In this work, the propagation of fast ionisation wave (FIW) discharges driven by negative nanosecond high-voltage (HV) pulses and the x-ray spectrum from the FIW discharges are investigated. The measurement using a capacitive probe shows that the peak axial electric field (E z peak ) at each position rises with the increase of the applied voltage amplitude (U max ), resulting in an increase of the FIW velocity (V FIW ). The influence of the pre-ionisation on the FIW propagation is estimated from the relationship between V FIW and E . z peak The pre-ionisation effect appears to be enhanced with the increase of U max and it is suspected to be due to the presence of more high energy electrons. This is supported by the measured x-ray spectrum, showing a higher count rate and a more elevated high energy tail with a larger U max . The spatially resolved measurement of the x-ray spectrum shows that, with the increase of the distance away from the HV electrode (as the cathode), the lower energy part (< ∼15 keV) of the x-ray spectrum keeps decaying, while its high energy tail (> ∼25 keV) rises at first and then decays. Based on the spatially resolved x-ray spectrum, it is inferred that, as high energy electrons move away from the HV electrode, the peak of their energy distribution is shifted toward a higher value, while their total number decreases. Further analysis implies that the electrons emitted from the cathode obtain a large percent of the applied potential in the vicinity of the cathode and become runaway electrons. The electrons with a high initial energy travel freely in the discharge region, due to a decreasing electron collisional cross section with its energy.

High-energy electron beam can be generated during the breakdown process in a gas diode at elevated pressures. The design of electrodes in the gas diode affects the parameters of these electron beams. In this paper, investigation on parameters of the runaway electron beam (REB) in air and nitrogen excited by nanosecond pulsed generators are presented. Experiments were carried out with gas diodes by using cathode and anode made of different materials, including stainless steel, aluminum, copper and titanium. Experimental results show that, in the case of nanosecond-pulse breakdown, the cathode material significantly affects the amplitude of the REB current pulse when either pressure or gap spacing are small. When the pressure is low, the REB current is highest for the stainless steel cathode due to its small electron work function. However, the REB current for the aluminum, copper and titanium cathodes is higher than that of stainless steel cathode at atmospheric pressure because of the Malter effect on their surfaces. The dependence of the amplitude of the REB current on the anode foils is investigated by using aluminum, titanium, copper and tantalum. The experimental results show that the atomic number of the metal, as well as mass mean free path of the electron in it contribute to the amplitude of the REB current pulse. Based on the REB currents of different anode foils, energy distribution of runaway electrons is estimated. Electron distribution with an average energy of ~55 keV is obtained when the amplitude of the applied voltage pulse across the discharge gap is 110 kV. The results may contribute to the design of the gas diode for the generation of high-energy electron beams in nanosecond-pulse breakdown.

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