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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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P. Evtushenko, J. Coleman, K. Jordan, M. Klopf, G. Neil, G. Williams
interferometer (step scan)
is used with CTR;
only tune (pulsed) beam
is used with CSR;
Requirements on phase space:
get slope and curvature right
Transition (synchrotron) radiation is produced when the electron bunch passes a boundary of two media
at 135 pC
essentially both are a modification
of the Michelson interferometer.
The two interferometers differ in
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
Used with CTR
(rapid scan device 2 sec/scan)
beam splitter (silicon)
focusing (parabolic mirrors)
mirror position is measured by
another built-in interferometer
Used with CSR
at the MPI entrance
longitudinal field profile
at the MPI exit
the autocorrelation function is
measured with the help of an
The Wiener-Khintchine theorem says:
“the Fourier transform of the autocorrelation function
is the power spectrum”.
raw data – interferogram
Fourier transform of the interferogram
low frequency cut-off
diffraction on the
detector input window
the losses are
the Gaussian shape
of the bunch is assumed
its power spectrum is
The fit function is used
The mod. Martin-Puplett interferometer (the Happek device) measurements.
The bunch length does not change with beam current.
The bunch compression is optimized for the nominal (135 pC) bunch.
It is also strongly bunch charge dependent.
The interferometer measurements also provide
an information about the beam stability!
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
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.