Role of the excited electronic states in the ionization of ambient air by a nanosecond discharge (original) (raw)
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Fully ionized nanosecond discharges in air: the thermal spark
Plasma Sources Science and Technology, 2020
The formation and decay of the thermal spark generated by a single nanosecond highvoltage pulse between pin electrodes are characterized in this study. The influence of air pressure in the range 50-1000 mbar is investigated at 300 K. By performing short-gate imaging and Optical Emission Spectroscopy (OES), we find that the thermal sparks exhibit an intense emission from excited electronic states of N + , in contrast with non-thermal sparks for which the emission is dominated by electronic transitions of N2. Spark thermalization consists of the following steps: (i) partial ionization of the plasma channel accompanied by N2 emission, (ii) creation of a fully ionized filament at the cathode characterized by N + emission, (iii) formation of a fully ionized filament at the anode, (iv) propagation of these filaments toward the middle of the interelectrode gap, and (v) merging of the filaments. The formation of the filaments, steps (ii) and (iii), occurs at subnanosecond timescales. The propagation speed of the filaments is on the order of 10 4 m/s during step (iv). For the 1-bar condition, the electron number densities are measured from the Stark broadening of N + and Hα lines, with spatial and temporal resolution. The electron temperature is also determined, from the relative emission intensity of N + excited states, attaining a peak value of 48,000 K. In the post-discharge, the electron number density decays from 4×10 19 to 2×10 18 cm-3 in 100 ns. We show that this decay curve can be interpreted as the isentropic expansion of a plasma in chemical equilibrium. Comparisons with previous experiments from the literature support this conclusion. Expressions for the Van der Waals and resonant broadenings of H, Hβ, and several lines of O, O + , N and, N + are derived in the appendix.
Journal of Applied Physics, 2010
In atmospheric pressure air preheated from 300 to 1000 K, the nanosecond repetitively pulsed ͑NRP͒ method has been used to generate corona, glow, and spark discharges. Experiments have been performed to determine the parameter space ͑applied voltage, pulse repetition frequency, ambient gas temperature, and interelectrode gap distance͒ of each discharge regime. In particular, the experimental conditions necessary for the glow regime of NRP discharges have been determined, with the notable result that there exists a minimum and maximum gap distance for its existence at a given ambient gas temperature. The minimum gap distance increases with decreasing gas temperature, whereas the maximum does not vary appreciably. To explain the experimental results, an analytical model is developed to explain the corona-to-glow ͑C-G͒ and glow-to-spark ͑G-S͒ transitions. The C-G transition is analyzed in terms of the avalanche-to-streamer transition and the breakdown field during the conduction phase following the establishment of a conducting channel across the discharge gap. The G-S transition is determined by the thermal ionization instability, and we show analytically that this transition occurs at a certain reduced electric field for the NRP discharges studied here. This model shows that the electrode geometry plays an important role in the existence of the NRP glow regime at a given gas temperature. We derive a criterion for the existence of the NRP glow regime as a function of the ambient gas temperature, pulse repetition frequency, electrode radius of curvature, and interelectrode gap distance.
Ionization Processes in Combined High-Voltage Nanosecond - Laser Discharges in Inert Gas
47th AIAA Plasmadynamics and Lasers Conference, 2016
Submitted for the GEC16 Meeting of The American Physical Society Ionization processes in combined high-voltage nanosecond-laser discharges in inert gas. ANDREY STARIKOVSKIY, MIKHAIL SHNEIDER, Princeton University, PU TEAM-Remote control of plasmas induced by laser radiation in the atmosphere is one of the challenging issues of free space communication, long-distance energy transmission, remote sensing of the atmosphere, and standoff detection of trace gases and bio-threat species. Sequences of laser pulses, as demonstrated by an extensive earlier work, offer an advantageous tool providing access to the control of air-plasma dynamics and optical interactions. The avalanche ionization induced in a pre-ionized region by infrared laser pulses where investigated. Pre-ionization was created by an ionization wave, initiated by high-voltage nanosecond pulse. Then, behind the front of ionization wave extra avalanche ionization was initiated by the focused infrared laser pulse. The experiment was carried out in argon. It is shown that the gas pre-ionization inhibits the laser spark generation under low pressure conditions.
Nonequilibrium discharges in air and nitrogen plasmas at atmospheric pressure
Pure and Applied Chemistry, 2002
Diffuse glow discharges were produced in low temperature (<2000 K) atmospheric pressure air and nitrogen plasmas with electron number densities in excess of 10 12 cm -3 , more than six orders of magnitude higher than in thermally heated air at 2000 K. The measured discharge characteristics compare well with the predictions of a two-temperature kinetic model. Experimental and modeling results show that the steady-state electron number density exhibits an S-shaped dependence on the electron temperature, a behavior resulting from competition between ionization and charge-transfer reactions. Non-Maxwellian effects are shown to be unimportant for the prediction of steady-state electron number densities. The power requirements of DC discharges at atmospheric pressure can be reduced by several orders of magnitude using short repetitive high-voltage pulses. Between consecutive pulses, the plasma is sustained by the finite rate of electron recombination. Repetitive discharges with a 100-kHz, 12-kV, 10-ns pulse generator were demonstrated to produce over 10 12 electrons/cm 3 with an average power of 12 W/cm 3 , 250 times smaller than a DC discharge at 10 12 cm -3 .
Plasma Sources Science and Technology, 2018
Non-equilibrium plasma generated from positive-pulsed nanosecond electrical discharges into desiccated air is simulated in this paper using a multi-dimensional, multi-physics plasma solver. A pin-to-pin electrode configuration is used with a fixed 5.2 mm gap spacing. Peak pulse voltages range between 10.2 and 22.5 kV. Care is taken to match the exact electrode profile from the experiments, and adjust the electron collision frequency so that breakdown limits closely match those from corresponding experimental results. The optimized numerical simulations predict qualitative streamer structure that is in close agreement with experimental observations. Quantitative measurements of atomic oxygen at the anode tip and qualitative estimates of streamer gas heating are closely matched by simulations. The model results are used to provide insight into the spatial and temporal development of the transient plasma. The work performed in this paper delivers a numerical tool that can be extremely useful to link the post-discharge plasma properties to low-temperature plasma ignition mechanisms that are of great interest for the automotive industry.
Ionization in strong electric fields and dynamics of nanosecond-pulse plasmas
Physics of Plasmas, 2006
The paper describes experimental and computational studies of air plasmas sustained by high repetition rate high-voltage nanosecond pulses. Current and voltage measurements, together with earlier microwave diagnostics, allowed us to determine the efficiency of ionization. The energy cost per newly produced electron in these diffuse volumetric plasmas was found to be on the order of 100 eV, two orders of magnitude lower than in diffuse quasineutral DC and RF plasmas, and comparable with or even lower than in the cathode sheaths of glow discharges. A plasma kinetic model was developed and tested against the experimental Paschen breakdown curve in argon. The kinetic model was found to adequately describe the Paschen curve, and the important role of ionization by fast ions and atoms near the cathode, as well as the increase in secondary emission coefficient in strong fields described in the literature, was confirmed. Modeling of plasma dynamics in highvoltage nanosecond pulses yielded the energy cost of ionization, which was found to agree well with the experimental values. Both experiments and modeling revealed that the ionization cost per electron in these plasmas is relatively insensitive to the gas density. Detailed investigations of the plasma dynamics revealed a critical role of the cathode sheath that was found to take up most of the peak voltage applied to the electrodes. The extremely high E/N, much higher than the Stoletov's field at the Paschen minimum point, results in a very high ionization cost in the sheath. In contrast, the E/N in the quasineutral plasma is closer to that associated with the Stoletov's point, resulting in a near-optimal electron generation. This behavior (the reversal of ionization efficiencies in the sheath and in the plasma) is opposite to that in conventional glow discharges. The positive space charge in the sheath and its relatively slow relaxation due to the low ion mobility was also found to result in reversal of electric field direction in the plasma at the tail of the high-voltage pulse.
Diffuse nanosecond discharges at elevated pressures in nonuniform electric fields
Russian Physics Journal, 2007
Pulsed volumetric discharges operating at elevated pressures were studied and are currently studied by many scientific groups in connection with their wide use in various fields of science and engineering [1]. In recent years interest in nanosecond volumetric discharges formed in a nonuniform electric field without a source of additional preionization at a pressure of 1 atm and more has quickened again . This is due to unique properties of such discharges, in particular to high input power densities (up to 0.8 GW/cm 3 ) and to the fact that x radiation and runaway electron beams emerging from the plasma of such discharges have been detected . It was proposed [5] to term this type of discharge the avalanche-electron-beam-initiated volumetric discharge (AEBIVD). However, the features of the volumetric nanosecond discharges formed in a nonuniform electric field without a source of additional preionization are poorly understood. This is related, in particular, to their short formative times. Furthermore, in a gap formed by a cathode of small radius of curvature and a plane anode there are some characteristic regions in which the character of the discharge and the processes involved vary substantially.
Thermal and hydrodynamic effects of nanosecond discharges in atmospheric pressure air
Journal of Physics D: Applied Physics, 2014
We present quantitative schlieren measurements and numerical analyses of the thermal and hydrodynamic effects of a nanosecond repetitively pulsed (NRP) discharge in atmospheric pressure air at 300 and 1000 K. The plasma is created by voltage pulses at an amplitude of 10 kV and a duration of 10 ns, applied at a frequency of 1-10 kHz between two pin electrodes separated by 2 or 4 mm. The electrical energy of each pulse is of the order of 1 mJ. We recorded single-shot schlieren images starting from 50 ns to 3 µs after the discharge. The time-resolved images show the shock-wave propagation and the expansion of the heated gas channel. Gas density profiles simulated in 1D cylindrical coordinates have been used to reconstruct numerical schlieren images for comparison with experimental ones. We propose an original method to determine the initial gas temperature and the fraction of energy transferred into ultrafast gas heating, using a comparison of the contrast profiles obtained from experimental and numerical schlieren images. This method is found to be much more sensitive to these parameters than the direct comparison of measured and predicted shock-wave and heated channel radii. The results show that a significant fraction of the electric energy is converted into gas heating within a few tens of ns. The values range from about 25% at a reduced electric field of 164 Td to about 75% at 270 Td, with a strong dependance on the initial gas temperature. These experiments support the fast heating processes via dissociative quenching of N 2 (B 3 g , C 3 u) by molecular oxygen.
Ignition of a nanosecond-pulsed near atmospheric pressure discharge in a narrow gap
Journal of Physics D: Applied Physics, 2011
The ignition phase and the transition to quasi DC glow operation of a narrow-gap near atmospheric pressure discharge in hydrogen are investigated experimentally. The discharge is ignited by a short 10 ns voltage pulse with a peak voltage of 1.3 kV followed by a 150 ns plateau of about 350 V. Pulsing is at 12 kHz which leaves a significant amount of residual charge between the individual pulses. Temporally resolved laser electric field measurement in the centre of the discharge employing a non-linear four-wave mixing scheme, ultra-high speed optical imaging by an ICCD camera at Balmer-alpha and Fulcher lines as well as the undispersed emission, and current and voltage measurements are performed. Special emphasis is put on a detailed analysis of the measured data by combining the results from the various diagnostics. This allows in addition to the directly measured quantities determination of the absolute evolution of the electron density, the development of space charge shielding, and the observation of the local electron dynamics. Pressure variations in a limited range indicate reasonable agreement with the Paschen law but raise also questions on the definition of the break down voltage under highly transient conditions.
Spatial evolution of the plasma kernel produced by nanosecond discharges in air
Journal of Physics D: Applied Physics, 2019
This work presents an experimental investigation of the hydrodynamic effects induced by nanosecond and conventional spark discharges. The energy deposited in sparks in the short breakdown time (~1 mJ/mm) induces hydrodynamic effects that redistribute the energy over a large volume (~1 cm 3) surrounding the initial plasma channel. This process influences the subsequent formation of the ignition kernel and the initiation of combustion. The experimental results presented in this paper were obtained with a set of synchronized diagnostics including Schlieren, OH Planar Laser Induced Fluorescence and electrical measurements of the energy deposited in the plasma. It is shown that the motion of the gas excited after the discharge breakdown depends not only on the total deposited energy but also on the dynamics of the energy input in the plasma. Finally, the effects of nanosecond sparks are compared with those of conventional sparks used for internal combustion engines. We show that, with 20 times less energy, the nanosecond spark produces a twice bigger excited gas volume than the conventional spark. This is because the energy deposited by the nanosecond spark during the breakdown stage is three times higher than for the conventional spark.