Plasma Material Interactions Research Articles

Effect of net direct current on the properties of radio frequency sheaths: simulation and cross-code comparison

In order to understand, predict and control ion cyclotron range of frequency interactions with tokamak scrape-off layer plasmas, computational tools which can model radio frequency (RF) sheaths are needed. In particular, models for the effective surface impedance and DC rectified sheath potentials may be coupled with full wave RF simulation codes to predict self-consistent wave fields near surfaces and the resulting power dissipation and plasma–material interactions from ion sputtering. In this study, previous work assuming zero net DC current flow through the sheath is generalized to allow the surface to collect net positive or negative current, as is often observed in experiments. The waveforms, DC potential and RF admittance are investigated by means of analytical theory, nonlinear fluid and particle-in-cell codes. Cross-code comparisons provide detailed model verification and elucidate the roles of ion and electron kinetics. When the sheath draws negative (positive) DC current, the voltage rectification is reduced (increased) compared with the zero-current case, and the magnitude of both the real and imaginary parts of the admittance are increased (reduced). A previous four-input parametrization of the sheath rectification and admittance properties is generalized to include a fifth parameter describing the DC sheath current.

A New High Flux Plasma Source Testing Platform for the CRAFT Project

As a part of Comprehensive Research Facility for Fusion Technology (CRAFT) project, a large superconducting linear plasma testing facility will be built to evaluate candidate plasma-facing materials and component mockups for China Fusion Engineering Test Reactor (CFETR). Powerful plasma source is a key component to produce reactor-relevant particle flux for material irradiation purpose. To develop source technology, a new testing platform is built at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). This platform utilizes conventional water-cooled magnets to sustain a steady-state field strength of > 7000 G and is equipped with a pump train of 4500 m3h− 1, making it capable to evaluate various plasma sources with a fueling rate up to several standard liter per minute (SLM). The ion flux can be measured with a water-cooled target probe. At present, intense plasma beam with an ion flux of > 1023 m− 2 s− 1 has been achieved with a cascaded arc source in this platform. Further testing on the lifetime of source components is ongoing. New cascaded arc type sources based on non-cylinder plasma column concept has been proposed to increase irradiation area and will be tested as well. After finishing source R&D mission, this platform will be upgraded into a plasma-material interaction research facility and open to all researchers in the community.

The design of Multiple Plasma Simulation Linear Device

• The physical design of a plasma linear device called M ultiple P lasma S imulation L inear D evice (MPS-LD) at Dalian University of Technology is presented. • MPS-LD is characterized by replaceable plasma source, plasma auxiliary heating system and flexible target room. • The objective of MPS-LD is to provide the plasma environment to investigate the plasma-material interactions and divertor plasma physics. • The design of a linear plasma device called M ultiple P lasma S imulation L inear D evice (MPS-LD) at Dalian University of Technology is presented. The objective of MPS-LD is to investigate the key scientific issues in tokamaks, i.e. plasma-material interactions and the divertor plasma physics. The distinctive features of MPS-LD are the replaceable plasma sources, the capability of plasma heating and flexible target room. The length of the vacuum chamber is 3 m, and the device consists of two sizes of chambers with diameters of 0.4 m and 0.6 m, respectively. The steady-state magnetic field with maximum strength at axis of 0.3 Tesla (T) is generated by 11 conventional copper magnets, including 8 small coils and 3 big coils. The device is divided into three regions: plasma source region, auxiliary heating region and target region. Initially, the helicon wave plasma source will be installed to produce the plasma with density of 10 18 -10 19 m -3 , and temperature of 1-3 eV. The arrange of magnets and the magnetic field distribution are computed by the finite element method, and the required pumping system for the desired neutral pressure is calculated by the pressure balance method. The design of magnet system, three-stage differentially pumped vacuum system, replaceable plasma sources, cooling circulation system, target chamber system, diagnostic system and auxiliary heating system, are presented in this work.

A large-area planar helicon plasma source with a multi-ring antenna on Linear Experimental Advanced Device (LEAD)

A planar helicon plasma source with a four-ring antenna has been developed on Linear Experimental Advanced Device (LEAD) in Southwestern Institute of Physics. The diameter of the largest antenna ring is 320 mm. This source is located outside of the vacuum chamber without any vacuum interface and injects radio frequency power into the chamber through a 340-mm-diameter quartz window. A low power threshold of 150 W for electromagnetic-mode to wave-mode transfer is experimentally confirmed. A large volume plasma with a density of over 1019 m−3 and a plasma generation efficiency (total number of electrons diveded by input power) of over 30× 1013 W−1 indicate the high performance of this large-area helicon plasma source, promising LEAD a suitable device for fundamental plasma physics and plasma material interaction research.

A Domestic Program for Liquid Metal PFC Research in Fusion

While high-Z solid plasma-facing components (PFCs) are the leading candidates for reactors, it is unclear that they can survive the intense plasma material interaction (PMI). Liquid metals (LM) PFCs offer potential solutions since they are not susceptible to the same type of damage, and can be “self-healing”. Following the Fusion Energy System Study on Liquid Metal Plasma Facing Components study that recently was completed by Kessel et al. (Fusion Sci Technnol 75:886, 2019) a domestic LM PFC design program has been initiated to develop reactor-relevant LM PFC concepts. This program seeks to evaluate LM PFC concepts for a Fusion Nuclear Science Facility (FNSF) or a Compact Pilot Plant via engineering design calculations, modeling of PMI and PFC components and laboratory experiments. The latter involves experiments in dedicated test stands and confinement devices and seeks to identify and answer open questions in LM PFC design. The new national LM PFC program is first investigating lithium as the plasma facing material for a flowing divertor PFC concept. Several flow speeds will be evaluated, ranging from ~ cm/s to m/s. The surface temperature will initially be held below the strongly evaporative limit in the first design; higher temperatures with strong evaporation will be considered in future concepts. Other topics of interest include: understanding of the hydrogen and helium interaction with the liquid lithium; single effect experiments on wetting, compatibility and embrittlement; and prototypical experiments for control and characterization of flowing LM. A path to plasma and future tokamak exposure of these concepts will be developed.

An Overview of the Hybrid Illinois Device for Research and Applications Material Analysis Test-stand (HIDRA-MAT)

The Hybrid Illinois Device for Research and Applications (HIDRA) at the University of Illinois at Urbana-Champaign is a toroidal plasma device that enables fusion plasma-material interaction testing with both stellarator and tokamak plasmas. HIDRA’s long-pulse steady state stellarator plasmas provide a testbed for plasma facing component (PFC) plasma exposures. The HIDRA Material Analysis Test-stand (HIDRA-MAT) is a material characterization module attached to HIDRA that is being designed and fabricated to include thermal desorption spectroscopy and laser induced breakdown spectroscopy systems for in-vacuo PFC characterization. A specialized rotatable sample holder positions the sample for liquid metal droplet application from a liquid metal droplet injector on HIDRA-MAT. Early experiments look to investigate the effect liquid lithium has on porous tungsten samples’ retention of H, D, and He after plasma exposure. Preliminary results from a dual residual gas analyzer system show the ability to differentiate D2 and He in HIDRA-MAT. This work aims to advance the understanding of liquid metal PFCs and further the design and development of new fusion PFCs and technologies.

Testing and analysis of steady-state helicon plasma source for the Material Plasma Exposure eXperiment (MPEX)

Preparing for next-step fusion facilities will require developing materials that can withstand the high ion and neutron fluences that will be present in the divertor region. These fluences are inaccessible in current toroidal devices. The Material Plasma Exposure eXperiment (MPEX) is a steady-state linear plasma device, currently undergoing conceptual design, that proposes to reach ion fluences as high as 1031 m−2. It will also be able to receive neutron irradiated samples to examine the multivariate effects of plasma material interactions. A target exchange chamber will be employed so that the MPEX target can be removed and placed in a separate diagnostic station without leaving vacuum. To operate in steady-state, the MPEX plasma will be confined using superconducting magnets, with active cooling for all plasma-interacting and plasma-facing components. The plasma source will be a high-power (200 kW) helicon antenna, which will be placed outside of the vacuum chamber. The radio frequency-transparent window for this antenna must be water cooled and must have a very low dielectric coefficient to limit the dielectric losses. The water-to-vacuum seal should not be an elastomer seal, to limit impurities at the plasma source. It is proposed to use a ceramic-to-metal joint. A prototype water-cooled helicon antenna window and assembly have been manufactured and tested in long-pulse conditions up to 10 kW in the Controlled Shear De-correlation eXperiment at the University of California, San Diego. Thermal results have been correlated with computational fluid dynamics simulation.

Heat Flux Analysis From IR Imaging on Proto-MPEX

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear device used to develop the source and heating concept for MPEX, an experiment to study plasma-material interactions for future fusion reactors. This article will highlight recent analysis and results on Proto-MPEX using an infrared (IR) camera, a key tool for imaging heat fluxes measured at the material target. The target heat fluxes are crucial to understanding the source and heating concepts. The recent analysis includes the application of homography and 3-D finite-element methods to provide between-shot measurements of the 2-D target heat flux profile. The IR camera inferred heat flux is shown to correlate with Langmuir probe inferred heat fluxes for a range of parameters in helicon only and helicon + electron cyclotron heated discharges. Results show both desirable central and undesirable edge heat flux for helicon source and electron heating on the target. These heat flux measurements are shown to be important for better understanding of the source and heating physics.

Radial control of electron temperature gradient with optimized operational configuration of double plasma device

A double plasma device (DPD) is tested for different operational configurations to identify suitable control for plasma parameters in a weakly ionized, unmagnetized plasma discharge. A separation grid is installed between the two chambers of DPD, which allows filtering of plasma from the first to the second chamber. Investigations are carried out to control the plasma parameters, especially the electron temperature by exploring the role of the grid. The grid bias is varied between −25–0 V and 0–30 V to reflect cooling and heating of plasma. The electron heating and cooling is prominent for the maximum ratio of n source/n target. The electron energy distribution function (EEDF) is obtained to describe the role of grid biasing in controlling the electron temperature in the second chamber. We demonstrated control on the radial profile of electron temperature by charging different radial cross-sections of plasma differently by using a multi-grid assembly system (MGAS). We have also identified the suitable operational regime for DPD where exercising a radial control on electron temperature is possible. Such plasmas can facilitate investigations on electron temperature control for applications in plasma processing, cold–plasma material interaction, etc where low energy electrons are desired.

New application of hyperspectral imaging to steady-state plasma observations.

A new application of hyperspectral imaging (HSI) to steady-state plasma emission observations is proposed because of its prominent feature: an HSI camera records a two-dimensional image, and each spatial pixel contains spectral data typically with more than a hundred bands, while conventional digital cameras have only three bands. The characterization of an HSI camera (Specim IQ) has been performed during steady-state plasma-material interaction experiments using the linear plasma device PISCES-A. By easily subtracting the background/continuum emission in contrast to conventional filter cameras, two-dimensional images of multiple emission lines at different wavelengths are simultaneously obtained during a single measurement, demonstrating the advantage in plasma emission observations.

The Materials Plasma Exposure eXperiment: Status of the Physics Basis Together With the Conceptual Design and Plans Forward

The Materials Plasma Exposure eXperiment (MPEX) is a linear plasma device that will address the plasma–material interaction (PMI) science for future fusion reactors and will enable testing of plasma-facing components (PFCs). It is designed as a steady-state device eventually delivering an ion fluence of up to 1031 m−2 to the target. The device will be designed to handle neutron-activated materials. These capabilities should push the technical readiness level of PFCs up to six for some end-of-life aspects. In order to achieve the relevant plasma conditions, as they are expected in future fusion reactor divertors, MPEX will utilize a novel plasma source concept. This plasma source concept uses a high-power helicon (200 kW, 13.56 MHz), an electron cyclotron heating (ECH) system, which will heat electrons via electron Bernstein wave (EBW) heating (about 400 kW, 70 GHz), and an ion cyclotron heating (ICH) system (400 kW, 6–9 MHz). The physics basis for this plasma source concept including the heating physics and transport is based on experiments on Proto-MPEX. An overview of the experimental results and the physics basis will be given. The physics basis directly translates into functional requirements of MPEX and the conceptual design. The status of the conceptual design of MPEX will be shown.

On the role of integrated computer modelling in fusion technology

• We analyse critically current capabilities and maturity levels of existing simulation tools relevant to fusion blanket physics and design. • The progress in the development of computational tools in each particular is found to be significant. • The requirements that need to be addressed for code integration are common to all areas and tools. • The integration into one single computational tool is currently premature and looser coupling ways need to be considered. Computer modelling is expected to play an increasingly important role in fusion design and technology, where the complexity of the physical processes involved (plasma, materials, engineering), and the highly interconnected nature of systems and components (“system of systems” design), call for support from sophisticated and integrated computer simulation tools. In this paper, we review the contribution of coupled computer modelling to the design of the reactor, breeding blanket and integrated first wall in terms of neutronics, materials behaviour (including plasma-materials interaction, radiation effects and compatibility with fluids), magnetohydrodynamics thermofluid issues and thermo-hydraulic aspects, as well as simulations of plasma transport out of the confinement region to determine heat and particle loads on plasma facing components. The current capabilities and levels of maturity of existing simulation tools are critically analysed, having in mind the possibility of integrating several tools in a single computational suite in the future and highlighting the perspectives and difficulties of such an endeavour.

Dynamics of sheath evolution in magnetized charge-fluctuating dusty plasmas

The evolution of sheath in plasma contaminated with varying dust charges under the effect of an external magnetic field is studied. Study of Sagdeev potential through pseudoptential approach has been attempted with a view to deriving the sheath equation. Numerical analysis has been carried out to study the potential variation with sheath-ward distance for various plasma parameters. A unique finding of the study is that the presence of dust particles as well as the magnetic field drastically modifies the Bohm sheath criterion for plasma sheath formation as obtained earlier in unmagnetised two-component plasma. The results have more realistic interpretation in showing explicitly the interaction of magnetic field and impurity caused by dust charge variation, with the possibility of its impact in various technological applications including plasma-material interaction, material processing and electro-mechanical devices.

Simulations of Argon plasmas in the linear plasma device GyM with the SOLPS-ITER code

Edge plasma codes, such as SOLPS, are widely used to study plasma transport in tokamaks scrape-off layers (SOL). The possibility to apply these codes to non hydrogenic plasmas and to linear plasma devices is gaining the interests of the fusion community. These facilities play a pivotal role in plasma-material interaction studies for future fusion devices and may allow to test the code capabilities both in terms of geometry and simulated plasma species. In this contribution, we apply the SOLPS-ITER code for the simulations of Argon plasmas in the medium-flux linear machine GyM. A sensitivity scan over the pumping efficiency, transport coefficients and absorbed electron power was done, performing B2.5-EIRENE coupled simulations. A quantitative analysis of the different flux contributions is provided through the use of a two-point modelling. Comparison with experiments shows a promising qualitative and quantitative agreement, with the sole exception of the simulated neutrals pressure. Dedicated EIRENE standalone simulations were performed to investigate this issue, also highlighting the role of neutral–neutral elastic collisions at high values of the puffing intensity.

Fracture behavior of tungsten-based composites exposed to steady-state/transient hydrogen plasma

The fracture behavior of plasma-facing components (PFCs) under extreme plasma-material interaction conditions is of great concern to ITER and future fusion reactors. This was explored in the current study by exposing pure tungsten (W), W-1%TiC and W-2%Y2O3 composites to a combined steady-state/transient hydrogen plasma up to a base surface temperature of ~2220 K, and up to 5000 transient pulses for 1000 s using the linear plasma generator Magnum-PSI. The applied heat loads were characterized by combining sheath physics, thermographic information and finite element analyses, with which the thermal stress was evaluated. Combining microstructural investigation and thermo-mechanical numerical analyses, a physical picture of fracture is developed. The transient heat loads drive surface crack initiation, whose depth can be estimated by a simple analytical model for pure tungsten, while the cooling period following the steady-state heat load induces tensile stresses, opening existing surface cracks deeper. The fracture process is mediated by the microstructure whereby the ceramic particles stabilize the microstructure but promote surface crack initiation due to suppressed plasticity at the grain boundaries and the particle-matrix interfaces. The surface cracks relieve the subsequent cycles of transient thermal stress but intensify the steady-state thermal stress, therefore, promoting deep crack propagation. These results help to understand failure mechanisms in PFCs under extreme operation conditions which are valuable for developing advanced PFCs.

Response of yttria dispersion strengthened tungsten simultaneously exposed to steady-state and transient hydrogen plasma

W–Y2O3 alloy with low DBTT (ductile-brittle transition temperature) and high RCT (recrystallization temperature), processed by high energy rate forging (HERF) was exposed to ITER-like steady-state and transient hydrogen plasma in the linear plasma generator Magnum-PSI. The steady-state heat fluxes were in the range of 8.35–16.32 MW·m−2, resulting in surface base temperatures of the samples in the range of 1271 °C to 1982 °C. The applied transient peak heat flux with a frequency of 5 Hz (a total of 1000 pulses) was about 0.50 GW·m−2. The exposure time was ~220 s. No obvious morphological change of the exposed sample with a base temperature of 1271 °C was observed, except the preferential erosion of the W/Y2O3 interface. However, cracks along grain boundaries were formed on the surface of the exposed samples with base temperatures above 1389 °C. Pronounced recrystallization and grain growth also occurred for the samples with base temperatures of 1666 °C and 1982 °C. It is desirable to find that no wide and deep crack with preferential propagation direction with regard to the sample dimension was observed, even extensive recrystallization and cracking along grain boundaries occurred. This indicates that decreasing DBTT and increasing RCT simultaneously is desirable to broaden the safe operational temperature window of W as plasma-facing material, and therefore to increase the power handling capability of the plasma-facing units of a tungsten-based divertor in future fusion reactors. However, the formation of W/Y2O3 composite, melting and depletion of the Y2O3 particles gradually occurred with increasing the surface temperatures. It implies that the doping of Y2O3 particles complicates the plasma-material interaction, compared to the pure W case. For example, it raises concerns about the formation of complex dust which will potentially be a significant issue for the safe operation of future fusion reactors (e.g. core plasma contamination and fuel recycling).

The Material Plasma Exposure eXperiment: Mission and conceptual design

• New linear plasma device MPEX will address plasma material interaction gaps for fusion. • MPEX will have increased capabilities over existing linear plasma devices and will be world-leading when completed. • MPEX will expose apriori neutron irradiated material samples for fusion reactor divertor relevant heat and particle fluxes. • The conceptual design of MPEX is finished. Mastering Plasma Material Interactions (PMI) is key for obtaining a high performance, high duty-cycle and safe operating fusion reactor. Numerous gaps exist in PMI which have to be addressed before a reactor can be built. In particular the lack of data at high ion fluence, fusion reactor divertor relevant plasma conditions and neutron displacement damage requires new experimental devices to be able to develop plasma facing materials and components. This has been recognized in the community and the U.S. fusion program is addressing this need with a new linear plasma device—the Material Plasma Exposure eXperiment (MPEX). MPEX will be a superconducting linear plasma device with magnetic fields of up to 2.5 T. The plasma source is a high-power helicon source (200 kW, 13.56 MHz). The electrons will be heated via Electron Bernstein Waves with microwaves using multiple 70 GHz gyrotrons (up to 600 kW in total). Ions will be heated via ion cyclotron heating in the so-called “magnetic beach heating” scheme in the frequency range of 6−9 MHz (up to 400 kW in total). An overview of the conceptual design and the project/design requirements is given.

Plasma–metal junction

In this work, the concept of plasma–metal junction (pm-junction) is presented, in addition to well established physics of pn-junction, metal–semiconductor junction, semiconductor–insulator junction, etc. We demonstrate experimentally the dependence of I-V characteristics on work function of the metal for a pm-junction. The experiment is done using tungsten (W) and stainless steel (SS) having different work functions (Φm) of 4.55 eV and 4.30 eV, respectively. The W and SS planar metal probes of identical geometry were exposed to plasma simultaneously, making two distinct pm-junctions. The plasma exposed is identical for both the metals, and therefore, the difference observed in the I-V characteristics of these two pm-junctions is attributed only to the difference in an energy-band diagram of these junctions, owing to respective work function. The proposed concept of the pm-junction may simplify the complex plasma–material interactions in all laboratory plasmas, including cold glow discharges to hot fusion plasmas.

Considerations for in situ, real time measurement of plasma-material interactions using Digital Holographic imaging

Digital Holographic (DH) imaging is a laser-based measurement technique, which can be used to monitor a material surface as it is being exposed to fusion-relevant plasmas. Both single-laser and dual-laser DH measurements have been demonstrated ex situ at Oak Ridge National Laboratory (ORNL). A DH diagnostic system is being assessed for deployment at ORNL to make in situ, real-time measurements of plasma erosion/redeposition in the Prototype Material Plasma Exposure eXperiment (Proto-MPEX). In situ, real time measurements pose unique challenges to diagnostic systems, which are not encountered by ex situ analysis techniques. The absolute surface height (tracked at the nm to μm position) during plasma exposure can be modified by: 1) surface movement due to vibration or thrust exerted by the plasma device, 2) surface growth due to the thermal expansion of the material under steady MW/m2 or transient GW/m2 plasma heat loads, and 3) surface modification due to plasma erosion/redeposition of the material substrate. To assess these effects, 1) the vibration spectrum of the diagnostic and the pulsed plasma device have been measured, 2) the thermal growth of the surface has been measured for an applied heat flux, and 3) the surface modification has been measured ex situ post plasma exposure, and ex situ post laser ablation as a proxy for the plasma. This paper will provide an overview of the results that have been achieved in the development of a DH diagnostic for in situ real-time measurements of plasma exposed surfaces in the Proto-MPEX device.

Conceptual Design and Analysis of In-Vessel Components for the Materials Plasma Exposure eXperiment (MPEX)

The materials plasma exposure experiment (MPEX) is a linear plasma divertor simulator currently undergoing conceptual design. The facility will expose material samples to steady-state plasma fluxes to examine plasma–material interactions (PMIs) that are expected in the next generation of fusion devices. The plasmas will be generated by a helicon source, with electron and ion heating sources of up to 800 kW possible. The peak heat fluxes of the target are expected to be up to 10 MW/m2. The facility will be capable of handling low-activation neutron-irradiated samples in order to examine the multivariate effects of neutron damage and plasma fluence. Neutron-irradiated samples are planned to be roughly of 10-mm diameter; however, plasma-facing components up to $60 \times 600$ mm can be accommodated. The steady-state nature of the device will require the magnetic confinement of the plasma to be achieved with superconducting magnets, with a maximum on-axis field of 2.5 T. In addition, since MPEX will be a steady-state device, in-vessel components need to be water cooled. The primary in-vessel components will be the target, the dump plate, the limiter, the skimmers, and the microwave absorber. The conceptual design of these components is presented here, including analyses that confirm that the designs are adequate to meet the requirements of MPEX operation.