Nickolay M. Zubarev

Direct numerical simulation of the strongly nonlinear stages of instability development for a non-conducting liquid with a charged free surface in a normal electric field is performed. It is demonstrated that two main stages of the instability can be distinguished: an initial stage, during which dimples appear on the surface, and a developed stage, during which these dimples transform into expanding bubbles. The bubble size increases with increasing applied field, despite the fact that the scale corresponding to the dominant mode of the instability decreases.

Direct numerical simulation of the strongly nonlinear stages of instability development for a non-conducting liquid with a charged free surface in a normal electric field is performed. It is demonstrated that two main stages of the instability can be distinguished: an initial stage, during which dimples appear on the surface, and a developed stage, during which these dimples transform into expanding bubbles. The bubble size increases with increasing applied field, despite the fact that the scale corresponding to the dominant mode of the instability decreases.

Application of 2 MV, 100 ps, 100 GW pulse from a solid-state driver for powering vacuum diodes with explosive emission cathodes is described. The first sealed-off diode with a “tube-plane” electrode geometry operated in a mismatched mode. A tantalum foil cathode emitted a divergent electron flow with the current of ≈ 12 kA, energy of ≈ 3.2 MeV and peak power of ≈ 40 GW through a beryllium window. Acceleration gradient of 500 МeV/m was achieved in the 6-mm long diode gap. Another diode with a tubular graphite cathode and a strong guiding magnetic field was closer to matching with the driver. The bunch current of 22–27 kA and energy of ≈ 2 MeV were estimated as it passed aluminum filters with thickness of 0.015–5.7 mm shielding the collector sensor. The charge of recorded bunch exceeded 2500 nC. These parameters do not contradict the ones obtained in diode simulation by particle-in-cell method.

An experimental study was performed to analyze the emission of runaway electrons in atmospheric gaps in the presence of an advanced pulse, preceding the subnanosecond leading edge of the main voltage pulse, during which the electric field at the cathode reaches a strength sufficient for the occurrence of runaway electrons. The pulses of the runaway electron current vary due to the appearance and dynamics of the cathode plasma layer from the boundary of which the electrons run away. Plasma is generated by ionization of the gas by field emission electrons as early as during the advance pulse. It is shown that the characteristics of the runaway electron flow depend on the amplitude and duration of this pulse, as well as on the presence of an interval between it and the leading edge of the main voltage pulse.

The influence of a uniform axial magnetic field on the drift of charged particles from the apex of a conical cathode in the surrounding gas is studied. A self-consistent analytical model is developed to describe this influence within the space-charge-limited current regime. Assuming the shape of the drift region to be conical, it is possible to determine the dependence of its opening angle on the electrode angle, the magnitude of the applied magnetic field, and the mobility of charge carriers.