Optimization of a vacuum spark discharge for soft-X-ray emission

A 1-kJ vacuum-spark device has been developed as a pulsed X-ray source for applications in the field of microlithography, microscopy, flash radiography, etc. The optimization works for the pulsed soft-X-ray emission are presented. The optimization was made in two steps: 1) X-ray emission maximization and 2) X-ray emission reproducibility. For the X-ray emission maximization, the changed parameters were the main capacitor energy, the anode-cathode geometry, the anode material, and the trigger pulse amplitude. The best combination is 1 kJ for the main capacitor energy, a conical shape for the anode, 6-12 mm anode-cathode separation, iron for the anode material, and 14-15 kV for the trigger pulse amplitude. In these conditions, energies of up to 10 J per pulse are obtained in the 3-40 keV range for the X-ray pulses. In order to have a good reproducibility, a high-power, very fast, high-voltage trigger pulse is required. To fulfill these conditions, a pulse transformer and an air spark gap were added to the initial triggering device (a magnetic pulse compression circuit). With the new trigger pulse (20 J per pulse, 50-ns rise-time, 22-kV amplitude in an open circuit), an acceptable X-ray emission reproducibility was obtained.