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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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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

jlab fel layout and longitudinal matching

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)

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  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

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!

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

mathematics of michelson interferometer

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”.

interferogram example corresponding spectrum

Interferogram example & corresponding spectrum

raw data – interferogram

Fourier transform of the interferogram

bunch length estimation

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

pulsed beam measurements rms 148 fs

Pulsed beam measurements; RMS 148 fs

The mod. Martin-Puplett interferometer (the Happek device) measurements.

pulsed beam measurements vs cw beam

Pulsed beam measurements vs. CW beam

The bunch length does not change with beam current.

very short and very long bunches

Very short and very long bunches

The bunch compression is optimized for the nominal (135 pC) bunch.

It is also strongly bunch charge dependent.

beam stability

Beam stability

The interferometer measurements also provide

an information about the beam stability!

conclusion

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.