Author: Schroeder, C.B.
Paper Title Page
TUP026 Transverse Coherence Properties of a TGU-based FEL 429
  • P. Baxevanis, Z. Huang, R.D. Ruth
    SLAC, Menlo Park, California, USA
  • C.B. Schroeder
    LBNL, Berkeley, California, USA
  The use of a transverse gradient undulator (TGU) is considered an attractive option for FELs driven by electron beams with a relatively large energy spread. In this scheme, a dispersion is introduced in the beam while the undulator poles are inclined so that the undulator field acquires a linear dependence upon the transverse position in the direction of dispersion. By suitably selecting the dispersion and the field gradient, the energy spread effect can be significantly mitigated, thus avoiding a drastic reduction in the FEL gain. However, adding the dispersion typically leads to electron beams with large aspect ratios. As a result, the presence of higher-order modes in the output FEL radiation can become significant. To investigate this effect, we study the properties of the higher-order eigenmodes of a TGU-based, high-gain FEL, using both a simplified, analytically-solvable model and a variational technique. This formalism is then used to provide an estimate of the degree of transverse coherence for a representative soft X-ray, TGU FEL example.  
Short-period Undulators based on Laser-driven Wakefields in Plasma Channels  
  • C.B. Schroeder, E. Esarey, C.G.R. Geddes, W. Leemans, S.G. Rykovanov
    LBNL, Berkeley, California, USA
  Funding: Work supported by the U.S. DOE under Contract No.DE-AC02-05CH11231.
We present a novel type of short-period undulator based on controlling the focusing forces on a beam in a plasma wave excited by an intense laser pulse undergoing centroid oscillations in a plasma channel. The period of such a plasma undulator is proportional to the Rayleigh length of the laser pulse and can be sub-mm with an effective undulator strength parameter of order unity. The undulator period can further be controlled and reduced by beating laser modes, or using multiple colors, in the plasma channel. We present analytic expressions for the electron trajectories in the plasma undulator. Analytic work is compared to numerical modeling. Examples are presented of short-period laser-driven plasma undulators based on available laser and plasma channel parameters.