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A fiber-coupled Dispersion Interferometer (DI) is being developed to measure the electron density of plasmas formed in power flow regions, such as magnetically insulated transmission lines, on Sandia National Laboratories (SNL’s) Z machine [D. B. Sinars et al., Phys. Plasmas 27, 070501 (2020)]. The diagnostic operates using a fiber-coupled 1550 nm CW laser with frequency-doubling to 775 nm. The DI is expected to be capable of line-average density measurements between ∼1013 and 1019 cm−2. Initial testing has been performed on a well-characterized RF lab plasma [A. G. Lynn et al., Rev. Sci. Instrum. 80, 103501 (2009)] at the University of New Mexico to quantify the density resolution lower limits of the DI. Initial testing of the DI has demonstrated line-average electron density measurements within 9% of results acquired via a 94 GHz mm wave interferometer for line densities of ∼1 × 1014 cm−2, despite significant differences in probe beam geometries. The instrument will next be utilized for measurements on a ∼1 MA-scale pulsed power driver {MYKONOS [N. Bennett et al., Phys. Rev. Accel. Beams 22, 120401 (2019)] at SNL} before finally being deployed on SNL’s Z machine. The close electrode spacing (mm scale) on Z requires probe beam sizes of ∼1 mm, which can only be obtained with visible or near infrared optical systems, as opposed to longer wavelength mm wave systems that would normally be chosen for this range of density.

Power flow studies on the 30-MA, 100-ns Z facility at Sandia National Laborat ories (SNL) have shown that plasmas in the facility’s magnetically insulated transmission lines (MITLs) can result in a loss of current delivered to the load. 1 During the current pulse, thermal energy deposition into the electrodes (ohmic heating, charged particle bombardment, etc.) causes neutral surface contaminants layers (water, hydrogen, hydrocarbons, etc.) to desorb, ionize, and form plasmas in the anode-cathode (AK) gap. 2 Shrinking typical ele ctrode thicknesses (~1 cm) down to that of thin foils (5−200 µm) produces observable amounts of plasma on smaller pulsed power drivers (≤1 MA). 3 We suspect that as the electrode material bulk thickness decreases relative to the skin depth of the current pulse (50−100 µm for a 100−500-ns pulse in aluminum), the thermal energy delivered to the neutral surface contaminant layers increases, and thus more surface contaminants desorb from the current carrying surface.

Kinetic simulations of Sandia National Laboratories’ Z machine are conducted to understand particle transport in the highly magnetized environment of a multi-MA accelerator. Joule heating leads to the rapid formation of electrode surface plasmas. These plasmas are implicated in reducing accelerator efficiency by diverting current away from the load [M.R. Gomez et al., Phys. Rev. Accel. Beams 20, 010401 (2017)PRABCJ2469-988810.1103/PhysRevAccelBeams.20.010401, N. Bennett et al., Phys. Rev. Accel. Beams 22, 120401 (2019)PRABCJ2469-988810.1103/PhysRevAccelBeams.22.120401]. The fully-relativistic, electromagnetic simulations presented in this paper show that particles emitted in a space-charge-limited manner, in the absence of plasma, are magnetically insulated. However, in the presence of plasma, particles are transported across the magnetic field in spite of being only weakly collisional. The simulated cross-gap currents are well-approximated by the Hall current in the generalized Ohm’s law. The Hall conductivities are calculated using the simulated particle densities and energies, and the parameters that increase the Hall current are related to transmission line inductance. Analogous to the generalized Ohm’s law, we extend the derivation of the magnetized diffusion coefficients to include the coupling of perpendicular components. These yield a Hall diffusion rate, which is equivalent to the empirical Bohm diffusion.

The objectives of this tutorial are as follows: 1) to help students and researchers develop a basic understanding of how pulsed-power systems are used to create high-energy-density (HED) matter; 2) to develop a basic understanding of a new, compact, and efficient pulsed-power technology called linear transformer drivers (LTDs); 3) to understand why LTDs are an attractive technology for driving HED physics (HEDP) experiments; 4) to contrast LTDs with the more traditional Marx-generator/pulse-forming-line approach to driving HEDP experiments; and 5) to briefly review the history of LTD technology as well as some of the LTD-driven HEDP research presently underway at universities and research laboratories across the globe. This invited tutorial is part of the Mini-Course on Charged Particle Beams and High-Powered Pulsed Sources, held in conjunction with the 44th International Conference on Plasma Science in May of 2017.

Two-dimensional electromagnetic (EM) particle-in-cell (PIC) simulations of a radial magnetically-insulated-transmission-line are presented and compared to the model of E. M. Waisman, M. P. Desjarlais, and M. E. Cuneo [Phys. Rev. Accel. Beams 22, 030402 (2019)PRABCJ2469-988810.1103/PhysRevAccelBeams.22.030402 in the “high-enhancement” (WDC-HE) limit. The simulations use quasi-equilibrium current and voltage values based on the Sandia National Laboratories Z accelerator, with prescribed injection of an electron sheath that gives electron density profiles qualitatively similar to those used in the WDC-HE model. We find that the WDC-HE model accurately predicts the quasiequilibrium ion current losses in the EM PIC simulations for a wide range of current and voltage values. For the case of two ion species where one is magnetically insulated by the ambient magnetic field and the other is not, the charge of the lighter insulated species in the anode-cathode gap can modify the electric field profile, reducing the ion current density enhancement for the heavier ion species. On the other hand, for multiple ion species, when the lighter ions are not magnetically insulated and are a significant fraction of the anode plasma, they dominate the current loss, producing loss currents which are a significant fraction of the lighter ion WDC values. The observation of this effect in the present work is new to the field and may significantly impact the analysis of ion current losses in the Z machine inner MITL and convolute.

A challenge for the TW-class accelerators driving $Z$-pinch experiments, such as Sandia National Laboratories' $Z$ machine, is to efficiently couple power from multiple storage banks into a single multi-MA transmission line. The physical processes that lead to current loss are identified in new large-scale, multidimensional simulations of the $Z$ machine. Kinetic models follow the range of physics occurring during a pulse, from vacuum pulse propagation to charged-particle emission and magnetically-insulated current flow to electrode plasma expansion. Simulations demonstrate that current is diverted from the load through a combination of standard transport (uninsulated charged-particle flows) and anomalous transport. Standard transport occurs in regions where the electrode current density is a few $1{0}^{4}\ensuremath{-}{10}^{5}\text{ }\text{ }\mathrm{A}/{\mathrm{cm}}^{2}$ and current is diverted from the load via transport without magnetic insulation. In regions with electrode current density $g{10}^{6}\text{ }\text{ }\mathrm{A}/{\mathrm{cm}}^{2}$, electrode surface plasmas develop velocity-shear instabilities and a Hall-field-related transport which scales with electron density and may, therefore, lead to increased current loss.

The reliable design of magnetically insulated transmission lines (MITLs) for very high current pulsed power machines must be accomplished in the future by utilizing a variety of sophisticated modeling tools. The complexity of the models required is high and the number of sub-models and approximations large. The potential for significant analyst error using a single tool is large, with possible reliability issues associated with the plasma modeling tools themselves or the chosen approach by the analyst to solve a given problem. We report on a software infrastructure design that provides a workable framework for building self-consistent models and constraining feedback to limit analyst error. The framework and associated tools aid the development of physical intuition, the development of increasingly sophisticated models, and the comparison of performance results. The work lays the computational foundation for designing state-of-the-art pulsed-power experiments. The design and useful features of this environment are described. We discuss the utility of the Git source code management system and a GitLab interface for use in project management that extends beyond software development tasks.

Power-flow studies on the 30-MA, 100-ns Z facility at Sandia National Laboratories have shown that plasmas in the facility's magnetically insulated transmission lines (MITLs) and double post-hole convolute can result in a loss of current delivered to the load. To study power-flow physics on the 1-MA, 100-ns MAIZE facility at the University of Michigan, planar MITL loads and planar post-hole convolute loads have been developed that extend into the lines of sight for various imaging diagnostics on MAIZE. These loads use 3D-printed dielectric support structures lined with thin foils of either aluminum or stainless steel. The metal foils serve as the current-carrying power-flow surfaces, which generate plasma during the current pulse. The foil thickness (50 µm) and widths (11.5-16 mm) are selected to ensure a sufficient linear current density (0.5-0.7 MA/cm) for plasma formation. Laser backlighting (532 nm) and visible-light self-emission imaging capture the overall plasma evolution in the anode-cathode gaps, including the gap closure velocities (1-4 cm/μs).

Pulsed power accelerators compress electrical energy in space and time to provide versatile experimental platforms for high energy density and inertial confinement fusion science. The 80-TW “Z” pulsed power facility at Sandia National Laboratories is the largest pulsed power device in the world today. Z discharges up to 22 MJ of energy stored in its capacitor banks into a current pulse that rises in 100 ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15 years in the use of pulsed power as a precision scientific tool. This paper reviews developments at Sandia in inertial confinement fusion, dynamic materials science, x-ray radiation science, and pulsed power engineering, with an emphasis on progress since a previous review of research on Z in Physics of Plasmas in 2005.

Absolute calibration of streaked visible spectroscopy systems has been performed at Z-machine at Sandia National Labs in order to determine temperatures of electrode surfaces during the current pulse. The ability to calibrate the full system, including all fiber optic runs and probes is crucial to understanding errors in the calibration process. The calibration procedure involves imaging a blackbody light source, with a known spectral radiance which is coupled to an integrating sphere. This source is streaked slowly over a few ns using Sydor streak cameras. The slow sweep is converted to a $100-500ns$ sweep by imaging a bright light source on both sweep rates, and obtaining wavelength and time dependent correction curves. Any broadband light source or several laser lines of differing wavelengths can be used for this correction. This technique has yielded temperature estimates of several eV in the Z convolute.

We have developed a physics-based transmission-line-circuit model of the Z pulsed-power accelerator. The 33-m-diameter Z machine generates a peak electrical power as high as 85 TW, and delivers as much as 25 MA to a physics load. The circuit model is used to design and analyze experiments conducted on Z. The model consists of 36 networks of transmission-line-circuit elements and resistors that represent each of Zs 36 modules. The model of each module includes a Marx generator, intermediate-energy-storage capacitor, laser-triggered gas switch, pulse-forming line, self-break water switches, and tri-plate transmission lines. The circuit model also includes elements that represent Zs water convolute, vacuum insulator stack, four parallel outer magnetically insulated vacuum transmission lines (MITLs), double-post-hole vacuum convolute, inner vacuum MITL, and physics load. Within the vacuum-transmission-line system the model conducts analytic calculations of current loss. To calculate the loss, the model simulates the following processes: (i) electron emission from MITL cathode surfaces wherever an electric-field threshold has been exceeded; (ii) electron loss in the MITLs before magnetic insulation has been established; (iii) flow of electrons emitted by the outer-MITL cathodes after insulation has been established; (iv) closure of MITL anode-cathode (AK) gaps due to expansion of cathode plasma; (v) energy loss to MITL conductors operated at high lineal current densities; (vi) heating of MITL-anode surfaces due to conduction current and deposition of electron kinetic energy; (vii) negative-space-charge-enhanced ion emission from MITL anode surfaces wherever an anode-surface-temperature threshold has been exceeded; and (viii) closure of MITL AK gaps due to expansion of anode plasma. The circuit model is expected to be most accurate when the fractional current loss is small. We have performed circuit simulations of 52 Z experiments conducted with a variety of accelerator configurations and load-impedance time histories. For these experiments, the apparent fractional current loss varies from 0% to 20%. Results of the circuit simulations agree with data acquired on 52 shots to within 2%.

We have developed a conceptual design of a next-generation pulsed-power accelerator that is optimized for advanced high-energy-density-physics experiments. The prime power source of the machine consists of 210 impedance-matched Marx generators (IMGs). Each IMG drives a 150-ns-long coaxial-transmission-line impedance transformer. The coaxial lines provide a minimum of 300 ns of transit-time isolation between each pair of IMGs. The lines in turn drive six radial-transmission-line impedance transformers, which transport the power generated by the IMGs to a six-level vacuum-insulator stack. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs); these are joined in parallel at a 12-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator's physics load. Since the accelerator would be the largest and most-powerful pulsed-power machine developed to date, we refer to it as Jupiter. The conceptual design of Jupiter is 72 m in diameter, stores 140 MJ of electrical energy, and generates 960 TW of peak electrical power at the output of the IMG system. The design delivers 2700 TW, 67 MA, and 9.2 MJ in a 110-ns pulse to a 0D magnetized-liner inertial-fusion (MagLIF) target. The principal goal of the design is to achieve high-yield thermonuclear fusion; i.e., a fusion yield that exceeds the energy initially stored by the accelerator's capacitors.

Magnetized Liner Inertial Fusion is a magneto-inertial fusion concept in which a metal cylindrical shell containing deuterium gas is axially magnetized, the fuel is heated with a laser, and the metal cylinder is imploded by the current of the Z Machine. In previous experiments, the axial magnetic field was 9-10 T, the laser energy absorbed in the fuel was less than 1 kJ, and the peak current flowing through the target was about 17 MA. These experiments produced fusion-relevant temperatures and densities at stagnation, up to 3e12 primary DD neutrons, and a magnetic field in the fuel sufficient to trap a sizable fraction of the fusion-produced tritons.

The Magnetized Liner Inertial Fusion concept (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is being studied on the Z facility at Sandia National Laboratories. Neutron yields greater than 1012 have been achieved with a drive current in the range of 17–18 MA and pure deuterium fuel [Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]. We show that 2D simulated yields are about twice the best yields obtained on Z and that a likely cause of this difference is the mix of material into the fuel. Mitigation strategies are presented. Previous numerical studies indicate that much larger yields (10–1000 MJ) should be possible with pulsed power machines producing larger drive currents (45–60 MA) than can be produced by the Z machine [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. To test the accuracy of these 2D simulations, we present modifications to MagLIF experiments using the existing Z facility, for which 2D simulations predict a 100-fold enhancement of MagLIF fusion yields and considerable increases in burn temperatures. Experimental verification of these predictions would increase the credibility of predictions at higher drive currents.The Magnetized Liner Inertial Fusion concept (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is being studied on the Z facility at Sandia National Laboratories. Neutron yields greater than 1012 have been achieved with a drive current in the range of 17–18 MA and pure deuterium fuel [Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]. We show that 2D simulated yields are about twice the best yields obtained on Z and that a likely cause of this difference is the mix of material into the fuel. Mitigation strategies are presented. Previous numerical studies indicate that much larger yields (10–1000 MJ) should be possible with pulsed power machines producing larger drive currents (45–60 MA) than can be produced by the Z machine [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. To test the accuracy of these 2D simulations, we present modifications to MagLIF experiments using the existing Z facility, for which 2D simulations predict a 100-fold enhancement of MagLIF fusion yields and considerable increase...

Experimental observation of photoionization capable extreme ultraviolet and vacuum ultraviolet emission from nanosecond timescale, developing low temperature plasmas (i.e. streamer discharges) in atmospheric air is presented. Applying short high voltage pulses enabled the observation of the onset of plasma formation exclusively by removing the external excitation before spark development was achieved. Contrary to the common assumption that radiative transitions from the b1∏u (Birge-Hopfield I) and b′1∑+ u (Birge-Hopfield II) singlet states of N2 are the primary contributors to photoionization events, these results indicate that radiative transitions from the c′4 1∑+ u (Carroll-Yoshino) singlet state of N2 are dominant in developing low temperature plasmas in air. In addition to c′4 transitions, photoionization capable transitions from atomic and singly ionized atomic oxygen were also observed. The inclusion of c′4 1∑+ u transitions into a statistical photoionization model coupled with a fluid model enabled streamer growth in the simulation of positive streamers.

There is a growing demand for understanding the physics of surface flashover, as it relates to the breakdown of electric fields on high power systems in the aerospace community. Specifically, the quantitative role of vacuum ultraviolet (VUV) radiation which is self-produced during the initial nanoseconds of surface flashover is virtually unknown. An experiment was constructed which allows detailed electrical and optical measurements of VUV emission during the timescales in which streamers are propagating before the transition into spark discharge. Repeated surface flashover events are generated using a solid-state high voltage pulser, with breakdown recorded in a number of gases at atmospheric pressure. Streamers are photographed using fast optical imaging with 3 ns resolution. Fast voltage and current diagnostics revealed a number of distinct stages of streamer development ranging from the onset of cathode directed streamers to the sharp current rise during final voltage collapse. The emission of VUV radiation is discussed in context to the observed streamer and electrical characteristics.

There is a growing interest in the physics of surface flashover between the interface of atmosphere and vacuum in some high-power systems. More specifically, the quantitative role of vacuum ultraviolet (VUV) radiation for the photoionization leading to a streamer development during the initial stages of a breakdown is unknown. This paper describes an experimental setup used to measure the VUV radiation emitted from atmospheric flashover as well as time-resolved imaging of the flashover event. A pulser providing the voltage to the gap was designed with special considerations in mind, including long lifetime, low noise, and high reproducibility. This enabled the study of the flashover in various background gases with an emphasis on spectroscopic measurements. The calculated spectra are compared with the measured spectra, and it is found that atomic oxygen and nitrogen are responsible for most of the VUV production in an air breakdown at atmospheric pressure in the wavelength range of 115-180 nm. Time-resolved spectroscopy reveals that the VUV radiation is emitted during the initial stages while the streamers are developing.

By deploying a photon Doppler velocimetry based plasma diagnostic, we have directly observed low density plasma in the load anode/cathode gap of cylindrically converging pulsed power targets. The arrival of this plasma is temporally correlated with gross current loss and subtle power flow differences between the anode and the cathode. The density is in the range where Hall terms in the electromagnetic equations are relevant, but this physics is lacking in the magnetohydrodynamics codes commonly used to design, analyze, and optimize pulsed power experiments. The present work presents evidence of the importance of physics beyond traditional resistive magnetohydrodynamics for the design of pulsed power targets and drivers.

Photonic Doppler velocimetry tracks motion during high-speed, single-event experiments using telecommunication fiber components. The same technology can be applied in situations where there is no actual motion, but rather a change in the optical path length. Migration of plasma into vacuum alters the refractive index near a fiber probe, while intense radiation modifies the refractive index of the fiber itself. These changes can diagnose extreme environments in a flexible, time-resolved manner.

Recent Magnetized Liner Inertial Fusion experiments at the Sandia National Laboratories Z pulsed power facility have featured a PDV (Photonic Doppler Velocimetry) diagnostic in the final power feed section for measuring load current. In this paper, we report on an anomalous pressure that is detected on this PDV diagnostic very early in time during the current ramp. Early time load currents that are greater than both B-dot upstream current measurements and existing Z machine circuit models by at least 1 MA would be necessary to describe the measured early time velocity of the PDV flyer. This leads us to infer that the pressure producing the early time PDV flyer motion cannot be attributed to the magnetic pressure of the load current but rather to an anomalous pressure. Using the MHD code ALEGRA, we are able to compute a time-dependent anomalous pressure function, which when added to the magnetic pressure of the load current, yields simulated flyer velocities that are in excellent agreement with the PDV mea...

This paper describes an experiment to characterize ions generated by a pulsed vacuum arc by using a microwave resonant cavity (MRC) as a transient diagnostic. Specific information is desired on the various species which can drift into the beam during repetitive operations of arc plasma generation. The arc source reference voltage is elevated above ground (∼200V), which results in a separation of ion species in the beam due to the acceleration experienced by the ions. The cylindrical MRC used in this study has a resonant frequency of ∼2.8 GHz when excited by a continuous RF source in the TM 01 mode of operation. When the neutralized ion beam propagates through the MRC located downstream from the arc source, the resonant frequency of the MRC is shifted by the local disturbance in electric field inside the cavity due to the presence of the electron space charge in the beam. Coupled with the time-of-flight separation of various ion masses, the MRC resonance shift provides a temporally resolved measurement of beam species and density downstream from the vacuum ion source without the use of a potentially invasive diagnostic such as charge collector plates within the beam cross-section. This diagnostic technique should prove useful in a variety of pulsed ion beam studies and applications in research and industrial environments.

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 N2/H2 surface flashover plasma by passively monitoring the self-absorption of intrinsic radiation produced by the 2s2 2p2 3s→2s2 2p3 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% N2/H2 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 1018 cm3 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 2s2 2p3 3s→2s2 2p4 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.

Summary form only given. Limited experimental literature exists for measuring radiation with wavelengths shorter than 115 nm primarily due to the difficulty associated with measuring VUV radiation due to the high absorption in molecular oxygen and optical materials. Furthermore, there is an increasing interest in quantifying the role of this radiation in the generation of low temperature plasmas. This paper describes a system for studying the early mechanisms of vacuum UV generation below 115 nm by inducing breakdown in an air environment near atmospheric pressure. However to avoid the attenuating effects of O2, the experiment is set up such that the pressure drops to deep vacuum (i.e. ~10-6 torr) within a distance of several millimeters from the location of breakdown without the use of a solid barrier. In order to perform this measurement, the experiment consists of an enclosure at high vacuum, a pair of electrodes, and a micro puff valve. The puff value releases an above atmospheric pressure wave of air in the vicinity of the electrodes, whose differential electrical potential is then raised to about 20kV with a rise-time on the order of 100 nanoseconds. With application of the pulsed electric field, the radiation emitted from the excited species escapes the very localized puff of air and propagates through the high vacuum largely unattenuated where it is resolved by vacuum UV-sensitive optics and detectors. With this setup, the generation of vacuum UV radiation below 115 nm can be studied in detail and in the future the data recorded with this system will be used in our modeling approaches for further understanding of low temperature plasma generation.

Summary form only given. A GPU-accelerated 3-dimensional PIC/MCC simulation code was developed using the CUDA environment to study the physical processes involved in the development of a low-temperature plasma. The simulation results aid in quantifying transient plasma development as it is often inaccessible experimentally in detail even with modern noninvasive techniques such as non-linear laser spectroscopy or high-speed electrical diagnostics. Hence, computational methods, such as Particle-in-Cell (PIC) and Monte Carlo Collision (MCC), provide a complementary approach to determining the mechanisms leading to plasma development. However, fully modeling the physics of the plasma development is made difficult by the number of plasma processes that must be tracked simultaneously, and only recently have computing resources provided the capability to track tens of millions of particle interactions. Furthermore, the introduction of graphics processing unit (GPU) computing provides an attractive means for economical and efficient parallelization of scientific codes through a framework such as NVIDIA CUDA. As such, a GPU-accelerated 3-dimensional PIC/MCC simulation was developed using the CUDA environment to provide characteristics during the initial stage of plasma development in atmospheric pressure nitrogen. The simulation was run on a NVIDIA GTX 580 with 3 GB of memory and 512 CUDA cores. The simulated geometry consists of two paraboloid electrodes with a gap distance of 5 millimeters with Dirichlet boundary conditions, and 22 unique electron interactions with molecular nitrogen are considered. The electrodes are excited with a step voltage pulse of several thousand volts also assuming a uniformly distributed initial electron density of 104 cm-3 in the volume. For instance, results from a 5 nanosecond simulation reveal the development of positive ion space charge channels near the anode and cathode regions. These channels appear consistent with high-speed streamer photographs captured during plasma formation. The electron energy distribution function (EEDF) indicates a non-Maxwellian velocity distribution during the application of the high electric field. Furthermore, a typical electron density near the cathode is on the order of 7 × 108 cm-3. The results from numerical simulation will be compared in detail to experimentally accessible parameters such as electron temperature and dissociation degree.

High energy photons released during the early stages of pulsed surface flashover are believed to be a contributing factor to the overall plasma formation. It is clear that only radiation with wavelength in the Ultraviolet (UV) and Vacuum Ultraviolet (VUV) regimes is capable of ionization and excitation processes of atmospheric gases, thus potentially providing seed electrons for secondary electron avalanches. To investigate the role of VUV photons in the breakdown process, an experiment was designed enabling detection of photons with energies greater than 7 eV with high temporal and spatial resolution. A transient low temperature plasma is generated on an MgF 2 surface between two point-to-point stainless steel electrodes using a 200 ns rise-time high voltage pulser. Light emitted during the plasma formation is transmitted through the VUV transmissive MgF 2 window into vacuum avoiding the strong absorption of VUV radiation in the atmosphere. High resolution measurements of the flashover event are acquired using a VUV-sensitive ICCD camera and photomultiplier tube (PMT) mounted to the exit ports of the VUV spectrograph. Detailed analysis of the two electrode regions clearly shows the presence of VUV emission lines prior to voltage collapse in atmospheric gases. Although a symmetric field geometry was chosen, it was seen in all cases that initial VUV emission originates from the anode. Furthermore, measurements of the 120 nm nitrogen ground transition taken at locations away from the anode demonstrated peak emission that coincided with streamer head locations. The observed behavior is consistent with a build-up of positive ion space charge in the gap attracting electrons from the surrounding medium causing excitations and ionizations in this high field region. Further time-resolved spectroscopy of the developing plasma in a high purity nitrogen environment captured the presence of the second positive system from molecular nitrogen as well as atomic emission lines. It was seen that the lifetime of the second positive system is much shorter (nanoseconds) than that of the atomic nitrogen emission lines (microseconds). The experimentally observed behavior will be discussed is it relates to the transient formation of a low temperature plasma along a dielectric surface.

The development of streamers during the initial stage of a pulsed atmospheric discharge, which carries a high content of vacuum UV (VUV) emission, is investigated. Due to the high spectral absorptivity of atmospheric air in the VUV regime, few experiments have been conducted that observe the wavelength range shorter than 180 nm. However, direct photoionization is believed to play an important role in streamer formation during this phase of breakdown. VUV radiation (hν > 7 eV) is energetic enough to promote step-ionization and directly ionize background gas mixtures. Utilizing a VUV-sensitive experimental apparatus, spatially-resolved photomultiplier tube (PMT) measurements were recorded showing that initial VUV emission is dependent on the inhomogeneous field distribution near the electrodes. It was revealed that further into the developing streamer the position of instantaneous VUV emission is propagating from anode to cathode away from the initial VUV emissions. From these observations, the low temperature plasma is capable of producing high energy photons during the streamer phase as well as after voltage collapse.

This paper describes recent advances in the study of self-generated emission of vacuum ultraviolet (VUV) radiation which is produced during the early time period leading to high voltage breakdown at atmospheric pressure. Previous studies of air breakdown showed the presence of 121.5 nm radiation which is spontaneously emitted by excited hydrogen atoms, HI. Since this Lyman-α line is self-absorbed, it enabled inferring various plasma parameters from recording emission spectra from 115–135 nm for species of HI and NI. For instance, measurements in H 2 /N 2 mixtures have revealed that the highest amount of absorption via HI atoms occurs in the high field region near the anode, implying that significant H 2 dissociation for radiation-trapping is occurring in this zone. Selective spatial measurements further showed that the apparent VUV emission centers (i.e. streamer heads) move away from the anode and the mechanisms leading to line broadening (i.e. Stark effect from space charge) are a function of streamer position. The presented self-absorption mechanisms are essential in quantitatively understanding the role of VUV radiation transport via absorption for photo-ionization during streamer breakdown, in which re-absorption of high energy photons is inherently a requirement.

This paper describes recent experiments to study selfproduced vacuum ultraviolet (VUV) emission from pulsed atmospheric plasma structures. While it has been classically believed that photo-ionization plays a significant role on plasma generation during fast timescales (i.e. streamers), the exact role of VUV radiation (energy greater than 7 eV) has only recently been explored and is currently an area of interest for the development of high power devices in the aerospace community. Since VUV emission is heavily absorbed by molecular oxygen and most optical materials, the direct observation of VUV radiation produced by atmospheric pressure plasmas is challenging. Experiments at Texas Tech University were performed with multiple vacuum monochromators, custom designed VUV transparent optical instruments, VUV sensitive intensified CCD and photomultiplier time-resolved diagnostics, and nanosecondtimescale electrical probes of the plasma. Previous studies were limited due to the non-linearity of the focusing optics used for VUV transmission, and thus the current experiment was designed to minimize chromatic abberation of recorded emission in the VUV regime of interest (115 – 135 nm). Quantitative observation of VUV emission from surface flashover in air revealed that the majority of emission is due to radiation from atomic oxygen and nitrogen in the wavelength range 130 – 135 nm, which has been confirmed by spectral calculation for an estimated Boltzmann temperature of 10 eV. High resolution spectral measurements in the range 115 – 130 nm also led to observation of various impurities along the surface, which were only observable due to the upgraded focusing system. Finally, time resolved measurements showed that the earliest VUV emission occurs during the streamer phase, where the recorded signal-to-noise ratio of the observed emission has been significantly increased due to more efficient optical diagnostics.

Vacuum Ultraviolet (VUV) radiation is commonly thought to enhance streamer formation, as it is energetic enough to cause photoionization in the gaseous volume. Light with wavelengths below 180 nm, i.e. VUV, is highly absorbed in the atmosphere which increases the difficulty of measuring any VUV emission from gaseous breakdown at atmospheric pressure. Nevertheless, VUV emission from pulsed surface flashover at atmospheric conditions was previously recorded at Texas Tech. A second generation system was designed to image VUV and visible emission directly while also preserving the spatial profile. The visible emission is imaged through an air-side focused ICCD, while VUV emission is imaged through a vacuum spectrograph. The variable length gap was excited with a pulser designed for a 100 ns rise time and 50 kV peak output. Captured images of visible light emission from streamers produced in oxygen are diffuse whereas nitrogen produces streamers that are segmented. VUV spatial images taken in oxygen reveal stronger emission closer to the cathode region, while nitrogen produces a more distributed intensity profile across the gap. While MgF 2 enabled transmission and measurement of VUV, streamer characteristics recorded in the visible light spectrum of surface flashover on BK7 dielectric windows were also investigated. In this paper, the observed streamer images in both visible and VUV wavelength range will be discussed as it relates to surface flashover at atmospheric pressure.

Summary form only given. The role of self-produced VUV emission (i.e. energies greater than 7 eV) on photo-ionization processes during the early nanoseconds of pulsed discharges is a new area of interest in the aerospace community, and could play a significant role in the theoretical understanding of plasma generation at short (nanosecond) timescales. Our previously reported experiments have shown VUV emission from atomic species of oxygen and nitrogen, which were excited during breakdown, but detailed analysis of emission in the range 115 - 135 nm was difficult due to chromatic aberration of VUV transparent optics in this regime. These limitations were recently alleviated by fitting the spectral apparatus with a custom set of MgF2-Aluminum coated off-axis parabolic mirrors used in conjunction with a high resolution vacuum monochromator. VUV emission is observed by either VUV sensitive intensified CCD or photomultiplier sensors, with additional current/voltage monitors and externally focused fast-frame (nanosecond capable) imaging complimenting the diagnostic setup. High resolution spectroscopy has been achieved in the excitation range of interest (8 - 10 eV), where species of atmospheric gases and electrode metal have been identified during the early nanoseconds of plasma generation. Temporal studies have shown that most VUV emission occurs during the time before voltage collapse and subsequent power flow, while the majority of visible emission is released after breakdown when the electron energy distribution has shifted to lower energies. This experiment also allowed for direct emission imaging of VUV radiation, where the spatial profile relative to plasma position is still intact. While the observed metal species are only emitted in the regions close to the electrodes as expected, significant differences were observed for species of NI (which is released throughout the plasma volume) and OI (which is released strongly in regions of high electric field) atoms during breakdown. This is most likely due to the accumulation of ionic space charge from photo-ionization in the gas volume, coupled with the high absorption cross section of molecular oxygen in the VUV range. These measurements will be discussed in context to current physics models of electric breakdown and plasma generation at atmospheric pressure.

This paper describes a 2nd-generation system for directly studying the emission of vacuum ultraviolet (VUV) light from pulsed dielectric surface flashover at atmospheric pressure. The role of self-produced VUV emission (i.e. energies greater than 7 eV) on photo-ionization processes during the early nanoseconds of pulsed discharges is virtually unexplored, and yet could be a significant factor in the physics of fast breakdown of directed energy systems (such as MW-class high power microwave devices) in the aerospace community. First generation experiments at Texas Tech University have shown that VUV emission corresponding to nitrogen and oxygen excitation in the energy range 8-10 eV is easily produced, but the use of MgF2 optics inhibited future work with existing hardware due to the transmission cutoff of this dielectric material and chromatic aberration if used as a lensing medium. In an effort to enhance the detection capabilities of our hardware in the wavelength range from 115-135 nm, the current system utilizes a custom designed set of off-axis parabolic MgF2-Aluminium coated mirrors as the primary focusing element. High resolution spectroscopy with the upgraded system resulted in the observation of the nitrogen doublet at 149.5 nm, leading to a better fit for the appropriate line broadening parameters for an approximate 10 eV Boltzmann electronic temperature. Evidence of self-absorption for HI (121.5 nm) provides new insight into the generation of space charge in these plasma structures, which has been investigated quantitatively in both SF6-H2 and N2-H2 mixtures.

Summary form only given. It is commonly accepted that vacuum ultraviolet (VUV) radiation, corresponding to emission from 180 nm to 115 nm and below, is responsible for photoionization contributing to streamer propagation during the initial stages of atmospheric discharges. An experimental setup was constructed to observe the VUV emission of pulsed surface flashover along a dielectric surface between atmosphere and vacuum. However, VUV radiation is highly attenuated in the atmosphere, which makes observation of detailed spectra difficult. For VUV transmission down to 115 nm the light emitted by surface flashover across an MgF2 window (front side of window in air, backside in vacuum) was focused by an MgF2 lens onto the entrance slit of the spectrograph. The high speed detection scheme consists of a VUV sensitive ICCD camera and a photomultiplier, both with nanosecond temporal resolution. Spectra were measured in various gas mixtures at atmospheric pressure with a flashover spark length of about 8 mm with a 35 kV pulsed excitation, and spectral calibration was done utilizing a VUV calibration lamp with a known emission spectrum. Virtually all lines from 115 to 180 nm can be identified as atomic oxygen and nitrogen transitions during flashover in dry air, with most VUV emission occurring during the initial breakdown stage (current rise). The extremely fast decay of VUV emission intensity following this initial stage is evidence of radiationless quenching of the excited energy levels associated with the observed spectral lines. Flashover studies were also performed in pure oxygen and nitrogen environments to reinforce the observed emission trends. Spectroscopy must be carefully detailed, for instance, the Oxygen-I line at 130.2 nm (which corresponds to a ground level transition) is shown to be strongly self absorbed in the atmospheric spark when compared to a similar oxygen emission line at 130.4 nm. Full spectra were simulated using SpectraPlot, a temperature dependent spectral software suite developed at Texas Tech. It has been concluded from the comparison of simulated and measured Nitrogen spectra between 140 and 150 nm that the electronic temperature is about 4.5 eV, assuming that the electronic nitrogen energy level population density is Boltzmann distributed. The measured spectra will be discussed in relation to the physics of surface flashover and volume breakdown at atmospheric pressure.

In an attempt to identify the mechanisms leading to pulsed dielectric surface flashover in atmospheric conditions, surface flashover across a magnesium fluoride (MgF 2 ) window was studied. The electrode configuration and the applied pulsed voltage level were chosen such that the generated electric field was symmetric with respect to the centerline between the electrodes. That is, neither electrode was favored with respect to flashover/breakdown initiation. A semiconductor-switched 32 kV pulse with 140 ns rise time was applied to the 8 mm wide flashover gap in air, nitrogen, and oxygen at atmospheric pressure. Fast voltage and current measurements along with nanosecond imaging revealed four stages of flashover development: (1) Onset of a cathode directed streamer with a charge on the order of 100 pC and traveling with a speed of ∼ 1 mm/ns at a macroscopic field level of ∼ 10 kV/cm associated with a slow current rise (on the order of 10−3 A/ns) temporarily augmented by (2) a 5 ns wide current spike at the moment when the streamer reaches the cathode followed by (3) a cathode directed streamer focused toward the center of the flashover gap with a slow rising current leading to (4) a sharp current rise (on the order of 10 A/ns) reaching roughly a circuit limited 45 A about 20 ns after the return strike meets an anode directed streamer. Although present in all tested gases, the current spike at the end of stage (2) is most different for all three gases and having the greatest impact in air.

Spectroscopic measurements in the vacuum ultraviolet (VUV) regime are difficult to make due to extremely large absorption of VUV radiation in most materials. This paper describes an experimental setup designed for studying the optical emission during pulsed surface flashover for the wavelength range between 115 nm to 180 nm at atmospheric pressures with a focus on the scheme used to excite the spark gap. The surface flashover of interest occurred on an MgF 2 window (front side of window in air, backside in vacuum) imaged onto the entrance slit of a 1 m vacuum spectrograph. Emission spectra were recorded with an Andor DH740 series ICCD camera. All data was taken at atmospheric pressure with a flashover spark length of about 8 mm created by a pulser designed for a 500 ns pulse, max 50 kV output. The centerpiece of this pulser is the CCSTA14N40 thyristor by Solidtron / Silicon Power which features a rate of change current of maximum 30 kA/µs and a hold-off voltage of up to 4 kV. A pulse transformer with Metglas® core was used to elevate the voltage to 50 kV with a rise time of 180 ns and a peak current of 500 A. The pulser was designed for a repetition rate of 10 Hz and is triggered by TTL pulses. Discussed in this paper, along with the measured spectra and their relation to the physics of surface flashover at atmospheric pressure, will be the design of the pulser.

Summary form only given. Spectroscopic measurements in the vacuum ultraviolet (VUV) regime are difficult to make due to extremely large absorption of VUV radiation in most materials. This paper describes an experimental setup designed for studying the optical emission during pulsed surface flashover for the wavelength range between 115 nm to 300 nm at atmospheric pressures. A vacuum monochromator VM 505 from Acton Research Corporation was used as the spectrograph. For VUV transmission down to 115 nm the light emitted by surface flashover across an MgF2 window (front side of window in air, backside in vacuum) was focused by an MgF2 lens onto the entrance slit of the spectrograph. A quartz window with sodium salicylate coating exposed to the spectrograph's vacuum was placed in the exit focal plane of the collimating mirror of the spectrograph. This fluorescent coating down-converts the VUV light to longer wavelengths that were recorded with an Andor DH520 series ICCD camera in combination with a Nikon 105 mm lens. Spectra were measured at atmospheric pressure with a flashover spark length of about 9 mm and DC excitation with a capacitance of 4.1 nF. Emission spectra were measured from 300 nm down to 130 nm. In parallel, theoretical spectra were calculated primarily for the identification of radiating species and their temperature. Utilizing the NIST Atomic Spectra Database (ASD) data a library of temperature dependent optical emission spectra was generated with SpectraPlot, a spectral software suite developed at TTU. VUV spectral lines of nitrogen, carbon, magnesium and silicon were identified. In pure nitrogen, for instance, the nitrogen I double line at 174.3 nm and 174.5 nm is clearly visible in the spectrum along with a strong double line at 279.6 nm and 280.4 nm, which is emitted by Magnesium II, eroded from the surface exposed to flashover. Spectra were measured in ambient air, pure nitrogen, and argon. An experiment upgrade is currently underway, increasing the VUV sensitivity of the setup. The measured spectra will be discussed in relation to the physics of surface flashover and volume breakdown at atmospheric pressure.

The quantitative role of self-produced vacuum-ultraviolet (VUV) light on photoionization-dominated gas discharges is currently an area of interest in the aerospace community. In this paper, we present the images of the VUV spectroscopic analysis of a pulsed atmospheric flashover, where the spatial content of emission relative to electrode geometry has been preserved. The observed spatial profile of emission is dependent on radiating species in the range of 120-125 nm and is discussed in relation to the physics of nanosecond discharges.

As the size of a positively biased electrode increases, the nature of the interface formed between the electrode and the host plasma undergoes a transition from an electron-rich structure (electron sheath) to an intermediate structure containing both ion and electron rich regions (double layer) and ultimately forms an electron-depleted structure (ion sheath). In this study, measurements are performed to further test how the size of an electron-collecting electrode impacts the plasma discharge the electrode is immersed in. This is accomplished using a segmented disk electrode in which individual segments are individually biased to change the effective surface area of the anode. Measurements of bulk plasma parameters such as the collected current density, plasma potential, electron density, electron temperature and optical emission are made as both the size and the bias placed on the electrode are varied. Abrupt transitions in the plasma parameters resulting from changing the electrode surface area are identified in both argon and helium discharges and are compared to the interface transitions predicted by global current balance [S. D. Baalrud, N. Hershkowitz, and B. Longmier, Phys. Plasmas 14, 042109 (2007)]. While the size-dependent transitions in argon agree, the size-dependent transitions observed in helium systematically occur at lower electrode sizes than those nominally derived from prediction. The discrepancy in helium is anticipated to be caused by the finite size of the interface that increases the effective area offered to the plasma for electron loss to the electrode.

We demonstrate a method for determining the dissociation degree of atmospheric pressure air discharges by measuring the self-absorption characteristics of vacuum ultraviolet radiation from O and N atoms in the plasma. The atom densities are determined by modeling the amount of radiation trapping present in the discharge, without the use of typical optical absorption diagnostic techniques which require external sources of probing radiation into the experiment. For an 8.0 mm spark discharge between needle electrodes at atmospheric pressure, typical peak O atom densities of 8.5 × 1017 cm−3 and peak N atom densities of 9.9 × 1017 cm−3 are observed within the first ∼1.0 mm of plasma near the anode tip by analyzing the OI and NI transitions in the 130.0–132.0 nm band of the vacuum ultraviolet spectrum.

We demonstrate a method for determining the dissociation density of N and H atoms present in a developing low temperature plasma, based on the emission and self-absorption of vacuum ultraviolet radiation produced from the plasma. Spark plasmas are produced via pulsed discharge in N2/H2 mixtures at atmospheric pressure, where information on the dissociated densities of the constituent gas molecules is desired without employing invasive diagnostic techniques. By analyzing the self-absorption line profile of 121.5 nm Lyman-α H radiation emitted within the first ∼1.0 mm of plasma near the anode tip, a peak dissociated H atom concentration of 5.6 × 1017 cm−3 was observed ∼100 ns into spark formation, with an estimated electron density of 2.65 × 1018 cm−3 determined from Stark broadening. Similarly, simultaneous line fitting of the N 120.0/124.3 nm emission profiles revealed a peak dissociated N atom concentration of 3.8 × 1017 cm−3 during the same discharge period.

The mechanisms leading to the development of an atmospheric low temperature plasma along a surface under pulsed conditions is of current interest. In the early plasma phase, high energy photons are a contributing factor to the process of generating electron avalanches resulting in surface flashover. Since only photons in the vacuum ultraviolet (VUV) regime are energetic enough to cause step-ionization or direct ionization of atmospheric gases, an experiment has been set up to enable observations of photons with wavelengths shorter than 200 nm while still allowing observation up to 800 nm. A spectrum simulation software package has been developed to allow for temperature analysis on the developing plasma in the VUV region. Observations below 200 nm revealed a Boltzmann distributed excited state population corresponding to a temperature of 3.1 eV. Time-resolved emission spectroscopy measurements of the entire electrode region during the streamer phase of breakdown demonstrate the presence of molecular nitrogen emission lines from the second positive system. Further photomultiplier tube measurements of the spark phase showed a rapid decrease in intensity of the second positive system compared to that of a representative atomic emission line in the VUV regime. This emission dominates the ultraviolet–visible (UV–VIS) spectrum during the initial phases of breakdown with little detection of other sources of emission during this phase.

Vacuum ultraviolet (VUV) emission is believed to play a major role in the development of plasma streamers in pulsed atmospheric discharges, but detection of VUV light is difficult in pulsed experiments at atmospheric pressures. Since VUV light is absorbed in most standard optical materials as well, careful attention must be given to the selection of the lens and mirror optics used in these studies. Of highest interest is the VUV emission during the initial stage of pulsed atmospheric discharges, which has a typical duration in the nanosecond regime. An experiment was designed to study this fast initial stage of VUV emission coupled with fast optical imaging of streamer propagation, both with temporal resolution on the order of nanoseconds. A repetitive solid-state high voltage pulser was constructed which produces triggered flashover discharges with low jitter and consistent pulse amplitude. VUV emission is captured utilizing both photomultiplier and intensified charge-coupled device detectors during the fast stage of streamer propagation. These results are discussed in context with the streamer formation photographed in the visible wavelength regime with 3 ns exposure time.