In Situ Measurements In A Crossed-field Amplifier and Comparisons with Numerical Simulations

In situ measurements of the electron density, electron energy distribution, and rf electric field have been made in a low frequency (100 to 200 MHz) crossed-field amplifier. The amplifier is physically large with a length of 40 cm, a width of 20 cm, and an anode to sole spacing of 2 to 3 cm. The slow wave circuit consists of a meander line on a dielectric with a slow wave wavelength of 6 cm at 150 MHz. The sole voltage is -1 to -1.3 kV, and the magnetic field is typically 50 to 55 Gauss. The amplifier is an injected beam type with a cathode of tungsten filaments biased independently of the sole. The beam current is as high as 350 mA, and the beam width is 10 cm. This device is typically operated with a highly cycloidal beam. The gain is as high as 7 dB with a 10 W rf input. High gain at higher rf input power is limited by the electron beam power. Two dimensional (input to output and sole to anode) measurements have been made of the electron density and the rf electric field using collection probes and rf pick-up probes. These measurements have been made during amplification. The rf measurements clearly show gain in the local rf field comparable to the measured circuit gain. Electron density measurements of the highly cycloidal beam show that the beam trajectories shift spatially during amplification. This shift is believed to be due to the loss of energy the electrons experience during the amplification process.