Internal injection for a microtron driving a terahertz free electron laser

A terahertz free electron laser (FEL) driven by a high-current classical $S$-band 12-orbit microtron with internal injection and a magnetron-based radio frequency system has been developed. The laboratory-size, inexpensive facility is widely tunable in the terahertz range. This makes it attractive for application in research laboratories and universities. Stability and reliability in operation of such microtron-based FEL is determined generally by the microtron injection system. Operation of the injection system employing a thermionic cathode has been analyzed. The analysis was performed using two-dimensional tracking simulations in which we considered bombardment of the cathode emitting surface with the back-streaming electrons. The analysis showed that the bombardment causes pulse overheating of the emitting surface and, as a result, an increase of beam loading of the accelerating cavity during the macropulse. The phenomenon affects the intrapulse stability of the accelerated current and the FEL operation. The analysis and measurements show how to optimize the microtron operation minimizing affects of the back-streaming electrons. The developed injection system based on $LaB6$ thermionic cathode provides operation of the widely tunable terahertz FEL in the ordinary regime with radiated macropulse power of $40–50W$ at the pulse duration of $2–4μs$⁠. The standard deviation of the lasing macropulse energy is less than 10% for long-time operation.