The dynamics of discharge propagation and x-ray generation in nanosecond pulsed fast ionisation wave in 5 mbar nitrogen

Abstract

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 () at each position rises with the increase of the applied voltage amplitude (Umax), resulting in an increase of the FIW velocity (VFIW). The influence of the pre-ionisation on the FIW propagation is estimated from the relationship between VFIW and The pre-ionisation effect appears to be enhanced with the increase of Umax 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 Umax. 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.