Magnetron-driven microtron injector of a terahertz free electron laser

A magnetron-driven microtron injector has been developed for a terahertz free electron laser (FEL). An internal injection system was chosen for the microtron to achieve a compact and inexpensive design. The system provides acceleration of electrons with low emittance and energy spread that is highly important for the FEL. However, the intrapulse instabilities of the accelerated current and the bunch repetition rate inherent to the injection system make problems for the FEL operation. Simulations of the beam dynamics and the transient process allow one to compute the load characteristic of the accelerating cavity and the time-dependent accelerated current. The simulation techniques also allow one to calculate time-dependent deviations of the magnetron frequency in the coupled system of the accelerating and magnetron cavities, as well as deviations in the bunch repetition rate. The computations validate proposed concepts for increasing the intrapulse current stability with appropriate time-dependent variation of the magnetron power and decreasing the bunch repetition rate instability through a simple microwave scheme utilizing the microtron accelerating cavity concurrently as an external stabilizing resonator for the magnetron. The realized concepts and optimization of the microtron regimes using the simulated phase motion of the accelerated bunch provide stable operation of the terahertz FEL, tunable in the range of 1-3 THz with extracted macropulse power up to 50 Watts at the macropulse energy of similar to 0.2 mJ.