Coherent transition radiation from a laser wakefield accelerator as an electron bunch diagnostic
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Coherent transition radiation from a laser wakefield accelerator as an electron bunch diagnostic

  • Author(s): van Tilborg, J.
  • Geddes, C.G.R.
  • Toth, C.
  • Esarey, E.
  • Schroeder, C.B.
  • Martin, M.C.
  • Hao, Z.
  • Leemans, W.P.
  • et al.
Abstract

The observation and modeling of coherent transition radiation from femtosecond laseraccelerated electron bunches is discussed. The coherent transition radiation, scaling quadratically with bunch charge, is generated as the electrons transit the plasma-vacuum boundary. Due to the limited transverse radius of the plasma boundary, diffraction effects will strongly modify the angular distribution and the total energy radiated is reduced compared to an infinite transverse boundary. The multi-nC electron bunches, concentrated in a length of a few plasma periods (several tens of microns), experience partial charge neutralization while propagating inside the plasma towards the boundary. This reduces the space-charge blowout of the beam, allowing for coherent radiation at relatively high frequencies (several THz). The charge distribution of the electron bunch at the plasma-vacuum boundary can be derived from Fourier analysis of the coherent part of the transition radiation spectrum. A Michelson interferometer was used to measure the coherent spectrum, and electron bunches with duration on the order of 50 fs (rms) were observed.

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