Generation of runaway electrons and X-ray emission during breakdown of atmospheric-pressure air by voltage pulses with a
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Generation of Runaway Electrons and X-ray Emission during Breakdown of Atmospheric-Pressure Air by Voltage Pulses with an ~0.5-μs Front Duration I. D. Kostyrya and V. F. Tarasenko Institute of High-Current Electronics, Siberian Branch, Russian Academy of Sciences, Akademicheskii pr. 2/3, Tomsk, 634055 Russia e-mail: [email protected] Received August 19, 2014
Abstract—Results are presented from experiments on the generation of runaway electron beams and X-ray emission in atmospheric-pressure air by using voltage pulses with an ~0.5-μs front duration. It is shown that the use of small-curvature-radius spherical cathodes (or other cathodes with small curvature radii) decreases the intensity of the runaway electron beam and X-ray emission. It is found that, at sufficiently high voltages at the electrode gap (Um ~ 100 kV), the gap breakdown, the formation of a spark channel, and the generation of a runaway electron beam occur over less than 10 ns. At high values of Um behind the anode that were reached by increasing the cathode size and the electrode gap length, a supershort avalanche electron beam with a full width at half-maximum (FWHM) of up to ~100 ps was detected. At voltages of ~50 kV, the second breakdown regime was revealed in which a runaway electron beam with an FWHM of ~2 ns was generated, whereas the FWHM of the X-ray pulse increased to ~100 ns. It is established that the energy of the bulk of runaway electrons decreases with increasing voltage front duration and is ≤30 keV in the first regime and ≤10 keV in the second regime. DOI: 10.1134/S1063780X15030058
1. INTRODUCTION Runaway electron beams (REBs) and X-ray emission behind a thin-foil anode during high-voltage breakdown in atmospheric-pressure air and other gases are easy to produce and record by supplying voltage pulses with a subnanosecond front duration to the electrode gap with a small-curvature-radius cathode (see, e.g., [1–9]). A decrease in the voltage pulse front (all other conditions being the same) allows one to achieve the highest REB intensities and the highest X-ray exposure doses. As the duration of the voltage pulse front increases, the number of electrons in the supershort avalanche electron beam (SAEB) substantially decreases, which makes them difficult to detect. To date, only one scientific group has reported [10, 11] the detection of REBs behind a foil during gas breakdowns initiated by microsecond voltage pulses. The REB measurements in [10, 11] were carried out during breakdown of atmospheric-pressure air. In [10], the presence of an REB was determined from the trace it left on an RT-1 film. The number of runaway electrons was estimated from the blackening of the film and was found to be about 107. Fast electrons were recorded only at the front of the conduction current pulse. According to [10], the runaway electron energy at a breakdown voltage of
70 kV corresponded to the applied voltage. It was also asserted in [10] that REBs were produced at voltage front durations of 0.4, 1.2, and 2 μs. In [11], the duration of
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