Time-resolved electron beam energy spectrum diagnostics for Vanderbilt FEL

A fast electron energy spectrometer has been built using a photodiode array measuring the backward optical transition radiation from a thin film of aluminum. The resolution of the electron energy spectrometer is about 0.2% with a time resolution of 50 ns. The maximum energy spread that can be measured is 6.4%. We present the measurements of the time-resolved electron beam energy spectrum on the Mark III linear accelerator at Vanderbilt University, while lasing at different wavelengths and while not lasing. We also discuss the effects of different parameters, such as cathode heating, alpha magnet strength and RF phase, on the electron energy spectrum and optical spectrum. The diagnostics of time-resolved electron energy spectrum and time-resolved laser spectrum provide the technology to understand the physical process of the FEL interaction. Based on these diagnostics, the FEL facility can realize some special modes of operation, such as macropulse chirping and macropulse two color lasing.