The Gas Loaded Free Electron Laser.

The free-electron laser (FEL) has generated considerable interest in recent years as a tunable source of coherent radiation. FEL's have the potential of providing high power radiation at wavelengths from microwaves to the ultraviolet. However, the practicality of free-electron lasers as research devices accessible to a large number of users is greatly limited by the size, complexity, and cost of the required electron beam accelerator. In a free-electron laser, the frequency of the coherent radiation, and thus the photon energy, increases as the energy of the electrons increases. To obtain a high flux of high frequency photons necessitates the construction of such costly equipment as electron storage rings, pushing the cost of FEL facilities into a multi-million dollar range. The gas-loaded free-electron laser (GFEL) is motivated by the desire to produce high energy radiation without requiring very high energy electrons. Introduction of a gas into the oscillator cavity of an FEL allows the frequency to be increased without requiring an increase in electron energy, thereby greatly reducing the overall cost of producing radiation at a given wavelength. The greatest potential limitation on the use of the GFEL concept is the effect which passage through atmospheric pressures of gas has on the electron beam. The beam ionizes the gas, forming a plasma which can disturb the stable propagation of subsequent electron beam pulses. While preliminary propagation experiments showed no sign of plasma effects, actual experiments using hydrogen in an FEL cavity did show degraded laser performance due to the presence of a plasma. Later experiments used an electron-attachment gas as a dopant in the hydrogen fill-gas in order to eliminate plasma effects. With this mixture, saturation of the laser signal has been achieved at pressures over 1/4 atm, with measured gain and wavelength tuning in accord with theory.