Effects of introducing a gas into the free-electron laser

The introduction of a gas into the wiggler section of a free electron laser (FEL) alters the phase velocity of the electromagnetic wave, and so changes the synchronism condition relating wavelength to wiggler parameters and beam energy. This provides a means for tuning the frequency of an oscillator, with the addition of 200 torr of hydrogen gas, the wavelength of a FEL operating in the near infrared without gas was reduced by 0.73 microns. The plasma generated from ionization of the hydrogen molecules by collisions with the electron beam diminished the oscillator gain, but this effect was eliminated by the addition of less than 0.1 percent of an electron attachment gas. Gain is also reduced by multiple scattering of the beam electrons, but this effect is not severe for a 1-m wiggler length. When hydrogen is used, a FEL with fixed wiggler parameters and electron energy can be tuned from the near infrared to about 1200 A, and, with helium, the wavelength can be reduced to 600 A.