X-ray emission from subnanosecond gas breakdown

The x-ray emission of highly overvolted spark gaps under electron runaway conditions is investigated. The pulse source, a RADAN 303 A, is connected to a test chamber through an oil-filled coaxial line, a coupling lens, and a biconical transmission line section, with a symmetrical arrangement attached on the opposite side of the chamber with a matching load. The test chamber allows pressure variation from 10⁻⁶ – 670 torr with argon or dry air used as a background gas. Voltage pulses with amplitudes of 40–150 kV, risetimes less than 200 ps, and FWHM less then 300 ps are applied across hemispherical electrodes with 1 mm spacing. A scintillatorphotomultiplier combination with a temporal resolution of 2 ns is used as X-ray detector. Metallic absorber foils of different thicknesses are used to obtain a rough energy spectrum of the x-rays and electrons in the range of about 10 to 150 keV. Results show a high electron-energy component (≫60 keV) existing up to atmospheric pressure, and an intense soft component (5 to 20 keV) at pressures around 100 torr. The observations are compatible with gaseous ionization and runaway conditions for extremely high E/p.