The high-power operation of a terahertz free-electron laser based on a normal conducting RF linac using beam conditioning

The feed-forward control of the amplitude and phase of a high-power RF pulse based on the overdrive technique is applied to an L-band electron linac to ensure that the energy of the electron beam is uniform and constant enough for the recommissioning of a terahertz (THz) free-electron laser (FEL) at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The quality of the electron beam is evaluated using the time-resolved measurement of its energy spectrum; the energy is constant within 0.1% during the electron macropulse for 15 MeV energy except for the transient period in the leading edge. A high-power, stable FEL operation that reaches saturation is obtained using beam conditioning. The characteristics and performance of the FEL for THz radiation are measured at the end of an evacuated transport line. The frequency ranges from 2 to 12 THz, from 25 to 150 μm in wavelength, and the maximum energy in the FEL macropulse with a ~3 μs duration is 3.7 mJ at 67 μm; using these values, the peak power in the macropulse is calculated to be 1.2 kW, while that in the micropulse with a 20 ps duration is calculated to be 0.5 MW. These values are comparable to those in the THz range of other FELs that are based on normal conducting RF linacs.