Linear Plasma Device Research Articles

Radial control of plasma uniformity and electron temperature by external plate biasing in a back diffused partially magnetized plasma

The possibility of achieving uniform and controllable plasma distribution remain a fundamental challenge in low pressure plasma discharges, which has numerous applications in plasma technologies. An external object when introduced in a magnetized plasma can significantly alter the spatial plasma distribution. In this paper, the effect of external plate biasing on both axial as well as radial characteristics of an expanding, partially magnetized plasma column, created by hot cathode filament inside a linear plasma device is presented. It is found that by applying a positive potential to a conducting external electrode, placed at a remote location away from the primary discharge region; the back diffused plasma tends to become uniform. The plate biasing also results in an overall increase in electron temperature of the expanding plasma; however, it does not appear to alter the radial plasma characteristics inside the source and the biasing region. A possible mechanism behind observing this effect has been provided.

A point plasma model for linear plasma devices based on SOLPS-ITER equations: application to helium plasma

Linear plasma devices represent an essential tool for nuclear fusion research, whereby understanding crucial aspects related to plasma-wall interactions or edge plasma behaviour. Simplified models are of great importance to complement and integrate experimental and simulation results of complex systems such as plasmas in linear machines, because they are fast and simple to employ. In this work, we present a global volume-averaged (0D) model for plasma investigation in linear machines. The 0D model equations are based on the space integration of the state of the art edge plasma model implemented in the SOLPS-ITER code. Comparisons between helium plasmas described with 2D simulations performed with SOLPS-ITER and with the 0D model highlight that contributions often neglected in tokamak edge models, e.g. electron-neutral excitation, may be relevant when describing weakly ionized plasmas in linear devices. The model is used to perform sensitivity studies with respect to several parameters and to analyse the time evolution of the system, leading to the identification of two relevant time scales governing the system. Lastly, a comparison of 0D results with experimental data from the linear device GyM is performed, showing satisfactory agreement. Our methods and results provide crucial interpretative keys in the investigation of the physics of edge plasmas.

ERO2.0 modelling of nanoscale surface morphology evolution

Plasma–material interaction (PMI) in tokamaks determines the life-time of first-wall (FW) components. Due to PMI, FW materials are eroded and transported within the device. Erosion is strongly influenced by the original morphology of the component, due to particle redeposition on near surface structures and to the changing of impact angle distributions, which results in an alteration of the sputtering effects. The Monte-Carlo impurity transport code ERO2.0 is capable of modelling the erosion of non-trivial surface morphologies due to plasma irradiation. The surface morphology module was validated against experimental data with satisfactory agreement. In this work, we further progress in the validation of the ERO2.0 capabilities by modelling both numerically generated surfaces as well as real surfaces, generated using atomic force microscopy (AFM) measurements of reference tungsten samples. The former are used to validate ERO2.0 against one of the morphology evolution models present in literature, in order to outline the conditions for reliable code solutions. Modifications induced in AFM-generated surfaces after argon and helium plasma irradiation are compared, showing a similar post-exposure morphology, mostly dominated by surface smoothing. Finally, the ERO2.0 morphology retrieved after He plasma exposure is compared to experimentally-available scanning electron microscopy and AFM measurements of the same surface morphology exposed in the linear plasma device GyM, showing the need for further improvements of the code, while a good agreement between experimental and simulated erosion rate is observed.

Bubble formation in liquid Sn under different plasma loading conditions leading to droplet ejection

Liquid metals have been proposed as potential divertor materials for future fusion reactors, and surface stability is a vital requirement for such liquid metal divertors (LMDs). Capillary porous structures (CPSs) have been applied to the design of liquid metal targets as they can avoid MHD instability by surface tension and provide a stable liquid surface. However, our previous work has found that liquid Sn surfaces can be very unstable in hydrogen plasma even in cases without magnetic fields. To increase our understanding of the interaction of liquid Sn surfaces with plasmas, in this work we systematically investigated the surface behaviors of liquid Sn in different plasma exposures in linear plasma devices, either in Nano-PSI at low flux and without magnetic field, or in Magnum-PSI with strong magnetic field strength. Surface instability leading to droplet ejection has been observed and recorded in the experiments. The ejection of droplets is not dependent on magnetic fields and plasma currents, and is found to be dependent on the plasma species and plasma flux and surface temperature. The CPS meshes applied in the experiments cannot completely avoid droplet ejection but can decrease droplet size and lower droplet production rate. In H plasma, droplets were observed once Sn melted even at low fluxes. For the case of N plasma, the appearance of droplets started at a temperature marginally higher than tin–nitride decomposition temperature. Only at high fluxes (∼1023–24 m−2 s−1) and high temperatures (900–1000 °C) were a few droplets observed in Ar or He plasma. For all cases, the ejection velocities of most droplets were around 1–5 m s−1. Bubble formation, growth and bursting in the plasma-species-supersaturated liquid Sn is proposed as the primary mechanism for the ejection of droplets. Plasma-enhanced solubility is responsible for the achievement of H/N-supersaturated liquid Sn, while high plasma flux implantation is responsible for Ar/He-supersaturated liquid Sn. Once the concentration of plasma species in liquid Sn reaches a certain supersaturation level, nucleation and growth of bubbles occur due to the desorption of dissolved plasma species from the liquid Sn. The formation and bursting of bubbles have been directly observed in the experiment. The sizes of most bubbles were estimated in the range of 40–400 μm or even smaller. A bubble growth model based on Sievert’s and Henry’s laws is invoked to describe bubble growth in liquid Sn.

Performance of liquid-lithium-filled 3D-printed tungsten divertor targets under deuterium loading with ELM-like pulses in Magnum-PSI

A fusion reactor divertor must withstand heat flux densities <10 MW m−2. Additionally, it may have to withstand millisecond pulses on the order of 0.5 to 30 MJ m−2 due to (mitigated) edge-localized modes (ELM) occurring with 30 to 60 Hz. We investigate if these requirements can be met by capillary porous system (CPS) liquid lithium divertors (LLD). 3D-printed tungsten CPS targets were exposed in the linear plasma device Magnum-PSI, to deuterium plasma discharges lasting 15 s, generating 1.5 to 16 MW m−2, and T e ∼ 1.5 eV. Additionally, ELM-like pulses were superimposed on top of the steady state for 3 s with a frequency of 2 and 100 Hz, power flux densities of 0.3 to 1 GW m−2, and T e up to ∼14 eV. All Li targets survived without damage. The surface temperature (T s) was locked at ∼850 °C, which was attributed to power dissipation via vapor shielding. Meanwhile, unfilled reference targets melted during the severest pulsed loading. A blue grayish layer of presumably LiD formed when T s < 500 °C, but disappeared when the locking temperature was reached. This implies that LiD formation can be avoided, but that it may require a surface temperature at which Li evaporation excessively contaminates the core plasma in a tokamak. During pulsed loading the plasma facing surface remained wetted in all conditions. Bolometry indicated that, only during pulses, there was a large increase in radiative power dissipation compared to targets without Li. A high speed camera with a Li-I filter showed that strong Li evaporation continued up to 5 ms after a pulse. Overall, the liquid-lithium-filled 3D-printed tungsten targets were found to be highly robust, and 3D-printing can be considered as a promising manufacturing technique for LLDs. Further research is needed particularly on the formation of LiD and the associated tritium retention, as well as the impact of enhanced evaporation during and after ELMs on the core plasma.

The Polaris Linear Plasma Device

We report the construction of Polaris, a new linear plasma device with an axial magnetic field dedicated to research and education at the American University of Beirut in Lebanon. The goal is to initiate the investigation of topics related to basic magnetized plasma physics and nuclear fusion. We discuss the novel technical solutions found to set up various components of the device. We start with the magnetic coils where we use power cables to generate the required axial magnetic field. They are included in a stainless steel case for water-cooling. Then, we present the vacuum and the gas inlet systems with which we control the neutral pressure. The way we manufacture the water-cooled radio frequency (RF) antenna allows the production of a variety of designs. Polaris is thus built to operate in a steady state. Using a reciprocating Langmuir probe, we determine the plasma properties as a function of the various control parameters. As the plasma remains in the “blue mode,” we show that the increase in the main magnetic field leads to an increase in the density but a decrease in the plasma temperature. The increase in neutral pressure yields the same effects. However, the increase in RF power leads to an increase in both density and temperature. Finally, plans for future investigations are briefly described.

Application of multiple regression for sensitivity analysis of helium line emissions to the electron density and temperature in Magnum-PSI

Helium line intensities have been utilized to measure the electron density, n e , and temperature, T e , by comparing measured line intensities to a collisional-radiative model (CRM). In this study, we use multiple regression analysis to train a model of the helium line intensities and n e /T e obtained from a Thomson scattering system in the linear plasma device Magnum-PSI; based on the trained model, we predict n e and T e from line intensities. We show that this method can also obtain radial profiles of n e and T e . We discuss appropriate selections of line pairs for the prediction based on the multiple regression analysis. A big advantage of this method against the standard technique using CRM is that modeling of atomic population distributions is not required, which sometimes needs to take into account various effects such as radiation trapping, transport of helium atoms in metastable states, etc.

Development of the materials analysis and particle probe for Proto-MPEX.

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device being used in plasma source research and development (R&D) for the proposed MPEX. Once the R&D is completed, this device can also be used to perform plasma-material interaction studies. To perform these studies, a new materials analysis and particle probe (MAPP) has been constructed. The MAPP's components are a sample holder and manipulator and a custom vacuum chamber with ports to facilitate surface chemistry diagnostics. The MAPP's overall design enables rapid sample turnaround and in vacuo surface characterization. The surface analysis vacuum chamber has ports for x-ray photoelectron spectroscopy, thermal desorption spectroscopy, back-scatter ion scattering spectroscopy, forward-scatter ion scattering spectroscopy, and direct recoil spectroscopy. The sample manipulator and holder is a Lesker/UHV Multi-Centre Analytical Stage, which is used to place the samples in the exposure region of the Proto-MPEX or the analysis position in the MAPP vacuum chamber. The sample holder has a heating capability of up to 1200 °C for heated exposure and for desorption studies. In this work, we present the MAPP's design and the first tungsten sample exposure with ex situ analysis that shows a surface deposition layer on the exposed target, highlighting the need for additional in situ measurements on the Proto-MPEX.

The impact of surface morphology on the erosion of metallic surfaces – Modelling with the 3D Monte-Carlo code ERO2.0

• The micro-scale model for simulation of the surface roughness effect on the erosion of PFCs and transport of sputtered material implemented previously into the 3D Monte-Carlo code ERO2.0 was applied for modelling of sputtering and deposition in the JET-ILW divertor conditions (smooth Tile 5 and rough Tile 6). • Micro-scale simulations have been conducted using two different assumptions: with and without tracing of incident plasma/impurity ions near the rough surface. Tracing of incident ions near the surface forms more realistic shadowing patterns than the case when this pattern is calculated based on magnetic field lines. • Simulations without tracing of incident ions have shown that surface roughness leads to a reduction of the effective sputtering yield by up to 50% depending on surface parameters. • Simulations including tracing of incident ions near the surface have confirmed decrease of the net erosion from the rough surface of the Tile 6 in comparison to the smoother surface of the Tile 5 by up to 50%. The roughness of metallic surfaces has a vital impact on the erosion of plasma-facing materials. Roughness determines the effective sputtering yield Y eff of the facing material. The angular/energy distribution of sputtered particles, and the spatial erosion and deposition distribution. The model for simulation the effect of the surface roughness was earlier implemented into the 3D Monte-Carlo code ERO2.0 and validated using results of ion beam experiments and experiments in the linear plasma device PSI-2. In the present study the developed ERO2.0 surface morphology model was applied to the JET-ILW tungsten (W) divertor consisting of smooth bulk W and W-coated CFC components. Influence of the surface roughness on the W erosion as well as on the transport of sputtered material in conditions of inclined magnetic field was investigated. Simulation results are in a good agreement with existing experimental findings.

In-situ LIBS and NRA deuterium retention study in porous W-O and compact W coatings loaded by Magnum-PSI

The purpose of this study is to investigate the applicability of in-situ laser induced breakdown spectroscopy (LIBS) for deuterium retention measurements in tungsten coatings with different morphology and oxygen content. These were exposed to a Gaussian beam of deuterium plasma in the Magnum-PSI linear plasma device. The deuterium line intensities determined by LIBS were compared with the deuterium content measured by nuclear reaction analysis (NRA).Both LIBS and NRA results showed that higher deuterium retention was achieved in the coating region corresponding to the periphery of the plasma beam. This decreasing deuterium retention in the central region can be attributed to higher surface temperature. At the same time, the deuterium retention in different coating types assessed by LIBS D intensity was markedly different from the retention determined by NRA. Porous W-O coating with high oxygen content had the highest deuterium retention according to NRA while D intensity obtained by LIBS was an order of magnitude smaller when compared with other coatings. The deuterium retention in compact W coating and thick W coating was almost the same and LIBS D intensities were also comparable for these coatings. The results demonstrate the LIBS applicability and its limits in different coating types.

Recrystallization-mediated crack initiation in tungsten under simultaneous high-flux hydrogen plasma loads and high-cycle transient heating

Tungsten and tungsten-based alloys are the leading material choices for the divertor plasma facing components (PFCs) in future fusion reactors. Recrystallization may occur when they undergo high heat loads, drastically modifying the predesigned grain structures and the associated desired mechanical properties. However, the influence of recrystallization on the thermal fatigue behavior of tungsten PFCs still remains unclear. In this study, ITER-grade tungsten was simultaneously exposed to a high-flux hydrogen plasma (∼5 × 1024 m−2 s−1) and high-cycle (104–105) transient heat loads in the linear plasma device Magnum-PSI. By correlating the surface temperature distribution, obtained by analyzing temperature-, wavelength-, and surface-dependent emissivity, and the surface modifications of the plasma exposed specimens, the crack initiation heat flux factor threshold was found to be ∼2 MW m−2 s0.5 (equivalently, ∼0.07 MJ m−2 for a 1 ms pulse). Based on electron backscatter diffraction analyses of cross-sections near the crack initiation sites, faster recrystallization kinetics near the surface compared to literature was observed and the surface cracks preferentially initiated at high angle grains boundaries (HAGBs). Upon recrystallization, the yield strength decreases which entails increasing cyclic plastic strains. The HAGBs fraction is increased, which constrains the transfer of plastic strains at grain boundaries. The recrystallization decreases the dislocation density, which promotes heterogeneous deformation. All these mechanisms explain the reduced crack initiation threshold of recrystallized tungsten compared to its as-received counterpart. The results provide new insights into the structural failure mechanisms in tungsten PFCs exposed to extreme fusion plasmas.

Quench Protection Study of Superconducting Magnets for the Materials Plasma Exposure Experiment

To advance the understanding of plasma material interactions, the Material Plasma Exposure eXperiment (MPEX) is a new linear plasma device that will generate and deliver plasma relevant to future fusion reactor divertors. The operation of MPEX is planned to be steady-state in order to facilitate high fluence exposures of plasma facing materials and components. The desire for steady-state operation along with the magnetic field requires the utilization of superconducting coils. The superconducting magnet system for MPEX has been developed. The baseline model has six superconducting magnet and one room-temperature magnet subsystems. In order to protect multiple superconducting magnet systems, quench analysis was carried out to determine the best protection approach for each magnet type. Because the mutual inductance accounts for approximately 35% of the stored energy in the entire system, this must be considered when determining the peak voltages and temperatures during a quench. Two approaches for passive quench protection are considered: (1) self-protecting magnets and (2) use of diodes to sub-divide the coils. For both approaches, active quench detection will be used to ensure all coils are de-energized in the event of a quench. Results of the quench analysis for several quench scenarios are presented.

Cryogenic Considerations for Superconducting Magnet System Design for the Material Plasma Exposure eXperiment (MPEX)

The Material Plasma Exposure eXperiment (MPEX) has been proposed as a facility to address plasma material interaction knowledge gaps to qualify and develop materials and technologies that surround plasma environments for future fusion reactors. Utilizing different radio-frequency (rf) heating technologies, MPEX is a linear plasma device that will generate fusion reactor-like plasmas with energies and particle fluxes at the target materials with electron temperatures of 1 to 15 eV, electron densities of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , and ion fluxes greater than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">24</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Starting with the MPEX requirements with respect to magnetic fields between 0.1 and 2.5 T and warm bores of either 0.65 m or 1.56 m, conceptual designs for a superconducting magnet system have been developed that utilize multiple NbTi windings distributed across seven cryostats to accommodate rf heating, water cooling, and vacuum systems needed for MPEX. While the cryogenic and magnet technologies relative to the field and space requirements are mature, the integration of these technologies across multiple cryostats presents several technical and logistical challenges. An analysis of the preferred refrigeration approach, modular recondensing liquid helium cryocoolers, was performed. Utilizing a design margin of a factor of two, this approach is feasible within the current design requirements for MPEX with some considerations related to its implementation within the thermal shields and the magnet subsystem geometries.

Effect of tantalum doping on blistering behavior and retention in tungsten exposed to deuterium plasma

Pure tungsten and tungsten tantalum (5 wt% ta) were prepared by powder metallurgy method, and then 40 eV deuterium plasma was injected into a linear plasma device with 1.08 × 1026 D/m2 energy. The irradiation properties of pure W and W-5%Ta were compared by a set of supplementary analysis techniques. The results show that adding proper amount of tantalum can effectively reduce the foaming caused by deuterium. In addition, TDS results show that the main desorption peak of pure tungsten is about 650 K, and the shoulder is at 730 K. For W-5%Ta alloy, the main desorption peak is about 810 K, and the deuterium retention in pure W is 4 times of that in W-5%Ta alloy. In addition, we also measured the hardness of pure W and W-5%Ta, which are 4.3 Gpa and 7.3 Gpa, respectively. The above phenomenon is due to the change of the microstructure and composition of the material.

Quantifying erosion and retention of silicon carbide due to D plasma irradiation in a high-flux linear plasma device

Abstract Silicon carbide (SiC) may be a viable option for future plasma-facing components (PFCs) due to its low hydrogenic diffusivity, high temperature strength, and mechanical resilience to neutron damage (Causey et al., 1978). The erosion and retention properties of SiC were quantified via deuterium plasma exposures in the PISCES-E RF plasma source on SiC-coated graphite samples at impact energies between 20 eV and 90 eV, surface temperatures of 500 K and 950 K, and fluences between 0.4 and 1.0 × 1024 m−2. The chemical sputtering yield of carbon from SiC was estimated by optical spectroscopy, varying between 0.0012 and 0.0083 depending on the deuterium impact energy. Chemical sputtering yields from graphite were 4× higher, on average, than yields from SiC and were largely consistent with previous analytic formulations. Chemical erosion of silicon atoms from SiC was not detected from the SiD molecular band, but the lack of Si surface enrichment at low Ei suggests that a non-collisional Si erosion source may be present. The retention of implanted deuterium in SiC was ~2× higher than that in tungsten at 500 K. Most D retained in SiC was desorbed at a peak temperature ~1000 K, and the desorption rate only varied slightly with impact energy and surface temperature. Fundamental differences in desorption behavior between Si, graphite, and SiC samples suggested that the SiC cubic lattice possessed unique trapping sites that cannot solely be attributed to Si-D or C-D bonds. New questions regarding preferential erosion and uncharacterized defects motivate expanded testing in linear and toroidal devices.

Superconducting Magnet Qualification Methodology for the Material Plasma Exposure Experiment

The Material Plasma Exposure eXperiment (MPEX), currently under design, is a new linear plasma device to advance the understanding of plasma-material interactions through the generation and delivery of plasmas as they are expected in future fusion reactor divertors. MPEX will be a steady-state device to study high-fluence exposures of plasma-facing materials and components. The requirements for the magnetic field at the target and the heating stages make the application of superconducting coils necessary. Conceptual designs for the superconducting magnets have been developed, and multiple cryostats with warm bore diameters of either 65 cm or 156 cm are envisioned to facilitate their integrated and timely assembly with other systems such as vacuum, water cooling, and RF power. Although design, fabrication, and testing for the magnets as stand-alone units are straightforward, challenges will arise during the integration of the system. Two different field profiles will be used during operation. The magnetic field where the electron cyclotron heating occurs needs to operate at both 1.25 and 2.5 T. It is critical that the magnets all share the same magnetic axis and alignment. The mutual inductance between cryostats will affect the quench behavior of the system. Also, cryostat-to-cryostat forces can be as large as 700 kN, and the magnitude and direction will change depending on which coils are energized. The design of the system must take those characteristics into account along with the quench scenarios. This paper describes the qualification approach that will be used to determine whether stand-alone tests can be used to ensure the success of the integrated system. Fiducials will be used to define the location of the magnetic axis for each cryostat to ensure proper alignment. Quench tests of a single magnet will be performed at a current above the normal operating current to account for additional stored energy from the mutual inductance to adjacent cryostats. Also, a 1018 steel plate will be mounted on either end of a cryostat to simulate the cryostat-to-cryostat forces. Requirements for the size and location of the steel plates are described.

Erosion research of CX-2002U carbon composites under low-temperature high-flux hydrogen plasma

The net erosion yield of CX-2002U carbon fiber composites under high-flux low-temperature hydrogen plasma is investigated using a linear plasma device. It is found that the net erosion yield decreases rapidly first, and then tends to saturate with the increase of hydrogen–plasma flux. When the temperature of the sample eroded by hydrogen plasma is above 300 °C, the hybridization of electrons outside the carbon atom would change. Then the carbon atoms combine with hydrogen atoms to form massive spherical nanoparticles of hydrocarbon compounds and deposit on the surface at the flux condition of 1.77 × 1022 m−2·s−1. Under the irradiation of hydrogen plasma loaded with negative bias, the surface morphology of the matrix carbon is changed dramatically. Moreover, the energy dependence of mass loss does not increase in proportion to the increase of hydrogen–plasma energy, but reaches a peak around 20 V negative bias voltage. Based on the analysis of different samples, it can be concluded that the enhancement of energy could make a contribution to chemical erosion and enlarge the size of pores existing on the surface.

Developments on two lithium vapor-box linear test-stand experiments

Abstract The lithium vapor-box divertor is a possible fusion power exhaust solution. It uses condensation pumping to create a gradient of vapor density in a divertor slot; this should allow a stable detachment front without active feedback. As initial explorations of the concept, two test stands which take the form of three connected cylindrical stainless steel boxes are being developed: one without plasma at PPPL, to test models of lithium evaporation and flow; and one for the linear plasma device Magnum-PSI (at DIFFER in Eindhoven, The Netherlands) to test the ability of a lithium vapor cloud to induce volumetric detachment and redistribute the plasma power. The first experiment uses boxes with diameters of 6 cm, joined by apertures with diameters of 2.2 cm. Up to 1 g of Li is placed in one box, which is heated to up to 600 ° C . The Li evaporates, then flows to and condenses in the two other, cooler boxes over several minutes. The quantity of Li transported is assessed by weighing the boxes before and after the heating cycle, and is compared to the quantity predicted to flow for the box at its measured temperature using a Direct Simulation Monte Carlo code, SPARTA. With good experimental conditions, the two values agree to within 15%. The experiment on Magnum-PSI is in the conceptual design stage. The design is assessed by simulations using the code B2.5-Eunomia. They show that when the H + plasma beam, with n e = 4 × 10 20 m −3 , T e = 1.5 eV , and r = 1 cm , is passed through a 16 cm long, 12 Pa, 625 ° C Li vapor cloud, the plasma heat flux and pressure on the target are significantly reduced compared to the case without Li. With the Li present, the plasma is cooled by excitation of Li neutrals followed by radiation until it volumetrically recombines, lowering the heat flux from 3.7 MW m−2 to 0.13 MW m−2, and the pressure is reduced by 93%, largely by collisions of H + with Li 0 .

Effects of Helium Seeding on Deuterium Retention in Neutron-Irradiated Tungsten

Neutron-irradiated tungsten (W) samples were exposed to helium (He)–seeded deuterium (D) plasmas using a linear plasma device called Tritium Plasma Experiment in order to investigate the synergetic effects of neutron and He irradiations on D retention in W. Exposure to nonseeded D plasma was also performed for neutron-irradiated and nonirradiated W samples for comparison. Deuterium retention in neutron-irradiated W after D plasma exposure was two to three times larger than that in W without neutron irradiation. Nevertheless, He seeding in D plasma resulted in a drastic reduction in D retention. The cross-sectional observation by transmission microscopy showed formation of He bubble layers with a thickness of 10 to 20 nm. There is a possibility that alpha particles in fusion plasma reduce tritium retention in neutron-irradiated plasma-facing components with W layers.

Plasma-surface interaction experimental device: PSIEC and its first plasma exposure experiments on bulk tungsten and coatings

Abstract A new laboratory-scale linear plasma device, PSIEC (Plasma-Surface Interaction system under Extreme Conditions), has been designed and constructed at Hefei University of Technology, China, to study plasma-material interactions for fusion reactor application. The PSIEC facility can generate continuous plasmas of hydrogen, deuterium, helium, argon and nitrogen. The electron density of these plasmas ranges from the order of magnitude of 1017 to 1018 /m3 and the electron temperature ranges from 1 to 40 eV. The incident ion energy is adjusted by applying a negative bias up to 110 V to the target. The plasma bombarding flux is varied from the order of magnitude of 1021 to 1022 ions/m2/s. Using the PSIEC facility, first plasma exposure experiments have been conducted on bulk tungsten and coatings. The different microstructure of tungsten have been found to lead to different damage effects under helium plasma exposure. Preliminary results allow us to expect nanocrystalline tungsten to be a promising candidate for plasma-facing material, of which the high concentration of intrinsic defects such as grain boundaries and interfaces offers the possibility of improving their irradiation resistance.