An experimental investigation into the evolution of the output of a free electron maser oscillator

We have constructed a Free Electron Maser (FEM) oscillator that can be operated in single mode in the X-band range (9–11 GHz). The electron gun has two anodes, the first being at ground while the second is held at +60 kV. Electron beam current flows from the thermionic cathode when the cathode is pulsed with up to −3 kV. With this arrangement we have found that it is possible to generate an electron beam current pulse of up to 50 mA with extremely fast rise and fall times (45 ns) with less than 100 V droop of the accelerating voltage over a 100 μs pulse. As such this device can be used to make accurate measurements of the growth of the microwave output as a function of time. We have measured the exponential gain of a longitudinal mode up to saturation at a fixed accelerating voltage and found that the spread of times taken to reach a given power level can only be explained if it is assumed that there is an initial phase during which the cavity is filling with spontaneous radiation. For the low small signal gains achievable with this system, this phase lasts for about a microsecond.