The Output Performance of the BESSY Multi-stage HGHG-FEL

BESSY proposes a linac-based cascaded High-Gain Harmonic-Generation (HGHG) free electron laser (FEL) multi-user facility, covering the VUV to soft X-ray spectral range. A photoinjector and a superconducting CW linac will feed three independent FEL-lines delivering variable polarized light. As the efficiency of the interaction between the radiation and the electron beam is higher in a helical undulator, one would tend to prefer such a device for the HGHG stages. Also a higher K-value of the modulators seems to be advantageous. This is not necessarily the case. We present simulation studies for the BESSY-HGHG-FEL and discuss the output performance for “helical stages ” and increased K-values of the modulators