Electron beam characterization for a compact far-infrared free-electron laser

A compact, far-infrared free-electron laser (FIR-FEL) is operating at 85 μm at Stanford University, where the electron beam is obtained from a 1½ cell, thermionic RF (2.856 GHz) cavity gun. This gun was not designed for FEL operation, and under the condition at which it was intended to operate, with a peak acceleration gradient below 85 MV/m, it would not be suitable. We have explored new parameter ranges, and have found that at high peak gradients, from 100-140 MV/m, the gun will function satisfactorily as an FEL accelerator. For example, thirty-nine percent of the total gun current was transmitted through a 1% energy window at a beam energy of γ=9.72. At γ=9.56, a 304 mA macropulse current with an estimated rms micropulse length of 3.4 ps was obtained, and the normalized rms emittance was measured to be 11.6 π-mm-mrad for the 1% energy-spread electrons, corresponding to a beam brightness of 2.3×10¹¹ A/m². For these parameters, the calculated small small-signal gain for our 0.5 m-long wiggler is 110% at a wavelength of 85 μm. In this paper we characterize the electron beam from a 1½ cell, thermionic cathode, RF cavity gun in a parameter range where it can be used as the accelerator for a far infrared FEL