Summary form only given. There has been a continued interest in utilizing streamer and spark discharges for new technologies that require low temperature plasma generation at atmospheric pressure, but diagnostics of these plasmas typically require external sources of probing radiation such as UV lamps or laser systems. Specifically, it is desired to measure the dissociated gas density from pulsed surface flashover plasmas, without the use of potentially invasive optical techniques such as two-photon absorption laser induced fluorescence (TALIF) spectroscopy, which may artificially increase the dissociation degree. We demonstrate a method for determining the dissociated gas density of N and H atoms in an N 2 /H 2 surface flashover plasma by passively monitoring the self-absorption of intrinsic radiation produced by the 2s 2 2p 2 3s→2s 2 2p 3 NI transition(s) at 120.0 nm, and the 2p→ls HI Lyman-α transition at 121.57 nm. This radiation is partially trapped by the spark plasma, assumed to be of Gaussian cylindrical shape with 500 μm diameter. The resulting emission line shapes can be calculated by inferring the plasma temperature, gas mixture, and the estimated dissociated atom density of each species in the plasma volume of measurement. For example, 80%/20% N 2 /H 2 discharges with a measured electron temperature of ~3.0 eV produce peak dissociated concentrations of 2% and 9% for atomic N and H, respectively, during the spark phase ~100 ns after voltage collapse. By assuming the quasi-contiguous approximation of the Holtsmark micro-field due to local electron perturbation of the HI radiators, the Stark line width of Lyman-α radiation yields electron densities on the order of 10 18 cm 3 during the spark phase. This self-absorption method has been extended to provide density information of surface flashover plasmas in air environments by passively monitoring the 2s 2 2p 3 3s→2s 2 2p 4 OI transitions) at 130.2 nm / 130.5 nm / 130.6 nm, which yield peak dissociated concentrations of 20% and 7% for atomic O and N, respectively.
A system for time-discretized spectroscopic measurements of the vacuum ultraviolet (VUV) emission from spark discharges in the 60-160 nm range has been developed for the study of early plasma-forming phenomena. The system induces a spark discharge in an environment close to atmospheric conditions created using a high speed puff value, but is otherwise kept at high vacuum to allow for the propagation of VUV light. Using a vertical slit placed 1.5 mm from the discharge the emission from a small cross section of the discharge is allowed to pass into the selection chamber consisting of a spherical grating, with 1200 grooves/mm, and an exit slit set to 100 μm. Following the exit slit is a photomultiplier tube with a sodium salicylate scintillator that is used for the time discretized measurement of the VUV signal with a temporal resolution limit of 10 ns. Results from discharges studied in dry air, Nitrogen, SF6, and Argon indicate the emission of light with wavelengths shorter than 120 nm where the photon energy begins to approach the regime of direct photoionization.
A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 × 106 particles with dynamic weighting and a total mesh size larger than 108 cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPU-implementation versus a CPU-implementation is considered, and a speed-up factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speed-up is realized for parallelization of the electron processes.
The development of plasma in a medium pressure (50 torr) environment in nitrogen was studied by simulation and measurement under the influence of non-uniform, pulsed electric fields. A GPU-accelerated, 3-dimensional particle-in-cell (PIC)/Monte Carlo Collision (MCC) simulation code was written utilizing the CUDA platform to simulate pulsed plasma development in a nitrogen environment and uncover the transient plasma characteristics in detail. The simulation provides significant speed-up over the CPU equivalent implementations. Experimentally, a needle-protrusion (1.5 mm in length, 200 μm tip radius) opposite a brass ground plane with the distance between needle-tip and wall held at 1.5 mm provided a non-homogeneous field. Excitation of the needle-plane gap was achieved with a ~100 ns rise-time high-voltage pulser with a peak voltage of 30 kV. Diagnostics included time-resolved nanosecond gated imaging for light intensity measurements and high speed electrical probes for timing. A time series of the plasma formation captured with a 5 ns camera gate revealed a mostly uniform expanding plasma cloud from the needle tip.
Abstract Understanding the role of physical processes contributing to breakdown is critical for many applications in which breakdown is undesirable, such as capacitors, and applications in which controlled breakdown is intended, such as plasma medicine, lightning protection, and materials processing. The electron emission from the cathode is a critical source of electrons which then undergo impact ionization to produce electrical breakdown. In this study, the role of secondary electron yields due to photons ( γ ph ) and ions ( γ i ) in direct current breakdown is investigated using a particle-in-cell direct simulation Monte Carlo model. The plasma studied is a one-dimensional discharge in 50 Torr of pure helium with a platinum cathode, gap size of 1.15 cm, and voltages of 1.2–1.8 kV. The current traces are compared with experimental measurements. Larger values of γ ph generally result in a faster breakdown, while larger values of γ i result in a larger maximum current. The 58.4 nm photons emitted from He(2 1 P) are the primary source of electrons at the cathode before the cathode fall is developed. Of the values of γ ph and γ i investigated, those which provide the best agreement with the experimental current measurements are γ ph = 0.005 and γ i = 0.01. These values are significantly lower than those in the literature for pristine platinum or for a graphitic carbon film which we speculate may cover the platinum. This difference is in part due to the limitations of a one-dimensional model but may also indicate surface conditions and exposure to a plasma can have a significant effect on the secondary electron yields. The effects of applied voltage and the current produced by a UV diode which was used to initiate the discharge, are also discussed.
