laser

  22. Results s=8.7mm s=14.4mm • Dips now reduced, more features in trace • Experimental Method still rough

  23. Results (cont’) • Features indicate head or tail on beam • As well trace width scales with r56

  24. Next Steps: Further response Studies • Will return to LBNL • Take FIR measurements out to mm range • Take reflectivity measurements • Calibrate pyro detectors and energy meter vs. Bruker

  25. Far Future: Improvements and Single Shot • Next beam access, run current setup in Nitrogen or Helium purge environment • Acquire THz camera, look at radiation properties • Study feasability of single-shot measurement

  26. Conclusion • Have improved bunch length measurement with study of material properties • Will continue to develop this until it is a useful diagnostic tool

  27. U C L A Presented by the E167 Collaboration M. Berry, I. Blumenfeld, F.-J. Decker, P. Emma, M.J. Hogan*, R. Ischebeck, R.H. Iverson, N. Kirby, P. Krejcik, R.H. Siemann, and D. Walz Stanford Linear Accelerator Center C.E. Clayton, C. Huang, C. Joshi*, W. Lu, K.A. Marsh, W.B. Mori, and M. Zhou University of California, Los Angeles S. Deng, T. Katsouleas, P. Muggli* and E. Oz University of Southern California Work supported by Department of Energy contracts DE-AC02-76SF00515 (SLAC), DE-FG03-92ER40745, DE-FG03-98DP00211, DE-FG03-92ER40727, DE-AC-0376SF0098, and National Science Foundation grants No. ECS-9632735, DMS-9722121 and PHY-0078715.

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