The time-resolved electric field in a fast ionization wave discharge in a diffuse nanosecond pulse discharge plasma in atmospheric pressure air is measured using the Electric Field Induced Second Harmonic (E-FISH) diagnostic. The electric field is placed on an absolute scale by calibration against a Laplacian field. At relatively low peak voltages, when the plasma is generated only near the pin high-voltage electrode, the electric field is measured ahead of the ionization wave during the entire voltage pulse, exhibiting a strong field enhancement compared to the Laplacian field, by about an order of magnitude. As the peak voltage is increased and the ionization wave traverses the laser beam, the electric field is measured both ahead of the wave and behind the ionization front, where the field drops rapidly due to the charge separation and plasma selfshielding. When the wave reaches the grounded electrode, the discharge transitions into a conduction phase in which the potential is redistributed within the gap. The electric field in the vicinity of the pin then increases again, following the applied voltage waveform for the rest of the pulse. The effective time resolution of the present measurements is 150 ps. Based on the single shot data, we find that the peak electric field in the wave front is moderately influenced by the applied voltage and varies between 160 to 210 kV/cm. This study demonstrates the viability of the E-FISH diagnostic for this class of atmospheric pressure discharges and paves the way for future in-depth studies of this particular problem.

The present work is devoted to the study of the spatio-temporal distribution of the reduced electric field (REF) in a 10 ns diffuse atmospheric air discharge at very high overvoltage, in a pin-to-plane electrode geometry. The REF is derived through the intensity ratio of two wellknown transitions of molecular nitrogen: N 2 (C-B, v′=2, v″=5) and N 2 + (B-X, v′=0, v ″=0). The achieved temporal resolution is 500 ps, while the spatial resolution is better than 300 μm and 400 μm in the axial and radial direction, respectively. Due to the fast rise time of the voltage pulse, the total electric field is dramatically disturbed by the contribution of the Laplacian field, contrary to low-voltage streamer discharges. Electric field values above the ionization threshold are sustained all along the plasma channel. The dynamics of the high-voltage diffuse discharge seem similar to those of classical streamers with a very high field zone propagating towards the plane electrode then followed by a backward neutralization wave. However, some noticeable discrepancies are reported between the experimentally-obtained distributions of the axial electric field at 65 and 85 kV and those computed by means of a fluid model. They stress in particular the origin and the role of the background electrons in the discharge dynamics. Several limitations of the applicability of the intensity-ratio method for the study of very transient phenomena are also discussed. At first, the effect of the temporal integration of the signals is addressed by comparing them with an artificial averaging of the modeling results. Then, the effect of the non-stationarity of the collected signals is put forward by applying the intensityratio method under steady-state assumption or not. Lastly, the overestimation of the electric field in the discharge front due the relatively long effective lifetime of N 2 (C, v′=2) compared to the discharge dynamics is discussed.

Measurements are reported of electromagnetic emission close to the cyclotron frequency of energetic ions in JET plasmas heated by waves in the ion cylotron range of frequencies (ICRF). Hydrogen was the majority ion species in all of these plasmas. The measurements were obtained using a sub-harmonic arc detection (SHAD) system in the transmission lines of one of the ICRF antennas. The measured ion cyclotron emission (ICE) spectra were strongly filtered by the antenna system, and typically contained sub-structure, consisting of sets of peaks with a separation of a few kHz, suggesting the excitation of compressional Alfvén eigenmodes (CAEs) closely spaced in frequency. In most cases the energetic ions can be clearly identified as ICRF wave-accelerated 3 He minority ions, although in two pulses the emission may have been produced by energetic 4 He ions, originating from third harmonic ICRF wave acceleration. It is proposed that the emission close to the 3 He cyclotron frequency was produced by energetic ions of this species undergoing drift orbit excursions to the outer midplane plasma edge. Particle-in-cell and hybrid (kinetic ion, fluid electron) simulations using plasma parameters corresponding to edge plasma conditions in these JET pulses, and energetic particle parameters inferred from the cyclotron resonance location, indicate strong excitation of waves at multiple 3 He cyclotron harmonics, including the fundamental, which is identified with the observed emission. These results underline the potential importance of ICE measurements as a method of studying confined fast particles that are strongly suprathermal but have insufficient energies or are not present in sufficient numbers to excite detectable levels of γ-ray emission or other collective instabilities. * See the author list of "X. Litaudon et al 2017 Nucl. Fusion 57 102001" arXiv:1806.05149v1 [physics.plasm-ph]

This work presents experiments and modelling of OH densities in a radio-frequency driven atmospheric-pressure plasma in a plane-parallel geometry, operated in helium with small admixtures of oxygen and water vapour (He+O2+H2O). The density of OH is measured under a wide range of conditions by absorption spectroscopy, using an ultra-stable laser-driven broad-band light source. These measurements are compared with 0D plasma chemical kinetics simulations adapted for high levels of O2 (1%). Without O2 admixture, the measured density of OH increases from 1.0×10 14 to 4.0×10 14 cm -3 for H2O admixtures from 0.05% to 1%. The density of atomic oxygen is about 1×10 13 cm -3 and grows with humidity content. With O2 admixture, the OH density stays relatively constant, showing only a small maximum at 0.1% O2. The simulations predict that the atomic oxygen density is strongly increased by O2 addition. It reaches 10 15 cm -3 without humidity, but is limited to 10 14 cm -3 beyond 0.05% water content. The addition of O2 has a weak effect on the OH density because, while atomic oxygen becomes a dominant precursor for the formation of OH, it makes a nearly equal contribution to the loss processes of OH. The small increase in the density of OH with the addition of O2 is instead due to reaction pathways involving increased production of HO2 and O3. The simulations show that the densities of OH, O and O3 can be tailored relatively independently over a wide range of conditions. The densities of O and O3 are strongly affected by the presence of small quantities (0.05%) of water vapour, but further water addition has little effect. Therefore, a greater range and control of the reactive species mix from the plasma can be obtained by the use of well-controlled multiple gas admixtures, instead of relying on ambient air mixing.

Control of the plasma chemistry is essential for the effectiveness of atmospheric pressure plasmas in many applications. For this, the effects of the humidity of the feed gas on the discharge chemistry need to be considered. Detailed studies are scarce and many of them are dominated by surface interactions, obscuring any volume effects. Here, a negative nanosecond pulsed discharge is generated in a pin-pin 3 mm gap geometry in He+H2O that enables the study of volume kinetics due to minimal surface area. The effect of humidity on the discharge development, electric field and electron density is investigated through experiments and modelling. It is found that the presence of water vapour affects both the electron density at the start of the pulse (remaining from the previous pulse) and the ionisation rates during the ignition phase, leading to a complex dependence of the discharge development speed depending on the water concentration. The electron decay is studied using the 0D global kinetics model GlobalKin. The dominant reactions responsible for the electron decay depending on the concentration of water vapour are determined by comparing experimental and simulated results and these reactions are grouped in simplified kinetic models. It is found that with water concentrations increasing from 0 to 2500 ppm, the complexity of the dominant reactions increases with in particular O2 +, H2O3 + and water clusters becoming important for high water concentrations. This work also provides experimental data for validation of kinetic models of plasmas in controlled environments.

We report results on the influence of relative humidity on the propagation speed, the intensity of the emitted light, the energy and the gas temperature of a pin-to-plane nanosecond pulsed discharge at atmospheric pressure in synthetic air. The discharge is generated under very high overvoltage (several tens of kV) so that it propagates with a voluminous, diffuse, and stable pattern. It is shown that the water vapor content has a strong impact on the discharge dynamics for gas mixtures with high relative humidity and for the highest electric field values. In particular, for voltage pulse amplitudes higher than 65 kV and relative humidity higher than 30%, the propagation abruptly slows down and the light intensity profiles show a stronger emission at the pin which weakens in the rest of the gap. The electric energy is slightly lower in humid air, independently of water vapor concentration. Also, time and spatially resolved gas temperature measurements carried out for different voltages show a late and significant heating at the pin whatever the water vapor content. An evaluation of the energy consumed in fast heating processes is proposed, showing an increased energy consumption at the pin in highly humid air. Besides, the hypotheses allowing for the consideration of the rotational temperature of the second positive system of nitrogen (N2(SPS)) as the gas temperature under high electric field conditions are discussed.

The spatio-temporal distribution of O and H radicals in a 90 ns pulsed discharge, generated in a pin-pin geometry with a 2.2 mm gap, in He + H2O (0.1 and 0.25%), is studied both experimentally and by 1D fluid modelling. The density of O and H radicals as well as the effective lifetimes of their excited states are measured using picosecond resolution Two-Photon Absorption Laser Induced Fluorescence (ps-TALIF). Good agreement between experiments and modelling is obtained for the species densities. The density of O and H is found to be homogenous along the discharge axis. Even though the high voltage pulse is 90 ns long, the density of O peaks only about 1 μs after the end of the current pulse, reaching 2x1016cm-3 at 0.1% H2O. It then remains nearly constant over 10 μs before decaying. Modelling indicates that the electron temperature (Te) in the centre of the vessel geometry ranges from 6 to 4 eV during the peak of discharge current, and after 90 ns, drops below 0.5 eV in about 50 ns. Consequently, during the discharge (<100 ns), O is predominantly produced by direct dissociation of O2 by electron impact, and in the early afterglow (from 100ns to 1 μs) O is produced by dissociative recombination of O2 +. The main loss mechanism of O is initially electron impact ionisation and once Te has dropped, it becomes mainly Penning ionisation with He2* and He* as well as 3-body recombination with O+ and He. On time scales of 100-200 μs, O is mainly lost by radial diffusion. The production of H shows a similar behaviour, reaching 0.45x1016 cm-3 at 1 μs, due to direct dissociation of H2O by electron impact (<100ns) followed by electron-ion recombination processes (from 200 ns to 1.5 us). H is dominantly lost through Penning ionisation with He* and He2* and by electron impact ionisation, and by charge exchange with O+. Increasing concentrations of water vapour, from 0.1 to 0.25%, have little effect on the nature of the processes of H formation but trigger a stronger initial production of O, which is not currently reproduced satisfactorily by the modelling. What emerges from this study is that the built up of O and H densities in pulsed discharges continues after electron-impact dissociation processes with additional afterglow processes, not least through the dissociative recombination of O2 + and H2 +.

The spatio-temporal rovibrational excitation and relaxation mechanisms of N2(X) in the post-discarge of a 10 ns high-voltage diffuse discharge are studied by Spontaneous Raman Scattering. It is shown that the vibrational excitation of nitrogen molecules remains high despite the strong electric fields applied during the discharge itself and the relaxation processes are similar to lower voltage ns discharges. The main differences with the lower field discharges are rather visible at the beginning of the discharge with a specific spatial volume distribution and a significant vibrational non-equilibrium between v=0,1 and v>1. The spatial distribution of the rovibrational excitation of the diffuse discharge is very wide radially, consistent with the sustainability of fields greater than 100 Td over nearly 8 mm during propagation. The initial rovibrational excitation is inhomogeneous along the axis. The gas temperature reaches up to about 1200 K close to the pin (85 kV, ambient air) while it remains below 500 K in the rest of the volume. It is possible to control the heating of the discharge without greatly modifying the energy transfer mechanisms by adjusting the duration of the voltage pulse. In terms of reactivity, high atomic oxygen densities seem to be very localized in the vicinity of the pin (10 24 m -3 at 1.5 mm from the pin, corresponding to about 20 % dissociation). This inhomogeneity reflects the distribution of energy in the volume of the discharge. The main effects of humidity are also studied. It amplifies the fast heating and accelerates the decay of atomic oxygen in the post-discharge. No significant acceleration of the V-T relaxation of nitrogen due to the addition of water vapour was observed for the studied conditions. A shock wave was identified which is triggered at around 500 ns.