Electrothermal Plasma Research Articles

Radiation Heat Transfer and Vapor Shielding in a Two-Dimensional Model of an Electrothermal Plasma Source

Electrothermal (ET) plasma discharges are emerging as valuable mechanisms for pellet injection in magnetic confinement fusion reactors. They have been shown to be capable of achieving the required pellet velocities and pellet launch frequencies required for edge localized mode control. Another advantage of ET plasma discharges is their ability to simulate fusion disruption events by depositing large heat fluxes on exposed materials. A deeper understanding of the heat transfer processes occurring in ET plasma discharges will aid in this particular application. ET plasma discharges involve the passage of high currents (order of tens of kA) along the axis of a narrow, cylindrical channel. As the current passes through the channel, radiant heat is transferred from the plasma core to the capillary wall. Ablated particles eventually fill the plasma channel and the partially ionized plasma is ejected. It is well known that the ablated material separating the plasma core from the ablating surface can act as a vapor shield and limit the radiation heat flux reaching the ablating surface. In this work, the results from a two-dimensional simulation model for ET plasma discharges are presented. The simulation of the plasma in a two-dimensional domain combined with the diffusion approximation for radiation heat transfer is shown to successfully simulate the effects of the vapor shield layer that develops inside these devices.

Spatiotemporal study of gas heating mechanisms in a radio-frequency electrothermal plasma micro-thruster

A spatiotemporal study of neutral gas temperature during the first 100 s of operation for a radio-frequency electrothermal plasma micro-thruster operating on nitrogen at 60 W and 1.5 Torr is performed to identify the heating mechanisms involved. Neutral gas temperature is estimated from rovibrational band fitting of the nitrogen second positive system. A set of baffles are used to restrict the optical image and separate the heating mechanisms occurring in the central bulk discharge region and near the thruster walls. For each spatial region there are three distinct gas heating mechanisms being fast heating from ion-neutral collisions with timescales of tens of milliseconds, intermediate heating with timescales of 10 s from ion bombardment on the inner thruster tube surface creating wall heating, and slow heating with timescales of 100 s from gradual warming of the entire thruster housing. The results are discussed in relation to optimising the thermal properties of future thruster designs.

Temperature-Dependent Hypersonic Flow Patterns of Expanding High-Density Metal Vapor Plasma from Capillary Source Simulating Plasma Flow Following a Fusion Disruption

Capillary discharge devices generating electrothermal plasmas from ablation of a liner material exposed to high heat flux are adequate devices to simulate fusion disruptions. Expansion of capillary-generated plasma into large volume simulates the evolution of the aerosol and plasma particulates into the reactor vacuum vessel. Effect of non-linearity and plasma bulk temperature on the adiabatic compressibility index was previously investigated showing considerable effect on the bulk flow parameters of polycarbonate plasma formed by the ablation of the capillary inner wall. In a fusion reactor, metals in the plasma-facing components such as the divertor, limiter, and first wall, will experience evaporation and formation of metal-vapor plasmas. Mathematical models have been developed to investigate the adiabatic compressibility index of ionized bulk metal vapors taking into account atomic and cluster ionization of metals, in addition to the effect of plasma bulk temperature and other nonlinearities. An important aspect of this current work is the distinction of the ionized states of metallic species instead of temperature-dependent lumped effective atomic number.

Generalized Scaling Laws of Plasma Parameters in Electrothermal Plasma Sources for Fusion Disruption Erosion and Hypervelocity Launch Applications

Generalized operational scaling laws have been developed for the peak plasma parameters at the exit of an electrothermal plasma capillary discharge. These parameters are the total ablated mass, plasma kinetic temperature, pressure, bulk velocity, and the radiant heat flux. The obtained scaling laws have particular coefficients depending on the material of the ablating liner inside the capillary source. The generalized scaling laws include the magnitude of the peak discharge current, material property, and the dimensions of the ablating sleeve. The values given by the scaling laws are compared with the ones calculated by the computational ElectroThermal Flow (ETFLOW) code and against the experimental data. The obtained results of the scaling laws have shown low error, especially the scaling laws of the kinetic temperature and the plasma exit velocity.

Flow Characteristics and Charge Exchange Effects in a Two-Dimensional Model of Electrothermal Plasma Discharges

Electrothermal (ET) plasma discharges are capillary discharges that ablate liner materials and form partially ionized plasma. ET plasma discharges are generated by driving current pulses through a capillary source with peak currents on the order of tens of kA and pulse lengths on the order of $$100\,\upmu \hbox {s}$$ . These plasma discharges can be used to propel pellets into magnetic confinement fusion devices for deep fueling of the fusion reaction, ELM mitigation, and thermal quench of the fusion plasma. ET plasma discharges have been studied using 0D, 1D, and semi-2D fluid models. In this work, a fully 2D model of ET plasma discharges is presented. The newly developed model and code resolve inter-species interaction forces due to elastic collisions. These forces affect the plasma flow field in the source and impede the development of plasma pressure at the exit of the source. In this work, these affects are observed for discharge current pulses peaking at 10 and 20 kA. The sensitivity of the model to the inclusion of charge exchange effects is observed. The inclusion of charge exchange has little effect on the integrated, global results of the simulation. The difference in total ablated mass for the simulations caused by the inclusion of charge exchange reactions is <1 %. Differences in local plasma parameters are observed during discharge initialization, but after initialization, these differences diminish. The physical reasoning for this is discussed and recommendations are made for future modeling efforts.

Study of High-Enthalpy Electrothermal Energetic Plasma Source Concept

The electrothermal (ET) energetic plasma source (ETEPS) is a different concept in which the ablation mechanism is forced inside of the open-ended capillary that has energetic liner, a propellant. The generation of the ET plasma results from Joule heating and radiant heat transport to the liner. The discharge initiates erosive burn of the propellant, and the mixed plasma-propellant gasification produces high-enthalpy energetic flow. In the ablation-dominated source, the eroded materials from the solid propellant liner are mixed inside the source before they flow out as a result of the large pressure gradient. The energetic ET source also has another concept in which no ablation occurs and the plasma is generated from the injection of energetic gasses or liquids into the confined open-ended capillary. The ablation-free source generates the plasma from the dissociation of the gaseous/liquid components, which in turn releases the chemical energy of these propellants and mixes the energy with the electrical energy of the plasma. This concept is different from ET chemical (ETC) sources; it generates the propellant or energetic flow without requiring a combustion chamber. It also provides mixing at the ionic level not available in current configurations of ETC launchers, igniters, or thrusters. The ET plasma code ETFLOW-EN was developed to computationally simulate the plasma generation and flow in energetic ET capillary discharges to predict the behavior of the energetic source with the use of lined solid propellants. Operation with liquid/gaseous energetic forms in a nonablative capillary is also a character of this concept and is part of the ETFLOW code. The results of using different forms of energetic materials in solid, liquid, and gaseous mixtures have shown the applicability of ETEPS to produce high-enthalpy energetic plasma flows with sufficient parameters suitable for ETC launch applications. Plasma and flow parameters at the capillary exit were investigated at different mixing ratios.

Modeling and Analysis of a Dual-Channel Plasma Torch in Pulsed Mode Operation for Industrial, Space, and Launch Applications

Dual-channel thermal plasma torch can operate with air, argon, or combustible gases to produce high-temperature plasma flow. This plasma torch can be used in various important applications such as metal industry recycling, surface coating and hardening, space operations using controlled thrust, and macroparticle acceleration based on the electrothermal nature of thermal torches and electrical-to-thermal energy conversion. Power for this torch is supplied from the electric mains and the voltage is stepped up to 6 kV. However, the torch can also operate on dc or pulsed mode. The electrical operation is characterized by the voltampere relationship to determine the power rating of the torch as well as diagnosing the dynamic behavior of the plasma. Experiments on the torch using air and argon have shown plasma temperatures in the range of 0.4–0.6 eV with plasma number density in the range of $10^{24}$ – $10^{25}/\text{m}^{3}$ , indicating a dense plasma regime with the plasma tends to be weakly nonideal. Plasma kinetic temperature and electron number density were obtained from optical emission spectroscopy using the relative line method as the plasma is near local thermodynamic equilibrium condition. Plasma temperature has its peak for low flow rates and decreases for increased flow rates. The torch modeling was conducted using an electrothermal plasma code to simulate and predict the parameters for pulsed mode operation. Simulation was conducted on a single channel as the dual torch is symmetric. Code results for extended pulselength show a plasma temperature between 0.6 and 0.8 eV for nitrogen, oxygen, and helium; which are in good correlation with plasma temperatures obtained from optical emission spectra and measured plasma resistivity. A set of computational experiments using short pulses at higher discharge currents has shown temperature in the range of 2.0–2.5 eV for nitrogen and helium.

Effect of Plasma Temperature and Nonlinearity of the Adiabatic Compressibility Index on Flow Parameters for Hypersonic Aerosol Expansion Following a Plasma Disruption

Future fusion reactors are expected to experience hard current disruptions that quench the confined plasma and depositing the energy on the plasma facing materials, thus causing surface ablation and the evolving aerosol expands into the reactor chamber. The plasma flow following a disruption event can be simulated experimentally and computationally using confined capillary discharges, which generate heat fluxes (up to ~80 GW/m2) typical to those expected in fusion reactors during a disruption. Computational technique to investigate the effect of plasma temperature on the adiabatic compressibility index has been investigated using a pulsed disruptive capillary electrothermal plasma source with a converging–diverging transition nozzle at the exit attached to the expansion regions. The plasma is formed in the capillary due to thermal decomposition of the ablating inner wall followed by subsequent ionization of the decomposed material. This work is particular to the cases where the ejected particulates coming out from a heat-load affected surface or arc-triggered plasma source orifice are at the sonic speed. The converging section in the nozzle provides a transition from the capillary exit subsonic condition to sonic state, which allows for observing the effect of temperature on the supersonic isentropic expansion. Mathematical model has been developed for effective atomic number (Z eff ) as function of the plasma temperature and has been used to model the adiabatic compressibility index (γ p ) of the plasma. Plasma parameters along the axial direction of the flow in the transition and the expansion regions are obtained.

Effect of Changing Source Capillary Radius on Bulk Flow Parameter Scaling Laws for Hypersonically Expanding Arc-Ablated Polycarbonate Plasma for Fusion and Space Applications

Capillary pulsed electrothermal plasma source coupled to a linear converging–diverging nozzle was previously found to be adequate as a simulator for the supersonic bulk flow patterns of the aerosol particulates generated following a hard disruption event that damages the plasma facing components in a fusion reactor. A similar system can assume an important role in the computational studies related to space electric propulsion and electrostatic ion thrusters. Scaling laws for the prediction of bulk temperature, density, pressure, plasma bulk velocity and Mach number as a function of the axial length traversed measured from the source exit had been proposed previously for an arc-ablated polycarbonate plasma jet evolving into a vacuum chamber via the transition nozzle. In the present work, the source capillary radius has been varied and with the sectional lengths remaining fixed, the converging and diverging angles have also been varied with it, as the diverging exit Mach number was kept constant at a high-hypersonic value ~18. The effect of changing source capillary radius as well as the transition region geometry on the aforesaid bulk flow parameters has been studied computationally and new scaling laws have been proposed.

Application of low-cost electrothermal vaporization capacitively coupled plasma microtorch optical emission spectrometry for simultaneous determination of Cd and Pb in environmental samples

The study discusses the applicability and analytical capability of a laboratory miniaturized system including a small-sized electrothermal vaporizer with Rh filament and a low power (15W) and low Ar consumption (150mlmin−1) capacitively coupled plasma microtorch interfaced with a low resolution microspectrometer for the simultaneous determination of Cd and Pb in environmental samples by optical emission spectrometry. Ten-microliter samples were dried (100°C, 80s) and vaporized (1500°C, 6s) by filament heating, then sample vapor was introduced into plasma via an Ar stream and 20 successive emission episodes were recorded with 500ms integration time per episode. The operating parameters of the electrothermal vaporization device and plasma were optimized in order to achieve the simultaneous determination of the two elements with best analytical performance. The method was verified by analyzing CRMs of soil, and then applied to real samples digested in aqua regia. The standard addition method was used to compensate for the non-spectral matrix effects on the analytical signal. Under the optimized conditions the limits of detection were 130±35μgkg−1 (13pg) for Cd at 228.802nm and 2100±175μgkg−1 (210pg) for Pb at 368.346nm. Recovery was in the range 98±18% for Cd and 103±11% for Pb for 95% confidence interval, while the relative standard deviation of repeatability was 4–12% (n=3). The investigated instrumentation has analytical potential and prototyping perspective to be used for toxic element determination in environmental samples.

Scaling Laws of Bulk Plasma Parameters for a 1-D Flow through a Capillary with Extended Converging–Diverging Nozzle for Simulated Expansion into Fusion Reactor Chamber

A capillary-extended converging–diverging transition region was previously proposed to allow for the flow and expansion of plasma into a large volume simulating aerosol expansion and particle transport in the active volume of a fusion reactor. It has been shown that the pulsed electrothermal plasma source was adequate for the simulation, and the expansion into the chamber is at steady conditions for the main plasma parameters indicating a uniform expansion of the aerosol following a disruption event. These parameters are the bulk temperature, density, pressure, plasma bulk velocity and Mach number for the same system geometrical configuration. Scaling laws in 1-D for bulk plasma parameters have been developed for ranges of axial length traversed by the flow to predict these parameters along the axis of the expansion chamber.

Analysis of Atmosphere-Breathing Electric Propulsion

To extend the lifetime of commercial and scientific satellites in low Earth orbit (LEO) and below (100–250 km of altitude) recent years showed an increased activity in the field of air-breathing electric propulsion as well as beamed-energy propulsion systems. However, preliminary studies showed that the propellant flow necessary for electrostatic propulsion at these altitudes exceeds the mass intake possible within reasonable limits, and that electrode erosion due to oxygen flow might limit the lifetime of eventual thruster systems. The pulsed plasma thruster (PPT), however, can be successfully operated with smaller mass intake and at relatively low power. This makes it an interesting candidate for air-breathing application in LEO and its feasibility is investigated within this paper. An analysis of such an air-breathing PPT system shows that for altitudes between 150 and 250 km, drag compensation is at least partially feasible assuming a thrust-to-power ratio of 30 mN/kW and a specific impulse of 5000 s. Further, to avoid electrode erosion, inductively heated electrothermal plasma generator technology is discussed to derive a possible propulsion system that can handle gaseous propellant without unfavorable side effects. Current technology can be used to create an estimated 4.4 mN of thrust per 1 mg/s of mass flow rate, which is sufficient to compensate the drag for small satellites in altitudes between 150 and 250 km.

Experiments and Modeling of Gas Heating in a Radio-Frequency Plasma Jet

Small plasma sources have been developed for a variety of applications and are often known as plasma jets (e.g. biomedical applications using atmospheric pressure helium or argon plasmas), as micro-plasma sources or arrays [1] (e.g. surface processing using lower operating pressure plasmas), or as micro-thrusters (e.g. electric propulsion using hollow cathode thrusters or neutralizers). At the ANU we have designed a capacitively coupled radiofrequency (13.56 MHz) cylindrical plasma source which currently operates at pressures around a few Torr and at powers ranging from 1-100 Watts with or without a pressure gradient imposed along the 2 cm long cavity [2]. The plasma diameter ranges from 1 to 6 mm before expanding in a larger chamber. Neutral gas heating is measured for a variety of gases (argon, nitrogen, xenon) and cavity diameters using optical emission spectroscopy. The results are combined with electric probe measurements to develop a plasma heating model where the heating is assumed to come from ion-neutral collisions in the maximum of the discharge under the powered electrode. A peaked density on axis is well described by the plasma model [2]. Typically, gas temperatures around 1000K are measured for N2 and Ar in agreement with theoretical predictions from Fruchtman [3]. The results show that this type of source can be tailored to suit a variety of plasma applications: its effectiveness in dissociating species means that it can be used for remote plasma etching [1] or for surface passivation from atomic nitrogen using N2. It can also be developed as an electrothermal plasma micro-thruster for space propulsion[2]. All of these applications benefit from the neutral gas heating and from the ability of the source to be developed as an array of sources. REFERENCES: [1]S. Dixon, C. Charles, R. Boswell, W. Cox, J. Holland and R. Gottscho, “Interactions between arrayed hollow cathodes”, J. Phys. D: Appl. Phys. 46, 145204, 2013[2]C. Charles and R.W. Boswell, “Measurement and modeling of a radiofrequency micro-thruster”, Plasma Sources Sci. Technol. 21, 022002, 2012[3]A. Fruchtman, “Energizing and depletion of neutrals by a collisional plasma”, Plasma Sources Sci. Technol. 21, 022002, 2012

Volume and surface propellant heating in an electrothermal radio-frequency plasma micro-thruster

The temporal evolution of neutral gas temperature over the first 5 min of operation for an electrothermal radio-frequency micro-thruster with nitrogen (N2) propellant was measured using rovibrational band matching of the second positive N2 system. Three distinct periods of gas heating were identified with time constants of τ1 = 8 × 10−5 s, τ2 = 8 s, and τ3 = 100 s. The fast heating (τ1) is attributed to volumetric heating processes within the discharge driven by ion-neutral collisions. The slow heating (τ3) is from ion neutralization and vibrational de-excitation on the walls creating wall heating. The intermediate heating mechanism (τ2) is yet to be fully identified although some theories are suggested.

Characterization of Short Intense Pulsed Electrothermal Plasma Capillaries for Use as Fusion and Launchers Heat Flux Sources

Electrothermal plasma sources operating in the confined capillary arc regime are characterized by the magnitude and shape of the discharge current. The desired plasma parameters at the source exit, especially the pressure and heat flux, are highly dependent on the arc due to the effect of the arc radiant energy that ablates the inner wall of the source. These sources have applications in fusion as drivers for pellet injectors and as high heat flux sources for fusion materials studies. The high-pressure high heat flux flow is also of application in mass accelerators and launch technology systems. The 1-D, time-dependent ETFLOW capillary code models the plasma generation and flow inside the capillary discharges and determines the plasma parameters. The input file to the code is the discharge current density providing the Joule heating in the energy equation. A circuit module has been developed and incorporated in the code to generate desired current shapes and magnitudes. The current pulse length was varied between 5 and 100 μs at constant amplitude of 50 kA, and then the pulse amplitude was varied between 10 and 200 kA at a constant pulse length of 20 μs. Increasing the pulse length while maintaining its amplitude increases the plasma density and the total ablated mass, which have accumulation behavior by increasing the pulse length, and subsequently increases the exit pressure from 60 to 410 MPa in the cases studied herein. The pressure increase allows the thermalization of the plasma particles through more collisions, which reduces the plasma temperature by about 0.2 eV. The bulk velocity follows the trend of the plasma temperature, but at shorter pulse lengths the total ablated mass is lower and enables the plasma to carry the particles with increasing velocity. Increasing the pulse amplitude up to 200 kA increases the density to about 18 kg/m3 and the bulk velocity, which varies between 6.1 and 10.7 km/s. A sharp increase in most plasma parameters occurs as a result of the increase in the pulse amplitude.

Radiative Heat Transport through Vapor Plasma for Fusion Heat Flux Studies and Electrothermal Plasma Sources Applications

Radiative Heat Transport through Vapor Plasma for Fusion Heat Flux Studies and Electrothermal Plasma Sources Applications High heat fluxes of up to 100 GW/m2 and greater over a discharge period of 100 to 1000 μs can be generated from electrothermal (ET) plasma sources from the confined arc discharge. Sources with input energy of 10 kJ in a miniature capillary (4 mm radius and 9 cm length) are capable of producing 88.33 GW/m2 heat flux inside the capillary, higher heat fluxes can be generated for higher input energies. Such high heat fluxes are adequate to simulate the energy deposition during hard disruptions in future fusion tokamak reactors, which result in erosion and thermal deformation of the surfaces of the critical internal components of the reactor. Calculation of the eroded mass due to intensive transient radiative heat transport to the surfaces is very critical in terms of the determination of the performance, durability and the life time of these components.

Ablation of Fusion Materials Exposed to High Heat Flux in an Electrothermal Plasma Discharge as a Simulation for Hard Disruption

Electrothermal plasma sources operating in the confined controlled arc discharge regime produce heat fluxes in the range expected for hard disruptions in future large tokamaks. The radiative heat flux produced inside of the capillary discharge channel is from the formed high density (1023–1027/m3) plasma with heat fluxes of up to 125 GW/m2 over a period of 100 μs, making such sources excellent simulators for ablation studies of plasma-facing materials in tokamaks during hard disruptions. Graphite, beryllium, lithium, stainless steel, tungsten, copper, and molybdenum are among the materials proposed for use in fusion reactors. Computational experiments with the ETFLOW code using heat fluxes between 10 and 125 GW/m2 have shown low total erosion for the low-z materials Li, Be and C and higher erosion for high-z materials Fe, Cu, Mo and W. The time rate of material erosion for various ranges of heat fluxes shows increased erosion with time evolution over the 150 μs pulse length of the simulated disruption event. At the highest values of simulated heat flux, low-z materials were found to ablate almost identically. At all simulated values of heat flux, the ablation of high-z materials correlated positively with the z-number.

Optimization of Fusion Pellet Launch Velocity in an Electrothermal Mass Accelerator

Electrothermal mass accelerators, based on capillary discharges, that form a plasma propelling force from the ablation of a low-z liner material are candidates for fuelling magnetic fusion reactors. As lithium is considered a fusion fuel and not an impurity, lithium hydride and lithium deuteride can serve as good ablating liners for plasma formation in an electrothermal plasma source to propel fusion pellets. A comprehensive study of solid lithium hydride and deuteride as liner materials to generate a plasma to propel cryogenic fuel pellets is presented here. This study was conducted using the ETFLOW capillary discharge code. Relationships between propellants, source and barrel geometry, pellet volume and aspect ratio, and pellet velocity are determined for pellets ranging in volume from 5 to 100 mm3.

Supersonic Flow Patterns from Electrothermal Plasma Source for Simulated Ablation and Aerosol Expansion Following a Fusion Disruption

A pulsed electrothermal plasma source of a capillary discharge operating in the confined controlled arc regime is investigated to simulate the source term for ablation-induced regime of fusion reactor following hard disruption, in which ablation of diverter surface produces large aerosol transporting into the vacuum vessel. The source is attached to a converging–diverging micro-nozzle transition region to allow for the plasma flow and expansion into a large volume simulating large chamber of fusion reactor aerosol expansion to facilitate modeling of the plasma transport. This transition region connects to a 4 mm diameter capillary source and has a 3.33 mm converging section with a 2° converging angle, followed by a 146.7 mm diverging section with a 60° diverging angle, thus making an overall transition length of ~150 mm. The diverging section has an exit diameter of 50.82 cm to open into a large volume of the same exit diameter and a length of 1 m. Preliminary computation results indicate about 21 Mach number at the diverging exit and drops down to 0.7 Mach number after suffering from multiple shocks in the large uniform expansion volume. The plasma parameters entering the large chamber are maintained constant along the axis of the chamber for a simulated 1-D condition.

Nanosecond optical imaging spectroscopy of an electrothermal radiofrequency plasma thruster plume

Nanosecond optical imaging spectroscopy is employed to investigate the spatio-temporal dynamics of the plasma plume expanding from a 4.2 mm-diameter, 20 mm-long cylindrical capacitively coupled electrothermal radiofrequency (rf) driven thruster using 10 W of power at 12.50 MHz and an argon pressure of 1.5 Torr. On-axis, the plume exhibits four distinct peaks of optical emission intensity within the rf period. The plume has a spherical shape with a transient radial extension (during half of the rf cycle) at the thruster exit plane due to an rf current to ground when the grounded electrode acts as an anode.