Electrothermal Plasma Source Research Articles

Measurements of dynamic surface changes by digital holography for in situ plasma erosion applications.

There are currently few viable diagnostic techniques for in situ measurement of plasma facing component erosion. Digital holography is intended to fill this gap. Progress on the development of single and dual CO2 laser digital holography diagnostics for in situ plasma facing component erosion is discussed. The dual laser mode's synthetic wavelength allows the measurable range to be expanded by a factor of ∼400 compared to single laser digital holography. This allows the diagnostic to measure surface height changes of up to 4.5 μm in single laser mode and up to 2 mm in dual laser mode. Results include ex situ measurements of plasma eroded targets and also dynamic measurements of nm and μm scale motion of a target mounted on a precision translation stage. Dynamic measurements have successfully been made with the system operating in both single and dual laser modes, from ∼50 nm to ∼4 μm in single laser mode and up to ∼400 μm in dual laser mode (limited only by the stage speed and camera acquisition duration). These results demonstrate the feasibility of using digital holography to characterize plasma facing component erosion dynamically, i.e., during plasma exposure. Results of proof-of-principle in situ digital holographic measurements of targets exposed to an electrothermal arc plasma source are presented.

A repetitive high current pulse generator for high flux electrothermal plasma jets.

Power sources play an important role in the characteristics and the applications of the electrothermal (ET) plasma as an edge localized mode (ELM) heat flux simulator. A repetitive high current ET plasma source with the capability of working at a 10 Hz repetition rate and peak current 7.5 kA is presented in this paper. By controlling the sequence of discharge of ten pulse power modules, a repetitive high heat flux plasma jet can be generated. A two-stage capillary structure is presented, and its repetitive trigger driving circuit based on surface flashover ignition is designed to achieve reliable and repetitive discharge. The topology of the inductive and capacitive (LC) series resonant circuit is applied to the charging system of the pulsed power source. The charging current is limited to 500 A with a charging time of 3.5 ms, and the ratio of the charging voltage to the operating voltage is 1.85. A diode and a power resistor in series are used to suppress the negative overvoltage, which is helpful to increase the thyristors' operating reliability. Using the designed repetitive ET plasma source, the characteristics of the ET plasma jet are investigated by measuring the voltages and currents and by obtaining images of the discharges. Experimental results show that the repetitive ET plasma generator can be used as an appropriate way to simulate the ELM-like heat flux plasma.

Surface Erosion of Plasma-Facing Materials Using an Electrothermal Plasma Source and Ion Beam Micro-Trenches

Erosion characteristics of tungsten-alternative plasma-facing materials (PFMs) were tested under high heat flux conditions in the electrothermal plasma source facility at Oak Ridge National Laboratory. The PFMs of interest are high-purity β-3C chemical vapor deposition silicon carbide (SiC) and the MAX phases Ti3SiC2 and Ti2AlC [MAX = chemical formula Mn+1AXn, where M is an early transition metal (such as Ti or Ta), A is an A-group element (such as Si or Al), and X is carbon or nitrogen]. An erosion analysis method was developed using a combination of focused ion beam microscopy and scanning electron microscopy, carving micro-trench geometries into polished sample surfaces. Samples of SiC, Ti3SiC2, and Ti2AlC were exposed to the electrothermal plasma source alongside tungsten and monocrystalline silicon. Samples were exposed to a Lexan polycarbonate (C16H14O3) electrothermal plasma stream in a He environment, at a specified impact angle, with infrared camera diagnostics. Edge localized mode–relevant heat fluxes of 0.9 to 1 GW/m2 over 1-ms discharges were generated on the target surfaces. Tungsten samples exhibited pronounced melt-layer formation and deformation, with measured molten pits 2 to 10 μm in diameter and melt-layer depths of up to 7 μm deep. Surface erosion rates for Ti3SiC2 and Ti2AlC ranged from 80 to 775 μm/s and 85 to 470 μm/s, respectively. Both MAX phases exhibited extreme surface fracture and material ejection, with damage depths past 4 μm for Ti2AlC and 11 μm for Ti3SiC2. SiC displayed the best performance, in one case surviving 15 consecutive electrothermal plasma exposures with an average erosion rate of about 29 μm/s and no surface fracturing. SiC erosion rates ranged from 23 to 128 μm/s.

Characterization of an electrothermal plasma source for fusion transient simulations

The realization of fusion energy requires materials that can withstand high heat and particle fluxes at the plasma material interface. In this work, an electrothermal (ET) plasma source has been designed as a transient heat flux source for a linear plasma material interaction device. An ET plasma source operates in the ablative arc regime driven by a DC capacitive discharge. The current channel width is defined by the 4 mm bore of a boron nitride liner. At large plasma currents, the arc impacts the liner wall, leading to high particle and heat fluxes to the liner material, which subsequently ablates and ionizes. This results in a high density plasma with a large unidirectional bulk flow out of the source exit. The pulse length for the ET source has been optimized using a pulse forming network to have durations of 1 and 2 ms. The peak currents and maximum source energies seen in this system are 1.9 kA and 1.2 kJ for the 2 ms pulse and 3.2 kA and 2.1 kJ for the 1 ms pulse, respectively. This work is a proof of the principal project to show that an ET source produces electron densities and heat fluxes comparable to those anticipated in transient events in large future magnetic confinement fusion devices. Heat flux, plasma temperature, and plasma density were determined for each shot using infrared imaging and optical spectroscopy techniques. This paper will discuss the assumptions, methods, and results of the experiments.

Comparison of an Electrothermal Plasma Source to a Light Gas Gun for Launching Large Cryogenic Pellets for Tokamak Disruption Mitigation

Future large tokamaks, such as ITER, will require a reliable technique for rapid energy dissipation to mitigate harmful effects from disruptions. Two main methods developed for disruption mitigation are massive gas injection and shattered pellet injection (SPI). Argon and neon are favorable materials for both injection methods. When launching pellets with SPI, it has proven difficult to launch intact pellets of pure argon and/or neon owing to their high material strength at cryogenic temperatures. In this work, we compare two methods of launching relatively high-Z pellets. An electrothermal plasma source is an experimental alternative to the fast opening, high-pressure, gas valve. The electrothermal source was used to launch Lexan™ pellets with approximately the same size and mass of comparable mixed gas (D2 and Ne) cryogenic pellets launched by gas guns. We describe comparisons of achieved pellet velocities, energy efficiencies of each system, and the implications of implementing each respective method on an operating tokamak.

Material impacts and heat flux characterization of an electrothermal plasma source with an applied magnetic field

To produce a realistic tokamak-like plasma environment in linear plasma device, a transient source is needed to deliver heat and particle fluxes similar to those seen in an edge localized mode (ELM). ELMs in future large tokamaks will deliver heat fluxes of ∼1 GW/m2 to the divertor plasma facing components at a few Hz. An electrothermal plasma source can deliver heat fluxes of this magnitude. These sources operate in an ablative arc regime which is driven by a DC capacitive discharge. An electrothermal source was configured with two pulse lengths and tested under a solenoidal magnetic field to determine the resulting impact on liner ablation, plasma parameters, and delivered heat flux. The arc travels through and ablates a boron nitride liner and strikes a tungsten plate. The tungsten target plate is analyzed for surface damage using a scanning electron microscope.

Computational Investigation of Nonideal Higher Ionization Carbon Plasma Under Simulated High Heat Fluxes in Fusion Tokamak Reactors

Plasma-facing materials in future large tokamaks will suffer from ablation due to expected hard disruptions, which affects the reactor interior lining tiles and the divertor modules. Ablation and surface evaporation due to the intense heat flux from disruption is associated with ionization of the evolved particulates. Generated ions at such plasma conditions may allow for higher ionization states such that the plasma at the boundary can be composed of electrons, ions (first, second and third ionization) and excited atoms. The boundary layer is dense and tends to be weakly nonideal. The NC State University electrothermal plasma code ETFLOW used to simulate the high heat flux conditions in which the carbon liner tested for simulated heat fluxes for transient discharge period of 100 μs, with FWHM of ~50 μs, to provide a wide range for obtaining reasonable good fits for the scaling laws. Transient events with ~10 MJ/m2 energy deposition over short transient of 50–100 μs would produce heat fluxes of 100–200 GW/m2. The heat flux range in this simulation is up to 288 GW/m2 to explore the generation of carbon plasma up to the third ionization C+++. The generation of such heat fluxes in the electrothermal plasma source requires discharge currents of up to 250 kA over a 100 μs pulse length with ~50 μs FWHM. The number density of the third ionization is six orders of magnitude less than the first ionization at the lowest heat flux and two orders of magnitude less at the highest heat flux. Plasma temperature varies from 31,600 K (2.722 eV) to 47,500 K (4.092 eV) at the lowest and highest heat fluxes, respectively. The plasma temperature and number density indicate typical high-density weakly nonideal plasma. The evolution of such high-density plasma particles into the reactor vacuum chamber will spread into the vessel and nucleate on the other interior components. The lifetime of the PFCs will shorten if the number of hard disruptions at such extreme heat fluxes would be increasing, resulting in major deterioration of the armor tiles.

Investigation of Geometry and Current Density Effects in a Pulsed Electrothermal Plasma Source Using a 2-D Simulation Model

Electrothermal (ET) plasma discharges have application to mass acceleration technologies relevant to military ballistics and magnetic confinement fusion reactor operation. ET plasma discharges are initiated in capillary geometries by passing large currents (order of tens of kiloamperes) along the capillary axis. A partially ionized plasma then forms and radiates heat to the capillary walls inducing ablation. Ablated particles enter the capillary plasma source and cause a pressure surge that can propel pellets to velocities exceeding 2 km/s. These devices present several advantages over other mass accelerator technologies due to their simple design and ability to achieve high projectile launch frequencies. In order to investigate the operation of ET plasma discharges in more detail than previously possible, a 2-D, multifluid model has been developed to simulate the plasma-fluid dynamics that develop in these devices during operation. In this paper, the 2-D simulation model is used to investigate the effect of source geometry and current density on discharge characteristics. Peak pressure and electric field magnitudes for pulsed discharge operation are shown to scale well with theoretical and empirical scaling laws for steady-state discharge operation. The pulse shape of the source internal pressure is shown to change significantly with increasing source radius. The behavior of other plasma parameters is investigated. In addition, observations of the departure from the ablation-controlled arc regime are presented. This analysis suggests that, for the current pulse length investigated, source radii higher than 4 mm require significantly more current density in order to produce sufficient ablation to stabilize the plasma discharge.

Surface Ablation and Melting of Fusion Materials Simulated by Transient High Heat Flux Generated in an Electothermal Plasma Source

An electrothermal (ET) plasma source has been used to produce intense transient high heat flux to evaluate surface erosion and melting of plasma-facing materials. The source produces high heat flux relevant to expected conditions during disruption event in future large fusion devices. Surface vaporization has been evaluated for selected fusion-relevant materials under radiant energy deposition. Additional simulation included the effect of melting and splattering. Vapor and droplet formation and their associated shielding effect have been investigated in this paper to assess the difference between a developed boundary layer from only vapor or melt layer or the mixed vapor-melt layer with possible ejection of molten droplets away from the surface. Fully self-consistent erosion models are developed and implemented in the ET ETFLOW code in a new version ETFLOW-boundary layer to model the response of plasma-facing materials and their erosive behavior under transient ET plasma high heat fluxes closely similar to expected ones in future fusion large tokamaks.

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.

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.

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.

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.

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.

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.