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

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

3. 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. 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. 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. 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. 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. 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. 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. 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. 11 CTR Properties CTR differential energy angular distribution obeys the Ginzburg-Frank Formula

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

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

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

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

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

17. 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. 18 Material Properties Measurements done from 16 microns to ~320 microns Mylar and TPX appear to have uneven response in this range

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

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

21. 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. 22 Results Dips now reduced, more features in trace Experimental Method still rough  m  m

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

24. 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. 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. 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. 27 Presented by the E167 Collaboration U C L A 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.