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The results of a complex study of the destructive effect of a nitrogen plasma jet at atmospheric pressure onto the surface of the MPG-6 graphite are presented. The space-time changes in the rate of decrease in the samples material, the temperature of their surface, the electron temperature of plasma in the oncoming flow and the plasma composition in the region of 'injection' were all experimentally determined. Using spectroscopic methods with high spatial (50 μm) and time (0.5 s) resolutions changes in the concentration of emitting carbon atoms (line CI 247.9 nm) and CN (transition B2Σ+ − X2Σ+) near the surface of isotropic graphite were recorded, which occur in the process of intense and long (100–150 s) exposure of graphite to a nitrogen plasma jet. The change in the vibrational and rotational temperatures of the radical CN and the molecular ion in the near-surface region of the plasma is determined by the method of 2D matrix spectroscopy. An analysis of the relative populations of the excited NI states in the interaction region at the quasistationary stage of sample heating revealed spatial-temporal changes in the electron temperature in the near-surface plasma, indicating an increase in its enthalpy caused by the rapid exothermic CN + N → C + N2 substitution process, which leads to a decrease in the heat flow acting on sample. The analytical solution of the problem of temperature change in a relaxing nitrogen plasma jet in the absence of graphite confirmed the experimentally observed slow cooling of it, caused by a number of exothermic recombination processes taking place in the plasma.

We have carried out cross-correlated high-resolved electrodynamical and spectro-optical measurements of different characterizations in a high-stable regime of the dc positive streamer corona in short gap in atmospheric density laboratory air. Special attention was paid to the measurement of absolute intensities of the second positive (SPS) and first negative (FNS) systems of molecular nitrogen as well as for further improvement in the spatiotemporal resolution along with high synchronization and stability. In the cathode region, the FNS and SPS emission temporal waveforms during primary streamer development have been recorded with the cross-correlated synchronization not worse than 0.1 ns and spatiotemporal resolution around 0.01–0.1 mm along the axis and 0.2–0.4 ns in the multiphoton mode of the PMT-based detector and in the single pulse acquisition mode of the measuring system. The shortest rise-times of the corresponding voltage waveforms were found to be around 0.3–0.4 ns for the SPS (0,0)-band and 0.2–0.3 ns for the FNS (0,0)-band, the full widths at the half-altitude ranging around 1.4–1.5 ns and 0.5–0.6 ns, respectively. Again, the absolute values and the ratio of the synchronized temporal waveforms of the SPS to the FNS (0,0)-bands have been recorded across the entire gap in the accumulation and averaging acquisition mode of the measuring system with somewhat worse temporal resolution but with adequate stability and stochasticity. Various supporting characteristics such as the synchronous anode and cathode electric current waveforms, the streamer velocity and the gas temperature within the streamer channel region have been measured and presented as well. All the data in the aggregate allow reconstruction of the 2D structure of the streamer head generally and of the electric field and the electron number density partially.

In this second of two papers, we have developed a fully self-consistent method of diagnostics of streamer discharge plasmas based on the analysis of absolute intensities of the second positive (SPS) and first negative (FNS) systems of molecular nitrogen. The theory of the method combines a spectroscopic technique for the calculation of temporal waveforms of the nitrogen absolute SPS and FNS (0, 0)-band emission and a self-consistent semi-analytical parametric axially symmetric 1.5D model of the filamentary streamer head. The spectroscopic technique takes into account characterizations of all units of the measuring spectro-optical channel, including spatiotemporal resolution and resolution in wavelength. Also, chromatic aberrations of the focusing lighting lens, errors in the adjustment of the object image onto the input slit as well as the correction of the slit function at the non-uniformly illuminated input slit of the monochromator on width are taken into account. The model of the streamer head is characterized by some trial-consistent allowable parametric on-axis profile of the electric field combined with the electron and total space charge number density profiles fully self-consistent with each other and with the trial electric field. Additionally, the corresponding 2D configuration for the field and the electron number density is constructed via special ellipse-like geodesic lines. The absolute values and the synchronous ratio of the 1L spatially and time-resolved cross-correlated temporal waveforms of the SPS to the FNS (0, 0)-bands, the ratio of their altitudes and the total emission power for both the waveforms measured in paper I have been taken into account to reconstruct the 2D structure of the streamer head. With a procedure fitting the directly computed output voltage waveforms to the experimental ones, the peak values of the electric field and the electron number density within the streamer head at mid-gap distances have been found to be 430?500?Td and (2?3) ? 1014?cm?3, respectively. And the corresponding absolute electric field is thus 70?80?kV?cm?1 at the initial gas temperature within the repetitive streamer region of 450?K.

Sequences of picosecond pulses with a total energy in the pulse train of about 1 mJ are transmitted through a hollow-core photonic-crystal fibre with a core diameter of approximately 14 µm. The fluence of laser radiation coupled into the core of the fibre under these conditions exceeds the breakdown threshold of fused silica by nearly an order of magnitude. The laser beam coming out of the fibre is then focused to produce a breakdown on a solid surface. Parameters of laser radiation were chosen in such a way as to avoid effects related to the excitation of higher order waveguide modes and ionization of the gas filling the fibre in order to provide the possibility to focus the output beam into a spot with a minimum diameter, thus ensuring the maximum spatial resolution and the maximum power density in the focal spot.