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Electrodeless streamer inception on an epoxy surface under AC voltage stress was investigated for different gas compositions and pressures, with a focus on the pressure region below 1 bar. For this purpose, we used a set-up with cylindrical electrodes embedded out-of-axis in a cylindrical epoxy rod. Experiments were performed in N2, SF6, ambient air, Ar and CO2. The discharge inception voltage was measured, from which the critical value K of the ionization integral was reconstructed assuming a non-disturbed Laplacian field distribution. We have validated that for electropositive gases Ar an N2 the generally assumed value of K  =  10 is in good agreement with our measurements. For electronegative gases, however, the experimentally obtained values turned out to be considerably higher. We attribute this discrepancy mainly to the statistical time delay of the first electron; to increase the probability of discharge inception in a critical region, it was necessary to extend the critical area by means of applying an overvoltage to the system.

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When charged particles impact the surface of a solid material, they can be backscattered from that surface or cause secondary electrons to be emitted. The ratio of the number of emitted electrons (secondary and backscattered) to the incident electrons number is known as the total electron emission yield. Knowledge of this total electron emission yield (TEEY) is highly required in various modern technologies, and several experimental techniques were developed with the purpose of its determination. However, experimental data, obtained with different methods are not in good agreement and sometimes are even contradictive. The purpose of this paper is to review the existing experimental methods of investigating electron emission (EE) from dielectric surfaces, as well as to analyze their accuracy and reliability. The focus is on the processes of the emission from thick layers of insulators which is the case for modern high voltage technologies.

The influence of a small addition of argon (2-5%) on the parameters of a strongly non-uniform microwave discharge (with the electrode microwave discharge as an example) in nitrogen at reduced pressures was studied. Experiments showed that the small addition of Ar strongly affected the discharge: it increased in size, and the power absorbed in the plasma and the emission intensities of nitrogen bands reduced. A self-consistent 2D modeling of the discharge was carried out. The model included the Maxwell, Poisson and Boltzmann equations and a set of balance equations for neutral excited and charged plasma species. The processes involving vibrationally excited ground state molecular nitrogen were taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. The results of modeling and experiment were in qualitative agreement. Additional information about the discharge allowed us to explain the experimental results. It was shown that this effect was influenced by a strong spatial non-uniformity of direct electron impact ionization rate, difference in diffusion and mobility coefficients, and difference in diffusion and volume processes of the loss of the main ions. This effect can be observed in all types of discharges if these conditions are satisfied.