Signature of build-up of coherence in an indigenously built Compact Ultrafast Terahertz Free Electron Laser set-up

A Compact Ultrafast Terahertz Free Electron Laser based on a novel plane wave transformer (PWT) linear accelerator (linac) structure has been built at the Raja Ramanna Centre for Advanced Technology, Indore, and is presently undergoing commissioning. We report here a measured out-coupled terahertz (THz) radiation power more than 100 times higher than the expected spontaneous emission power for the beam parameters measured in the experiment. This is the first observed signature of build-up of coherence in the system, which has been made possible with the introduction of a fundamental frequency pre-buncher to increase peak accelerated electron beam current through the PWT linac. This note discusses these recent results and the ongoing efforts to increase the radiation frequency to > 1 THz with a further significant increase in THz power by increasing energy and peak current of the electron beam from the injector linac.