The results of the generation of a high-power microwave ($550 MW, 0.5 ns, $9.6 GHz) beam and feasibility of wakefield-excitation with this beam in under-dense plasma are presented. The microwave beam is generated by a backward wave oscillator (BWO) operating in the superradiance regime. The BWO is driven by a high-current electron beam ($250 keV, $1.5 kA, $5 ns) propagating through a slow-wave structure in a guiding magnetic field of 2.5 T. The microwave beam is focused at the desired location by a dielectric lens. Experimentally obtained parameters of the microwave beam at its waist are used for numerical simulations, the results of which demonstrate the formation of a bubble in the plasma that has almost 100% electron density modulation and longitudinal and transverse electric fields of several kV/cm.
We demonstrate a practical means by which one can inject more than the space-charge limiting current into a vacuum diode. This over-injection causes self-oscillations of the space-charge resulting in an electron beam current modulation at a fixed frequency, a reaction of the system to the Coulomb repulsive forces due to charge accumulation. Published by AIP Publishing.
The space charge limited current of an electron beam generated in a magnetically insulated foil-less diode by a cylindrical or hollow cathode and injected into a cylindrical drift tube depends on the energy of the electrons and on the ratio of the beam radius to the tube radius. When this beam drifts into a tube of larger radius, it exceeds the corresponding space charge limit. This leads to the formation of a virtual cathode and to the reduction of the energy of the beam. The latter is known as a squeezed beam state. When the cylindrical tube consists of more segments of different radii and lengths, the dynamics of the beam can be manipulated. We propose a simple configuration resulting in the production of a high-frequency periodic train of high-current electron beam pulses.
An experimental setup to demonstrate a recent scheme [Leopold et al., Phys. Plasmas 24, 073116 (2017)] to persistently over-inject a vacuum electron diode so that it self-oscillates in the GHz-frequency range and releases a periodic train of electron bunches is described. We present simulations in which we account for the finite rise time of the voltage accelerating electrons from a finite radius cylindrical thermionic cathode and finite grid electrode transparency, which make the experiment different from the original theoretical scheme. It was found that these experimental conditions do affect the expected results but a self-oscillating behavior is still possible. Our experimental results so far indicate the presence of the self-oscillatory behavior, but because the cathode used emits from a few hot-spots rather than uniformly, the beam current modulations are different from those obtained in the simulations.
A recently proposed scheme to persistently over-inject a vacuum electron diode so that it self-oscillates, releasing a periodic train of electron bunches [Leopold et al., Phys. Plasmas 24, 073116 (2017)] was only partially demonstrated [Siman-Tov et al., 26, 033113 (2019)] because of the presence of hot spots on the dispenser cathode surface. The results obtained utilizing a new dispenser cathode with uniform surface emission, produce self-oscillations close to the expected frequencies.
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