High-Voltage Nanosecond Discharge as a Means of Fast Energy Switching

The formation of a nanosecond discharge with the use of a Hamamatsu streak-camera and with simultaneously wideband (10 GHz) measurement of voltage and displacement current caused by a streamer in one pulse has been studied. Nanosecond voltage pulses of various amplitudes (16, 20, and 27 kV) were applied across a point-to-plane gap (8.5 mm) filled with air at various pressures (13, 25, 50, 100, and 200 kPa). It was found that the voltage across the gap drops as soon as a streamer appears in the vicinity of the pointed electrode. At the same time, a pre-breakdown current begins to flow. The magnitude of the pre-breakdown current, as well as the voltage drop, is determined by the rate of formation of dense plasma and, accordingly, by the rate of redistribution of the electric field in the gap. The streamer velocity determines the rise time and amplitude of the current. The higher the streamer velocity, the shorter the rise time and the higher the amplitude of the pre-breakdown current. The propagation of a backward and third ionization waves was observed both with the streak camera and by measuring the displacement current. As they propagate, the discharge current increases to its amplitude value.

Measurement of the expansion velocity of the plasma high-current vacuum arc discharge

In this work, we present experimental results on measuring the velocity of vacuum arc discharge plasma expansion. In the experiments, two designs of plasma guns were used. In the first version, the end of the arc discharge cathode was located below the plane of the anode, and the surface of the insulator separating them was parallel to the axis of symmetry of the plasma gun. In this design, the arc discharge plasma escapes the anode through a hole, the diameter of which coincides with the diameter of the cathode. In the second variant, the plane of the end face of the arc discharge cathode coincided with the plane of the anode, and the surface of the insulator separating them was located perpendicular to the axis of symmetry of the plasma gun. To obtain an image of plasma in the optical range, an FER-7 optical streak camera was used. Based on the results obtained, it can be concluded that the expansion velocity of the plasma of a high-current vacuum arc discharge does not depend on the design of the guns considered in this experiment.

Gas discharge sustained by powerful THz and sub-THz gyrotrons in the mixtures of noble gases with nitrogen

The discharge propagation velocity towards electromagnetic radiation of sub-THz and THz bands was measured in various noble gases (argon, krypton) mixtures with nitrogen in the wide pressure range (0.1 – 2 atm) for various field intensities into the focal spot (from dozen of kW/cm2 to several MW/cm2). In the experimental setups two different gyrotrons were used. In case of 263 GHz it was CW gyrotron with power up 1 kW, in case of 670 GHz – pulsed gyrotron (20 μs) with power up to 40 kW. In both cases the focusing system provided the size of the focal spot of (2–3)·λ, which ensured the investigation of discharge phenomena in a wide pressure range (0.1 – 2 atm). In both cases discharge appeared in the focal spot spread towards heating radiation into the area with the field intensity much less than one in the focal spot. Velocity of the discharge propagation was measured by using photos from speed camera with small exposure (down to 20 ns) and streak camera. It was demonstrated that discharge velocity increase along with pressure decrease and drops with electric field decrease as it moves away from the focal spot.