• S. Huang, J.Y. Li, Y.K. Wu
  FEL/Duke University, Durham, North Carolina
• S. Huang
  PKU/IHIP, Beijing

Funding: This work was supported by US Air Force Office of Scientific Research medical FEL grant FA9550-04-01-0086.

The Madey theorem is a valuable research tool for studying Free-Electron Lasers (FELs). The theorem relates the shape of the on-axis spontaneous radiation spectrum of FEL wigglers to the FEL gain. The theorem predicts that degradation of the spontaneous spectrum, for example as a result of the increase of the electron beam energy spread, provides a direct measure of the reduction of the FEL gain. Extensive experiments have been performed to study the validity of the Madey theorem for a storage ring base optical klystron FEL. The experimental data show that the lasing wavelength of the FEL is very close to the maximum slope of spontaneous spectra as predicted by the Madey theorem with a relative wavelength discrepancy less than 0.2%. Further analysis is underway to understand this wavelength difference. In addition, we have performed direct measurements of the start up gain of the FEL and compared it with the changing slope of the spontaneous spectra. The preliminary results show a good agreement between the measured FEL gain and the prediction by Madey theorem.

• J.Y. Li, Y.K. Wu
  FEL/Duke University, Durham, North Carolina
• S. Huang
  PKU/IHIP, Beijing

Funding: Work supported by US Air Force Office of Scientific Research medical FEL grant FA9550-04-01-0086.

The storage ring based free electron laser (FEL) oscillator serves as a photon driver for the High Intensity Gamma-Ray Source (HIGS) at Duke University. The FEL cavity consists of two concave mirrors with a large radius of curvature of more than 27 m. Both cavity mirrors see very high intensity intracavity FEL power; the downstream mirror also receives higher harmonic spontaneous UV-VUV radiation of wigglers. The large heat load by various types of radiation can deform the mirror surface, causing FEL mode distortion. The FEL mirror can also be damaged by intense UV-VUV wiggler harmonic radiation. To mitigate these problems, a pair of water-cooled, in-vacuum apertures have been installed inside the FEL cavity. These apertures are ideal for manipulating the FEL transverse profile. This paper reports our study on the FEL transverse mode shaping using these apertures, including the characterization of the transverse mode structure of the FEL beam under a variety of operation conditions. These studies allow us to minimize the diffraction loss of the fundamental mode of the FEL while effectively reducing the impact of off-axis UV-VUV wiggler radiation on the FEL mirrors.