Gain and efficiency of table-top terahertz free-electron lasers driven by a microtron accelerator

There are difficulties in developing high-power and small-size terahertz (THz) sources that can be used for field applications. In an attempt to design small-size THz free-electron laser (FEL) devices capable of producing higher output powers in the THz spectral region of 1–2 THz, we have developed a microtron accelerator that can accelerate electron beams to 3–6 meV, with a macropulse current of more than 40 mA. The new THz FELs use hybrid electromagnetic (EM) undulators that are two to four times shorter in length than the previous undulator and waveguide resonators with mode cross-sectional areas that are more than two times smaller than the parallel-plate waveguide in the existing FEL. We confirm that the gains and losses of the compact FELs are sufficient for lasing, and we estimate that average output power of approximately 1 W is possible with an efficiency approximately 10 times greater than the existing FEL. The minimum size of the THz FEL system, including a high-voltage pulse modulator, is estimated to be approximately 1.5 m × 2.0 m.