1. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Bunch length measurements at JLab FEL using coherent transition and synchrotron radiation P. Evtushenko, J. Coleman, K. Jordan, M. Klopf, G. Neil, G. Williams

2. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL JLab FEL (layout and longitudinal matching) Requirements on phase space: Long bunch in linac high peak current (short bunch) at FEL –bunch length compression at wiggler “small” energy spread at dump –energy compress while energy recovering –“short” RF wavelength/long bunch  get slope and curvature right E  E  E  E  modified Martin-Puplett interferometer (step scan) is used with CTR; only tune (pulsed) beam Michelson interferometer (rapid scan) is used with CSR; CW beam

3. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Bunch length measurements using coherent radiation Transition (synchrotron) radiation is produced when the electron bunch passes a boundary of two media (magnetic field). Response time is zero. Shape of the radiation pulse is a “copy” of the electron bunch shape. When the wave length of the radiation becomes more than the bunch length the radiation becomes COHERENT. (  L ) Power is proportional to: intensity of incoherent radiation  N intensity of coherent radiation  N 2 Measurements of the radiation spectrum give information about the bunch length. An interferometer could be used to measure the spectrum. at 135 pC N  8.4  10 8

4. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Interferometers We use two different interferometers; essentially both are a modification of the Michelson interferometer. The two interferometers differ in implementation; Beam splitter Polarizer Detector Focusing element Mirror position measurements! Modified Marin-Puplett interferometer: (step scan device 2 min/scan) beam splitter & polarizer (wire grids) detector (Golay cell) focusing (Plano-convex lens) mirror position is set by step motor Used with CTR Michelson interferometer: (rapid scan device 2 sec/scan) beam splitter (silicon) detector (pyroelectric) focusing (parabolic mirrors) mirror position is measured by another built-in interferometer Used with CSR

5. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Mathematics of Michelson interferometer the autocorrelation function is measured with the help of an interferometer The Wiener-Khintchine theorem says: “the Fourier transform of the autocorrelation function is the power spectrum”. longitudinal field profile at the MPI entrance longitudinal field profile at the MPI exit detectors measure intensity I  E 2

6. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Interferogram example & corresponding spectrum raw data – interferogramFourier transform of the interferogram

7. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Bunch length estimation the Gaussian shape of the bunch is assumed its power spectrum is also Gaussian low frequency cut-off diffraction on the detector input window the losses are approximated by The fit function is used

8. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Pulsed beam measurements; RMS 148 fs The mod. Martin-Puplett interferometer (the Happek device) measurements.

9. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL CW beam measurements 0.31 mA; RMS 147 fs

10. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL CW beam measurements 0.62 mA; RMS 151 fs

11. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL CW beam measurements 1.25 mA; 163 fs

12. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL CW beam measurements 2.51 mA; 145 fs

13. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Pulsed beam measurements vs. CW beam The bunch length does not change with beam current.

14. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Very short and very long bunches The bunch compression is optimized for the nominal (135 pC) bunch. It is also strongly bunch charge dependent.

15. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Beam stability The interferometer measurements also provide an information about the beam stability!

16. Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy Thomas Jefferson National Accelerator Facility FEL Conclusion Further developments A modified Martin-Puplet interferometer and Michelson interferometer are routinely used at JLab FEL for bunch length measurements. Michelson interferometer (rapid scan device) provides non-invasive measurements with high current CW beam. Results of the measurements of two interferometers agree within 15%, when the same (frequency domain) data evaluation approach is applied. Make the Michelson interferometer to work with the pulsed beam. Also the modified Martin-Puplett interferometer could be used with the CW beam if another detector is used (with synchrotron radiation). To eliminate the water vapor absorption the Michelson interferometer will be operated in vacuum.