| The Stanford Far-Infrared Free-Electron Laser (FIRFEL) is the smallest, least expensive FEL in the world to date. The primary reduction in cost came about from using a 1.5 cell, thermionic-cathode RF gun as the sole accelerator. The entire FEL was built for a component cost of US{dollar}300,000; the beamline can fit in a footprint of several square meters. Both the cost and the size of the FIRFEL are approximately an order of magnitude smaller than other machines operating in the same wavelength range.; The primary constraint on FIRFEL operation is the accelerator. The thermionic cathode gun used as the accelerator was not designed for FEL use; rather, it was intended for use as the injector for a higher-energy linac. It produces electron beams with large energy spreads which must be filtered before injection into the wiggler, and as the beam energy is lowered the usable electron beam current falls as well. Therefore, in order to cover a broad wavelength range the FIRFEL will require a modified accelerator.; It is possible to design a thermionic cathode RF gun capable of producing electron beams with very low energy spreads. According to simulations, it is possible to build a thermionic cathode RF gun which can produce a beam with more than 90% of the beam current in a small energy spread. Such a gun would be ideal for use with the FIRFEL project. it is also possible to enhance the present FIRFEL gun to make it more suitable for FEL use.; The results of simulation studies on several gun designs are presented. The behavior of thermionic cathode guns operating in various modes is examined in the light of FEL use. Calculated beam parameters, including energy spread, emittance, useful current, and microbunch length, are presented for several of the more promising gun designs. A method of allowing individual current micropulse selection from a thermionic cathode gun is proposed. Finally, an upgrade for the present FIRFEL is proposed, which should allow saturated lasing from 85-200 {dollar}mu{dollar}m. |
