Contrary to conventional lasers where the active medium which provides the optical gain is matter, the active medium and source of energy in FEL are accelerated electrons transported in vacuum. The FEL operates in principle as DC to optical frequency powe r converter, converting efficiently a big fraction of the large kinetic energy of the electrons into radiated power. Operation in vacuum eliminates the power limitation which in conventional lasers are due to material damage, heating and non-linear optical effects. This explains the high operating power capabilities of the de vice.

The operating principles of EA-FEL: A high current electron-gun (I > 1 Amp) injects an electron beam into the acceleration tube of a van de Graff accelerator. The electron beam is accelerated through the tube up to the voltage of the High Voltage (HV) terminal which is kept at a level of few million volts by the charging belts of the van de Graff accelerator. The high energy electron beam then goes at very high speed through a periodic magnetic structure called a "wiggler" which is placed inside the terminal. The wiggler causes the elect rons to oscillate (wiggle) along the wiggler length, and consequently emit electromagnetic radiation like every oscillating charge. The emission ferquency d epends on the electron speed and the wiggler period. This frequency may be very high (optical frequency) because of the Doppler effect that increases the forward emission frequency of a moving oscillator.

Because the radiation wavelength is determined only by the electron velocity and the wiggler period, any wavelength of operation can be attained by proper choice of FEL parameters, and tuning can be carried out by varying the e-beam energy (velocity). Thi s is a fundamental difference from conventional lasers where the emission frequencies are usually discrete, and are determined by the naturally occuring quantum energy levels of the active medium material. Thus the FEL may provide radiation energy in wide ranges of wavelengths where no other coherent radiation sources exist, predominantly in the IR-mm wave region. The virtual power of the electron beam at the HV terminal is very high. A beam of average current I of a few amperes accelerated to voltage V of a few million Volts carries a power P=V·I of many Megawatts. In a FEL oscillator the radiation emitted by the e-beam in the wiggler is trapped inside the laser resonator. When the radiative power stored between the resonator reflectors is high enough, a big fraction (few percent) of the e-beam power is extracted upon passage through the resonator and turned into radiative power by the process of stimulated emission. At saturation each electron which traverses the wiggler emits millions of photons. Thi s is again fundamentally different from the emission process in conventional lasers (where each excited electron emits a single photon) and provides another explanation for the high power capability of FEL. The electrons lose a few percents of their energy during passage through the wiggler, and transform it into radi ative energy. However, the e-beam still keeps most of its kinetic energy at this stage. In most conventional FELs the e-beam is dumped right after the wiggler and most of its energy is wasted (turned into heat). Not so in EA-FEL. As shown in Fig.1 the was ted electrons exiting from the wiggler are decelerated through the deceleration tube (in an energy conservative system) and are slowed down almost to zero velocity.

The beam is then collected externally by a multi-stage collector with little heat generation. This scheme called "depressed collector" or "energy retrieval" scheme enables the FEL to operate with very high energy conversion efficiency. In fact if the enti re electron beam is transported along the accelerator without interception then the dominant consumer of electric power in the FEL system is the collector power supply. The power it provides is transferred almost entirely into radiative power. This explai ns the high total (wall plug) conversion efficiency of EA-FEL as opposed to conventional FEL.