Computer based test bench for modulation of terahertz FEL radiation power.
We propose simple method to modulate output radiation power in FEL oscillator which allows to adjust smoothly the average radiation power as well as to obtain short (down to 35 microseconds) radiation macrobunches. This method is based on periodic shift of electron bunch phase with respect to FEL radiation bunch stored in optical cavity which results in lasing suppression. The phase shift frequency required to suppress lasing is relatively small and it does not change significantly electron bunch repetition rate. To demonstrate feasibility of the proposed method we created computer based test bench which is described in this paper. The test bench comprises standard CAMAC blocks: G0609 - modulator timer and G0601 – clock generator. The electron bunches phase shift is carried out by a programmable skipping of the reference frequency periods which are used to start current modulator in electron gun. The skipping is controlled by computer code which runs under real-time operating system LynxOS v3.1. This environment allows to switch modes with an accuracy of 16 microseconds. The clock-generator is used to send synchronization signals to radiation user equipment.