Volume x-ray radiation of fast electrons at high-voltage nanosecond breakdown of dense gases

X-ray spectra of fast electrons injected into a gas subjected to an external electric field are simulated. For weak fields, the simulated spectra are in good agreement with the Kramers theory. For strong fields, the x-ray spectrum differs considerably from the well-known spectra of x-ray tubes with massive cathodes and the radiated energy is much higher. As the voltage applied to a given discharge gap grows, the number of emitted photons reaches a maximum, while the radiated energy tends to saturate instead of decreasing.