We investigate structure and evolution of long positive spark breakdown; and we study at which stage pulses of hard X-rays are emitted. Positive high-voltage pulses of standardized lightning impulse wave form of about 1 MV were applied to about 1 meter of ambient air. The discharge evolution was imaged with a resolution of tens of nanoseconds with an intensified CCD camera. LaBr 3 (Ce + ) scintillation detectors recorded the X-rays emitted during the process. The voltage and the currents on both electrodes were measured synchronously. All measurements indicate that first a large and dense corona of positive streamers emerges from the high voltage electrode. When they approach the grounded electrode, negative counter-streamers emerge there, and the emission of hard X-rays coincides with the connection of the positive streamers with the negative counter-streamers. Leaders are seen to form only at later stages.

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Please cite as: P O Kochkin et al 2015 J. Phys. D: Appl. Phys. 48 025205 Abstract.We investigate the development of meter long negative discharges and focus on their X-ray emissions. We describe appearance, timing and spatial distribution of the X-rays. They appear in bursts of nanosecond duration mostly in the cathode area. The spectrum can be characterized by an exponential function with 200 keV characteristic photon energy. With nanosecond-fast photography we took detailed images of the pre-breakdown phenomena during the time when X-rays were registered. We found bipolar discharge structures, also called "pilot systems", in the vicinity of the cathode. As in our previous study of X-rays from positive discharges, we correlate the X-ray emission with encounters between positive and negative streamers. We suggest that a similar process is responsible for X-rays generated by lightning leaders.

A simple new method of characterization of porous materials is developed in this paper. In this method, we combine the traditional Kelvin equation with an improved statistical adsorbed film thickness to account for the finite dimension of pores. The adsorption process in a pore is viewed as a molecular layering process followed by a filling mechanism. Here, we argue that this mechanism is valid not just for mesopores but also micropores because of our allowance for the statistical film thickness to be a function of not only pressure but pore size as well. The model is tested against the data of nitrogen adsorption onto activated carbon samples:  the fitting is found to be excellent, and the results are comparable with those obtained by other methods such as the DFT method. The much shorter computation time required by this method compared to that by DFT makes this method a very attractive alternative in the PSD characterization.

A lightning surge generator generates a high voltage surge with 1.2 μs rise time. The generator fed a spark gap of two pointed electrodes at 0.7 to 1.2 m distances. Gap breakdown occurred between 0.1 and 3 μs after the maximum generator voltage of approximately 850 kV. Various scintillator detectors with different response time recorded bursts of hard radiation in nearly all surges. The bursts were detected over the time span between approximately half of the maximum surge voltage and full gap breakdown. The consistent timing of the bursts with the high-voltage surge excluded background radiation as source for the high intensity pulses. In spite of the symmetry of the gap, negative surges produced more intense radiation than positive. This has been attributed to additional positive discharges from the measurement cabinet which occurred for negative surges. Some hard radiation signals were equivalent to several MeV. Pile-up occurs of lesser energy X-ray quanta, but still with a large fraction of these with an energy of the order of 100 keV. The bursts occurred within the 4 ns time resolution of the fastest detector. The relation between the energy of the X-ray quanta and the signal from the scintillation detector is quite complicated, as shown by the measurements.PACS 52.70, La 52.80.Mg, 92.60.Pw