Study of the vacuum spark plasma soft X-ray sources

Abstract

Summary form only given, as follows. The vacuum spark plasma as a soft X-ray source has been extensively studied due to its high efficiency of X-ray emission when it is able to operate in the so-called micropinch regime. Applications of soft X-rays in the fields of lithography, biology, medicine, etc. usually require accurate dose controllability, besides proper spectrum and uniformity. To meet this challenging requirement, ALFT has been developing a unique low inductance vacuum spark X-ray source for several years now, which is mainly characterized by relative low stored electrical energy (∼2nf, ∼15 kV) and high repetition rate of operation (up to 30 kHz), as well as a combination of two types of X-ray emission, i.e., electron bombardment and plasma pinch. In this study, within the same high repetition regime, the X-ray output has been explored as a function of the storage capacitance. The experimental results show that the X-ray output per shot increases linearly with the storage capacitance from 1 nF up to 9 nF. This helps in better understanding the physics of X-ray emission in this low energy regime, and practically opens a new way to increase the total X-ray power without loss of efficiency by trying to enhance the pinch phenomenon at expense of electron bombardment. Another experimental study was carried out on anode erosion at different repetition frequencies, anode materials, and storage capacitance, in order to understand the mechanism of debris emission, and improve on debris mitigation. On the theoretical side, a simple model has been studied on the dynamics of the production of metallic plasmas as in the vacuum spark, as well as recombination, diffusion and recovery of the initial vacuum condition. The purpose was to see if there was a theoretical limit to the attainable frequency of operation of our radiation source. The results of the theoretical analysis indicate that there is a limit, which is in fair agreement with the limit found experimentally. As far as application of our radiation sources is concerned, a preliminary study has been carried out on the use of soft X-rays in the field of biology for protein analysis, and it appears that the VSX-400, our latest model, can become a useful diagnostic tool in this field.