Breakdown In Helium Research Articles

Nanosecond pulse breakdown in noble gases

The results of the investigation of the breakdown characteristics of the planar “open” discharge and open discharge with the generation of counter-propagating electron beams under excitation by pulses with nanosecond rise fronts are presented. The amplitude parameters of current and voltage and temporal characteristics of breakdown in helium, neon, and argon were measured. It is demonstrated that the breakdown in the open discharge is characterized by considerably larger electric field strengths at the same development delays as in the avalanche discharge. A similarity criterion based on the photoemission mechanism of electron generation, according to which the discharge development delay is inversely proportional to the squared working gas pressure, is obtained.

Theoretical study of threshold intensity dependence on gas pressure in the breakdown of helium by CO2 laser radiation: Influence of preionization mechanism

An electron cascade model adapted to provide a thorough understanding of the physical mechanisms involved in CO2 laser-induced helium plasma is presented. The model combines a time-dependent calculation of the electron energy distribution with rate equations, describing how the population of excited states changes [Y. E. E.-D. Gamal and G. Abdellatif, Appl. Phys. B 117(1), 103 (2014)]. It encountered the possible elastic and inelastic electron collisional processes that enhance the electrons' growth, leading to gas breakdown. The analysis explores the experimental threshold intensity dependency on gas pressure [J. J. Camacho et al., Spectrochim. Acta, Part B 66(1), 57 (2011)]. The measurements are carried out using 9.621 μm over pressure in the range from 12.0 to 87.0 kPa. Since multiphoton ionization is improbable, ionization proceeds via the inverse bremsstrahlung absorption. In this experiment, the ignition of this process is initiated by the experimentally assumed pre-breakdown approach. No experimental estimation was given for the initial electron density. The electron diffusion and the loss of electron energy through elastic collisions have no contribution to this experiment. The calculations of the threshold intensity are performed to determine the initial electron density. The model's validity is assured by the reasonable agreement between the calculated thresholds, and the measured ones are only achieved at a specific initial electron value for each gas pressure. Over pressure exceeding 30.0 kPa, the agreement was reasonable in the presence of recombination losses. The threshold intensity is controlled by the initial electron density for lower pressures. The analysis showed how the gain and loss of electrons control the breakdown threshold for helium concerning the determined initial electron density for the tested pressures.

Reaction chain used to describe the process and mechanism of helium breakdown under high pressure

In this study, we establish a two-dimensional axisymmetric fluid model and verify its efficacy via experiments. We analyze the reaction rates to introduce a concept that we have termed “reaction chain” to describe the process and mechanism of helium discharge at high pressure. We observe that the entire process of discharge can be perfectly described by the reaction chains, which indicates that the breakdown process of helium involves a series of chain reactions, just like the nuclear fission. The results revealed that ground-state ionization reaction predominates the early discharge stage, whereas with the quick accumulation of He2*, penning ionization reaction will eventually play a dominant role. Our study confirms that metastable atoms play an important role in helium discharge. We demonstrated that He* is converted into He2* as an intermediate and that the number density of He2* eventually influences the breakdown of helium.

Effect of ambient gas species on microwave breakdown pattern

The air, nitrogen, and helium breakdown caused by microwaves at atmospheric pressure are, respectively, investigated by solving the theoretical model, which consists of Maxwell’s equations and plasma fluid equations. The transport coefficients (e.g. an ionization frequency) are predicted by the Boltzmann equation solver BOLSIG+. The effective diffusion coefficient that describes properly the transition from the free diffusion to the ambipolar diffusion is introduced into the plasma fluid equations. The two-dimensional simulation results show that a filamentary plasma array is produced in the air and nitrogen breakdown, but it disappears completely in the helium breakdown. This is because the transport coefficients and wave scattering by the plasma differ from gas to gas. The simulated plasma patterns in the three different gases agree qualitatively with the experimental results. In addition, the plasma propagation speed in the nitrogen breakdown from the simulations is close to the experimental data.

High-fidelity modeling of breakdown in helium: initiation processes and secondary electron emission

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(21P) 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.

Experiments and simulation of helium breakdown at high pressure and temperature in a non-uniform electric field

Breakdown voltages of helium between needle-plane copper electrodes were measured to obtain the base data for insulation design of the electrical equipment working in high temperature and pressure helium gas in the High Temperature Gas-cooled Reactor (HTGR). The measurements were performed at a gap length between 1.02 mm and 5.30 mm, temperature and pressure varying from 0.01 to 7 MPa and 25–180 °C, respectively. Some abnormal “U-shapes” were found to appear especially at pressure over 3 MPa, which can be explained by the effects of space charge on the corona layer. COMSOL Multiphysics was used to simulate the discharge with a two-dimensional drift–diffusion model. The results agree well with the experimental data on the whole, but the errors rise when the pressure and gap length become very large. Evolutions of the current and particle number densities were also obtained. These breakdown voltages and characteristics of helium discharge is considered much valuable to the insulation design in HTGR.

Experiments on helium breakdown at high pressure and temperature in uniform field and its simulation using COMSOL Multiphysics and FD-FCT

In this paper, experiments on breakdown voltages of helium gas in a uniform field have been carried out at a separation from 0.25 to 3.02 mm. The pressure and temperature respectively vary from 1 to 7 MPa, 25 °C to 180 °C, which is the working condition for the main helium blower and its Active Magnetic Bearings (AMBs) in the High Temperature Reactor-Pebble-bed Module (HTR-PM). COMSOL Multiphysics and a self-programmed FD-FCT (Finite Difference-Flux Corrected Transport) code with a two-dimensional drift–diffusion plasma model have been used to simulate the breakdown process and the results agree well with the experimental data. This kind of theoretical and experimental work provides valuable references for helium insulation design of electric devices in HTR-PM and other helium gas-cooled reactors. It also helps understand the mechanisms in helium discharge at high temperature and pressure which is quite different from its low counterpart.

1D PIC-DSMC analysis of a high-pressure nanosecond pulse discharge breakdown in helium

Kinetic simulations of plasma phenomena during and after formation of the conductive plasma channel of a nanosecond pulse discharge are analyzed and compared to existing experimental measurements. Particle-in-cell with direct simulation Monte Carlo collisions (PIC-DSMC) modeling is used to analyze a discharge in helium at 200 Torr and 300 K over a 1 cm gap. The analysis focuses on physics that would not be reproduced by fluid models commonly used at this high number density and collisionality, specifically non-local and stochastic phenomena. Similar analysis could be used to improve the predictive capability of lower fidelity or reduced order models. First, the modeling results compare favorably with experimental measurements of electron number density, temperature, and 1D electron energy distribution function at the same conditions. Second, it is shown that the ionization wave propagates in a stochastic, stepwise manner, dependent on rare, random ionization events ahead of the ionization wave when the ionization fraction in front of the ionization wave is very low, analagous to the stochastic branching of streamers in 3D. Third, analysis shows high-energy runaway electrons accelerated in the cathode layer produce electron densities in the negative glow region over an order of magnitude above those in the positive column. Future work to develop reduced order models of these two phenomena would improve the accuracy of fluid plasma models without the cost of PIC-DSMC simulations.

Laser induced breakdown in pulsed helium atmospheric pressure plasma jet

This study connects two fields of research: laser induced plasmas and nonequilibrium low-temperature plasmas by studying effects of preionisation, made by a single-electrode atmospheric pressure plasma jet, on a laser induced breakdown (LIB) in helium. A laser pulse (1064 nm, 4 ns pulse width, 5 Hz repetition rate) was focused into the helium jet flowing into ambient air and synchronized with the discharge (10 kHz repetition rate). The level of preionisation in the focal volume at the moment when the laser pulse arrives was controlled by changing the time delay between voltage and laser pulse. The conditions of the discharge are strongly affecting laser induced plasma by providing initial seed of electrons and promoting the breakdown in helium. Measurements of the absorbed laser pulse energy and optical emission relative intensities were performed for different time delays between voltage and laser pulse. Study of LIB with preionisation is discussed as a potential tool for diagnostics of free plasma jets and jets interacting with various target materials.

Three regimes of high-voltage breakdown in helium

A quasi-analytical model was developed to map out the low-pressure (left-hand) branch of the Paschen curve at very high voltage when electrons are in the runaway regime and charge exchange/ionization avalanche sustained by ions and fast neutral atoms becomes important. The model was applied to helium gas between parallel-plate electrodes, at potentials ranging in magnitude between 10 and 1000 kV. The respective value of reduced electric field E/n varied in the range of 50−6000 kTd (1 kTd = 10−18 Vm2), with reduced density nd (where n is the gas density and d is the inter-electrode distance) on the order of 1020 m−2. Three regimes of the breakdown have been identified according to the relative share of impact ionization by electrons, by ions, and by fast neutrals. The analytically derived Paschen curve is compared to those obtained with a detailed particle-in-cell/Monte Carlo simulation, and also through experimental measurements (L Xu, A V Khrabrov, I D Kaganovich and T J Sommerer 2017 Phys. Plasmas 24 093511). The model provides accurate predictions for E/n up to ∼103 kTd, constrained by availability and quality of required input data.

Breakdown in helium in high-voltage open discharge with subnanosecond current front rise

Investigations of high-voltage open discharge in helium have shown a possibility of generation of current pulses with subnanosecond front rise, due to ultra-fast breakdown development. The open discharge is ignited between two planar cathodes with mesh anode in the middle between them. For gas pressure 6 Torr and 20 kV applied voltage, the rate of current rise reaches 500 A/(cm2 ns) for current density 200 A/cm2 and more. The time of breakdown development was measured for different helium pressures and a kinetic model of breakdown in open discharge is presented, based on elementary reactions for electrons, ions and fast atoms. The model also includes various cathode emission processes due to cathode bombardment by ions, fast atoms, electrons and photons of resonant radiation with Doppler shift of frequency. It is shown, that the dominating emission processes depend on the evolution of the discharge voltage during the breakdown. In the simulations, two cases of voltage behavior were considered: (i) the voltage is kept constant during the breakdown; (ii) the voltage is reduced with the growth of current. For the first case, the exponentially growing current is maintained due to photoemission by the resonant photons with Doppler-shifted frequency. For the second case, the dominating factor of current growth is the secondary electron emission. In both cases, the subnanosecond rise of discharge current was obtained. Also the effect of gas pressure on breakdown development was considered. It was found that for 20 Torr gas pressure the time of current rise decreases to 0.1 ns, which is in agreement with experimental data.

Compositional Analysis of Aerosols Using Calibration-Free Laser-Induced Breakdown Spectroscopy.

We demonstrate that the elemental composition of aerosols can be measured using laser-induced breakdown spectroscopy (LIBS) without any preliminary calibration with standard samples. Therefore, a nanosecond Nd:YAG laser beam was focused into a flux of helium charged with alumina aerosols of a few micrometers diameter. The emission spectrum of the laser-generated breakdown plasma was recorded with an echelle spectrometer coupled to a gated detector. The spectral features including emission from both the helium carrier gas and the Al2O3 aerosols were analyzed on the base of a partial local thermodynamic equilibrium. Thus, Boltzmann equilibrium distributions of population number densities were assumed for all plasma species except of helium atoms and ions. By analyzing spectra recorded for different delays between the laser pulse and the detector gate, it is shown that accurate composition measurements are only possible for delays ≤1 μs, when the electron density is large enough to ensure collisional equilibrium for the aerosol vapor species. The results are consistent with previous studies of calibration-free LIBS measurements of solid alumina and glass and promote compositional analysis of aerosols via laser-induced breakdown in helium.

Investigation of the threshold intensity versus gas pressure in the breakdown of helium by 248 nm laser radiation

An investigation of the unexpectedly strong dependence of the threshold intensity on the gas pressure in the experimental study on the breakdown of He by short laser wavelength (Turcu et al., in Opt Commun, 134:66–68, 1997) is presented. A modified electron cascade model is applied (Evans and Gamal, in J Phys D Appl Phys, 13:1447–1458, 1980). Computations revealed reasonable agreement between the calculated thresholds and the measured ones. Moreover, the calculated electron energy distribution function and its parameters proved that multiphoton ionization of ground and excited atoms is the main source for the seed electrons, which contributes to the breakdown of helium. The effect of diffusion losses over pressures <1,000 Torr elucidated the origin of the strong dependence of the threshold intensity on the gas pressure. Collisional ionization dominates only at high pressures. No evidence for recombination losses is observed for pressures up to 3,000 Torr.

Effect of light irradiation on Townsend breakdown in helium at 5.2 K

We report an experimental investigation of the light irradiation effect on the Townsend breakdown mechanism of plasma discharges in helium, at 5.2 K, for densities up to 23 mol l−1. The plasma discharges were generated in a pin-to-plane electrode geometry, separated by a gap distance of 5 µm. The pin electrode was the cathode. Two light sources, with wavelengths of 365 and 635 nm, were used. The influence of the pressure and the light flux was studied. Based on the results, a phenomenological interpretation of the effect of light irradiation on the breakdown mechanism is proposed. Photon-enhanced field emission of electrons could be of significant importance.

Influence of the breakdown mechanism to the left of the Paschen minimum on the breakdown temperature of the free electron gas Maxwell spectrum

Dc breakdown of helium in the range to the left of the Paschen minimum is examined. An expression relating breakdown voltage to the pd product (pressure × inter-electrode distance) and the breakdown temperature of the free electron gas Maxwell spectrum is derived. The type of breakdown mechanism is deduced by submitting the measured values of breakdown voltage in the range to the left of the minimum to a statistical analysis that treats the dc breakdown voltage as a random variable. Fitting the derived expression to experimental results allows breakdown temperatures to be determined, along with ways of applying them under different breakdown mechanisms.

Time-resolved optical emission spectroscopic measurements of He plasma induced by a high-power CO 2 pulsed laser

Laser-induced breakdown spectroscopy of helium plasma, initially at room temperature and pressures ranging from 12 to 101 kPa was investigated using a transverse excitation atmospheric CO 2 pulsed laser ( λ = 9.621 and 10.591 μm, a full width at half maximum of 64 ns, and an intensity from 1.5 to 5.36 GW cm −2). The helium breakdown spectrum is mainly due to electronic relaxation of excited He, He + and H. Plasma characteristics were examined in detail on the emission lines of He and He + by the time-integrated and time-resolved optical emission spectroscopy technique. Optical breakdown threshold intensities, ionization degree and plasma temperatures were obtained. An auxiliary metal mesh target was used to analyze the temporal evolution of the species in the plasma. The results show a faster decay of the continuum emission and He + species than in the case of neutral He atoms. The velocity and kinetic energy distributions for He and He + species were obtained from time-of-flight measurements. Electron density in the laser-induced plasma was estimated from the analysis of spectral data at various times from the laser pulse incidence. Temporal evolution of electron density has been used for the estimation of the three-body electron–ion recombination rate constant.

Characteristics of Gas Breakdown in Hollow-Core Fibers

We succeeded in obtaining gas discharge in 250-, 150-, and 50-mum inner-diameter (i.d.) hollow-core fibers (HCFs) by using longitudinal direct current excitation. Stable glow discharges of at least several minutes were observed for these HCFs. A flash glow was also observed for an HCF with an i.d. of ~20 mum. Breakdown of helium and argon gases in a 26.2-cm-long 250-mum-i.d. HCF was achieved with a voltage of less than 30 kV. Breakdown voltages of helium and argon gases were measured for various bore sizes, fiber lengths, and a pressure range from below 1 to 50 Torr. Experimental results deviated from previous theoretical models and further theoretical and experimental investigations are needed to understand the unique characteristics of gas-discharge in HCFs with i.d. below 250 mum.

Monte Carlo versus bulk conductivity modeling of RF breakdown of helium

A Monte Carlo collision model and a bulk conductivity model have been implemented in the finite-difference time-domain code Lsp to allow simulation of weakly-ionized plasmas. The conductivity model uses only mesh quantities derived from moments of the electron distribution function, while the Monte Carlo model uses particles to provide a detailed representation of the electric distribution function. The models are compared in simulations of Helium gas breakdown in an applied radio frequency radio frequency (RF) electric field. The conductivity model assumes that the free electron velocity distribution equilibrates instantly with the applied field, and transport coefficients for the model are obtained from steady-state solutions of the Boltzmann equation. For Helium near standard temperature and pressure (STP) and a 1-GHz applied electric field, the conductivity model is found to agree well with the Monte Carlo model and is orders of magnitude faster. The Monte Carlo model, which treats scattering and ionization of particles in a detailed way, captures transient effects associated with finite electron heating and cooling times which are absent from the conductivity model

Production of hollow cylindrical plasmas for laser guiding in acceleration experiments

One of the major tasks for the progress of laser acceleration of electrons in plasmas is the guiding of focused pulses along path lengths much larger than the depth of focus. We have tested experimentally the production of hollow plasmas in gas to be used as guiding medium. We induced optical breakdown in Helium with a nanosecond laser pulse similar to the Amplified Spontaneous Emission (ASE) pedestal of a powerful ultrashort laser pulse. The plasma produced in this way was carefully characterized by high resolution interferometry. Plasma channels several millimeters in length whose density, depth and width match the requirements for an efficient guiding have been obtained. Channels with a variety of parameters can be created according to the well-known dynamics of the plasmas produced by optical breakdown.

Plasma-based nitrogen tail pulse shutter for CO2-TEA lidar dial systems

This paper presents the construction, use and characterisation of a laser-induced sealed plasma shutter to clip off the nitrogen pulse tail of a CO2-TEA laser-based lidar dial system. Investigation of the optimum gas filling pressure, temporal profile of the clipped pulse, and the laser threshold power intensities to achieve ionisation growth and breakdown in helium, argon, and nitrogen are also presented. Values of these power density thresholds lie between 3×1011 W cm-2–5×1012 W cm-2, 2×1011 W cm-2–2×1012 W cm-2 and 3×1013 W cm-2–2×1014 W cm-2 for helium, argon, and nitrogen, respectively. The range resolution attainable with the present clipped pulses is 15 m, which is 30 times better than that readily obtained with the nitrogen-tailed pulses. Field measurements of the lidar returns with the clipped pulse from a co-operative target are presented.