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Bunch length measurements at JLab FEL using coherent transition and synchrotron radiation

Bunch length measurements at JLab FEL using coherent transition and synchrotron radiation. P. Evtushenko , J. Coleman, K. Jordan, M. Klopf, G. Neil, G. Williams. E. E. E. E. f. f. f. f. modified Martin-Puplett interferometer (step scan) is used with CTR; only tune (pulsed) beam.

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Bunch length measurements at JLab FEL using coherent transition and synchrotron radiation

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  1. 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. E E E E f f f f modified Martin-Puplett interferometer (step scan) is used with CTR; only tune (pulsed) beam Michelson interferometer (rapid scan) is used with CSR; CW beam 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 JLab FEL (layout and longitudinal matching)

  3. 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  N2 • Measurements of the radiation spectrum give information about the bunch length. • An interferometer could be used to measure the spectrum. Bunch length measurements using coherent radiation at 135 pC N  8.4108

  4. 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! Interferometers 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. longitudinal field profile at the MPI entrance Mathematics of Michelson interferometer longitudinal field profile at the MPI exit detectors measure intensity IE2 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”.

  6. Interferogram example & corresponding spectrum raw data – interferogram Fourier transform of the interferogram

  7. Bunch length estimation low frequency cut-off diffraction on the detector input window the losses are approximated by the Gaussian shape of the bunch is assumed its power spectrum is also Gaussian The fit function is used

  8. Pulsed beam measurements; RMS 148 fs The mod. Martin-Puplett interferometer (the Happek device) measurements.

  9. CW beam measurements 0.31 mA; RMS 147 fs

  10. CW beam measurements 0.62 mA; RMS 151 fs

  11. CW beam measurements 1.25 mA; 163 fs

  12. CW beam measurements 2.51 mA; 145 fs

  13. Pulsed beam measurements vs. CW beam The bunch length does not change with beam current.

  14. 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. Beam stability The interferometer measurements also provide an information about the beam stability!

  16. 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. Conclusion Further developments 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.

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