Temporal correlation between hard x rays and radio emissions in the MHz and GHz frequency ranges generated by a laboratory high-voltage discharge

We investigate the temporal correlation between very-high-frequency (VHF, at frequencies of the order of 10-100MHz), ultrahigh-frequency (UHF, at frequencies within 1-6GHz), and x-ray (with photon energies more than 10keV) emissions, which accompany the development of a high-voltage discharge initiated in a long gap at voltages up to 1MV. The x-ray and UHF emissions are found to emerge starting approximately from the prepulse current onset observed before the discharge gap breakdown and both gradually decay coming to this instant. The UHF emission spectrum is represented with frequencies up to 6GHz, with the highest spectral power being achieved within 1-2GHz. The radio emission power drops sharply at frequencies below 1GHz and increases closer to 150MHz with the highest spectral power of VHF radiation being reached within 60-90MHz. The VHF emission can appear before the discharge current and UHF emission onsets in the form of 100-ns-long prepulses, and its intensity significantly increases as the UHF emission starts. The analysis of the temporal correlation between x rays, VHF, and UHF radiations, discharge current, and voltage waveforms indicates that the generation mechanisms of the discharge electromagnetic radiations are difficult to be interpreted in terms of the developing or colliding streamer concepts.