Abstract
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 $\ensuremath{\sim}0.2\text{ }\text{ }\mathrm{mJ}$.
Highlights
A compact driven by MI-456A magnetron high-current microtron injector [1] has been developed for a laboratorysize terahertz free electron laser (FEL), tunable in the range of 1–3 THz [2]
Parameters of the magnetron and parameters of the microtron optimal for the FEL operation are shown in Tables I and II, respectively
The simulation and the measurements are in good agreement and showed that the developed microwave scheme effectively decreases the frequency instability of the magnetron autogenerator. This reduces the bunch repetition rate instability during the macropulse to a level that is acceptable for terahertz FEL operation
Summary
A compact driven by MI-456A magnetron high-current microtron injector [1] has been developed for a laboratorysize terahertz free electron laser (FEL), tunable in the range of 1–3 THz [2]. To keep the accelerated current constant during the macropulse, we tuned the modulator charging line to provide linear enhancement of the magnetron current In this case, as follows from simulation and measurements, the accelerated current has a flat top providing intrapulse stability of the beam current suitable for FEL operation. The simulation and the measurements are in good agreement and showed that the developed microwave scheme effectively decreases the frequency instability of the magnetron autogenerator This reduces the bunch repetition rate instability during the macropulse to a level that is acceptable for terahertz FEL operation. The concepts of the intrapulse stabilizations realized in the magnetron-based microtron driving the terahertz FEL at optimized regimes, 2D tracking, simulation of the stabilizations, and measured results including data obtained at the FEL operation are presented and discussed in this article
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