A new type of FEL with quasi-continuous spectrum by short-bunched electron beam

Using a pre-bunched electron beam from a linear accelerator, we have constructed a new type of FEL which has the output of quasi-continuous spectrum in the millimeter wave region. The FEL consists of an open cavity and a mono-pole bending magnet. Wave packets of successive coherent radiation from the electron bunches are stored in the cavity and stimulate another wave packet of coherent radiation. The accumulated wave packets are composed of overtones of the RF of the linear accelerator, which are quasi-continuously distributed over the wide millimeter region. Observed enhancement of the output intensity and its dependence on the magnetic field verified the laser action of coherent synchrotron radiation. (author)