Dependence of relativistic backward wave oscillator properties on effective beam gamma

The operation of a Backward Wave Oscillator (BWO) is shown to be critically dependent on the energy of the slow-space charge wave of the electron beam. Experimental work parameterizing the dependence of microwave frequency on effective beam energy, γbeam, reveals that through an understanding of electron beam dynamics a BWO could be systematically tuned through a desired frequency range while maintaining high power of a few hundred megawatts and narrow frequency bandwidth, which here was 400 MHz. Through variation of γbeam, 1.2 to 1.5 for their experiment, the authors observe the lack of scaling of peak microwave power with the kinetic energy of the electron beam for the γbeam >1.32. This effect was previously found in numerical simulation. The authors look for an explanation of this effect by examining the relationship of beam current to spacecharge-limiting current for increasing γbeam. Dramatic evidence of pulse shortening, a phenomenon known to relativistic oscillators is also seen and some preliminary remarks are made