Experimental observation of modulation modes of powerful microwave pulses produced by a 5-nanosecond width Ka-band backward wave oscillator

Summary form only given, as follows. The operating regime (stationary, self-modulated, or chaotic) of a relativistic backward-wave oscillator (BWO) depends on how much the generators operating current exceeds the starting current. A BWO in the self-modulation or chaotic mode can, upon a lapse of transit time generate a train of short microwave radiation spikes with a power level even higher than that in the stationary mode. For an oscillator pertaining to the Ka frequency band (38-GHz), the characteristic value of transit time lies in the range of hundreds of picoseconds. So, even 5 ns width beam current is enough for excitation of the train of subnanosecond microwave spikes. Practical experimental realization of the non-stationary generation regime of the microwave device was performed by invoking a compact high-current electron accelerator (/spl sim/200 to 250 keV, /spl sim/1.5 kA) built around a RADAN-303B pulser equipped with a subnanosecond peaker (slicer). The device features a smoothly controllable wavefront risetime (0.3-1.5 ns), and an accelerating-voltage pulse duration (0.5-5) ns. To attain self-modulation or chaotic regime for the beam current being constant, the ratio (operating current/starting current) was varied based on the strong dependence of a BWO´s starting current from the length of the slow-wave system. The report presents the comparison of experimental results and the data of numerical PIC-simulations that, for example, shows a different nature of modulation of the BWO microwave pulse generated at various magnetic fields applied for transportation of an electron beam.