Plasma-wall Interaction Studies Research Articles

Assessments of the key plasma parameters with different scenarios on HIT-PSI using EMC3-EIRENE

Linear plasma devices offer an effective way to conduct plasma-wall interaction studies and contribute to a basic understanding of edge plasma physics. A new platform at Harbin Institute of Technology for Plasma Surface Interaction experiments (HIT-PSI) is a newly-built linear device at the stage of commissioning that is capable of simulating high heat power deposition on divertor targets similar to tokamak conditions. Therefore, numerical simulations to evaluate the plasma characteristics are essential for designing and guiding the experimental conditions in HIT-PSI. In this work, the three-dimensional edge transport code EMC3-EIRENE has been used to investigate the plasma parameter distributions in HIT-PSI with the puffing and pumping systems involved. The effects of the heating power and target position on the distribution of electron density, electron temperature, and particle and heat fluxes have been investigated by EMC3-EIRENE. Particularly, the reduction in the electron density with the puffing fluxes has also been studied by analyzing individual atomic and molecular processes. Finally, the influence of varying pumping speeds on plasma parameters has been investigated in detail by adjusting the recycling coefficients at the two pumping ports.

Surface temperature measurement from infrared synthetic diagnostic in preparation for ITER operations

The protection of ITER in-vessel components and the plasma-wall interaction studies will be based on a large network of infrared (IR) cameras covering 70% of the tokamak. The surface temperature measurement from IR images remains challenging due to the presence of metallic targets, with changes in surface thermo-radiative properties (emissivity) and the presence of multiple reflections. The paper provides an overview of major progress to improve the interpretation of IR image and to get more reliable surface temperature from IR synthetic diagnostics. The paper presents the latest development of (1) the forward model to include the modelling of the edge localised modes and a new advanced camera that is better adapted to experimental data (2) the inverse model to retrieve the emissivity of the targets and the surface temperature from a neural network trained exclusively from synthetic IR images. Promising results have been obtained both from simulated test images with an estimated emissivity better than 0.05 and a surface temperature better than 10%, and from WEST experimental images of ITER-like wide-angle to filter reflection patterns.

Ex-situ quantification of impurity deposition depth on HL-2A divertor graphite tile by laser-induced breakdown spectroscopy

During tokamak discharges, the Plasma Wall Interaction (PWI) would lead to impurity deposition and fuel retention on the surface of the Plasma Facing Materials (PFMs). Laser-induced breakdown spectroscopy (LIBS) has emerged as a promising technique to facilitate in-situ, real-time, online characterization of the impurity deposition and fuel retention on the wall of tokamaks for PWI studies. The purpose of this investigation is to quantify the impurity deposition layer on the graphite tile removed from the dome region of the HL-2A tokamak lower divertor by using the ex-situ LIBS approach. The investigations indicate that the primarily deposited impurity elements are Fe, Cr, Ni, Mn, Ca, Al, Cu, Si, C, which may originate from the stainless steel first wall, divertor, limiter or the siliconized wall conditioning due to the PWI process in HL-2A tokamak. This has been further supported by observing a significantly positive correlation between the Cr, Ni, Mn, Al and Fe elements by spectral cumulative intensity linear regression analysis. The 3D morphology of the laser-ablated craters is reconstructed using a confocal microscope, revealing the laser ablation rates for the deposition layer and the substrate graphite tile to be 348 nm/pulse and 220 nm/pulse, respectively, under the laser fluence of 10 J/cm2. Then the thickness of the deposition layer is determined by using the correlation coefficient method. Along the poloidal direction of HL-2A tokamak, the deposited impurity shows an inhomogeneous pattern, with the thickness of the deposition layer varying from 1.74 μm to 5.92 μm. Finally, the absolute depth distribution of each element is also quantitatively measured.

Remote wide angle view broad wavelength viewing system compatible with D-T operations in JET

Imaging diagnostics in Joint European Torus have become essential in the study of plasma wall interactions and the protection of the plasma facing components. During deuterium operations, the location of these diagnostics was at close proximity to the vessel. However, for the 50–50 deuterium-tritium operation, the neutron yield impact onto the electronics of these imaging diagnostics would have caused them to fail completely at these locations. Shielding these systems from neutrons is achieved by relocating a selected number of these imaging systems to outside the Torus Hall (i.e. to the other side of the biological shield wall). The relocation encompasses the construction of two new lines of sight crossing through the biological shield wall and each of them using an optical relay which extends to separate ports in the machine. This article starts with a brief description of two new viewing systems. However, it focuses on the optical modelling and engineering design of one of them which is a multiple camera system producing a wide angle view (WAV) of the inside of the vessel. This WAV system has been used successfully during the recent D-T campaign. It operates in a broadband wavelength range extending from the visible to the middle infrared. It also incorporates a novel in-house horizontal kinematic mirror mount made from glass reinforced plastic as part of the optical relay. The operational stability of the WAV system has also been assessed and some post design improvements implemented.

Double pulse laser-induced breakdown spectroscopy for the analysis of plasma-facing components

Laser-induced breakdown spectroscopy is applied successfully for plasma-wall interaction studies in several fusion devices and post-mortem analyses of plasma-facing materials. However, the quantitative as well as qualitative analysis of low hydrogen isotope contents in tungsten plasma-facing components is still challenging. A promising approach to increase the optical signal in laser-induced breakdown spectroscopy is to apply a second laser pulse to the laser-produced plasma. We present two setups for post mortem plasma-facing component analyses using different laser pulse properties and different excitation geometries. The enhancement factors and changes in spectral line shapes for double pulse compared to single pulse laser-induced breakdown spectroscopy are presented.

Plasma-wall interaction studies in W7-X: main results from the recent divertor operations

Wendelstein 7-X (W7-X) is an optimized stellarator with a 3-dimensional five-fold modular geometry. The plasma-wall-interaction (PWI) investigations in the complex 3D geometry of W7-X were carried out by in situ spectroscopic observations, exhaust gas analysis and post-mortem measurements on a large number of plasma-facing components extracted after campaigns. The investigations showed that the divertor strike line areas on the divertor targets appeared to be the major source of carbon impurities. After multistep erosion and deposition events, carbon was found to be deposited largely at the first wall components, with thick deposits of >1 μm on some baffle tiles, moderate deposits on toroidal closure tiles and thin deposits at the heat shield tiles and the outer wall panels. Some amount of the eroded carbon was pumped out via the vacuum pumps as volatile hydrocarbons and carbon oxides (CO, CO2) formed due to the chemical processes. Boron was introduced by three boronizations and one boron powder injection experiment. Thin boron-dominated layers were found on the inner heat shield and the outer wall panels, some boron was also found at the test divertor unit and in redeposited layers together with carbon. Local erosion/deposition and global migration processes were studied using field-line transport simulations, analytical estimations, 3D-WallDYN and ERO2.0 modeling in standard magnetic field configuration.

Test Results of Active Thermography Method for Plasma-Wall Interaction Studies on the КTM Tokamak

• The developed method is intended to improve the accuracy of temperature measurements by a thermographic camera for researches on the KTM tokamak. • The method allows tracking the changes in the body emissivity during the plasma irradiation. • The method is based on the use of external infrared radiation. • The method was tested on a test bench with a plasma-beam facility with heating of the selected sample by an electron beam. • The test results have demonstrated the efficiency of the proposed method. The paper describes the main experimental results of testing the method of thermographic measurements on a test bench with a plasma-beam facility. The developed method is intended to improve the accuracy of surface temperature measurements of candidate materials of the first wall of future thermonuclear reactors during investigations on the KTM tokamak. The method is based on the use of an external infrared radiation to directly determine the changes in the body reflectivity and a corresponding increase in the accuracy of the temperature measurement by a thermographic camera by recalculating and correcting the value of the emissivity. Resulting curves of changes in reflectivity (emissivity) obtained from the measurements are close to the real values within the inaccuracy of the method. The results of the test demonstrated the efficiency of the proposed method.

Performance of DFT functionals for properties of small molecules containing beryllium, tungsten and hydrogen

Abstract A comparative study of bond lengths, atomization energies and vibrational frequencies for a set of neutral molecules Ben, BenHm, Wn, WnBem, and WnHm with m + n ≤ 4 in their ground state is presented. We compare 16 density functionals chosen from rungs 1 to 4 of Jacob's ladder and Hartree–Fock results to Coupled Cluster calculations. For atomization energies, ωB97XD, closely followed by B97D, M06, B3LYP and M11, shows the best performance while M11, ωB97XD and HSEH1PBE are most faithful in reproducing bond lengths. The ranking sequence for predicting vibrational frequencies is HSEH1PBE, B3LYP and M11. The range-separated hybrid mega-GGA functional M11 stands out to be accurate for all three properties for the studied Be/H/W molecules. The basis set and size dependence of the CCSD(T) energies and the influence of inner core electrons on the atomization energies of several tungsten-containing molecules are also discussed. CCSD(T)/cc-pVQZ energies are already close to the complete basis set limit from a cc-pVQZ/cc-pV5Z extrapolation level. Core correlation contributes with 3–5% for W2 and W hydrides in energies. The comparison of DFT functionals in this particular setting and the set of molecules allows to evaluate the accuracy of atomistic plasma-wall interaction studies for the ITER environment, where Be, W, and H/D/T interact with each other.

Experience on divertor fuel retention after two ITER-Like Wall campaigns

The JET ITER-Like Wall experiment, with its all-metal plasma-facing components, provides a unique environment for plasma and plasma-wall interaction studies. These studies are of great importance in understanding the underlying phenomena taking place during the operation of a future fusion reactor. Present work summarizes and reports the plasma fuel retention in the divertor resulting from the two first experimental campaigns with the ITER-Like Wall. The deposition pattern in the divertor after the second campaign shows same trend as was observed after the first campaign: highest deposition of 10–15 μm was found on the top part of the inner divertor. Due to the change in plasma magnetic configurations from the first to the second campaign, and the resulted strike point locations, an increase of deposition was observed on the base of the divertor. The deuterium retention was found to be affected by the hydrogen plasma experiments done at the end of second experimental campaign.

Development of laser-based technology for the routine first wall diagnostic on the tokamak EAST: LIBS and LIAS

A laser based method combined with spectroscopy, such as laser-induced breakdown spectroscopy (LIBS) and laser-induced ablation spectroscopy (LIAS), is a promising technology for plasma-wall interaction studies. In this work, we report the development of in situ laser-based diagnostics (LIBS and LIAS) for the assessment of static and dynamic fuel retention on the first wall without removing the tiles between and during plasma discharges in the Experimental Advanced Superconducting Tokamak (EAST). The fuel retention on the first wall was measured after different wall conditioning methods and daily plasma discharges by in situ LIBS. The result indicates that the LIBS can be a useful tool to predict the wall condition in EAST. With the successful commissioning of a refined timing system for LIAS, an in situ approach to investigate fuel retention is proposed.

Plasma–wall interaction studies within the EUROfusion consortium: progress on plasma-facing components development and qualification

The provision of a particle and power exhaust solution which is compatible with first-wall components and edge-plasma conditions is a key area of present-day fusion research and mandatory for a successful operation of ITER and DEMO. The work package plasma-facing components (WP PFC) within the European fusion programme complements with laboratory experiments, i.e. in linear plasma devices, electron and ion beam loading facilities, the studies performed in toroidally confined magnetic devices, such as JET, ASDEX Upgrade, WEST etc. The connection of both groups is done via common physics and engineering studies, including the qualification and specification of plasma-facing components, and by modelling codes that simulate edge-plasma conditions and the plasma–material interaction as well as the study of fundamental processes. WP PFC addresses these critical points in order to ensure reliable and efficient use of conventional, solid PFCs in ITER (Be and W) and DEMO (W and steel) with respect to heat-load capabilities (transient and steady-state heat and particle loads), lifetime estimates (erosion, material mixing and surface morphology), and safety aspects (fuel retention, fuel removal, material migration and dust formation) particularly for quasi-steady-state conditions. Alternative scenarios and concepts (liquid Sn or Li as PFCs) for DEMO are developed and tested in the event that the conventional solution turns out to not be functional. Here, we present an overview of the activities with an emphasis on a few key results: (i) the observed synergistic effects in particle and heat loading of ITER-grade W with the available set of exposition devices on material properties such as roughness, ductility and microstructure; (ii) the progress in understanding of fuel retention, diffusion and outgassing in different W-based materials, including the impact of damage and impurities like N; and (iii), the preferential sputtering of Fe in EUROFER steel providing an in situ W surface and a potential first-wall solution for DEMO.

Plasma-wall interaction studies in the full-W ASDEX upgrade during helium plasma discharges

Plasma-wall interactions have been studied in the full-W ASDEX Upgrade during its dedicated helium campaign. Relatively clean plasmas with a He content of >80% could be obtained by applying ion cyclotron wall conditioning (ICWC) discharges upon changeover from D to He. However, co-deposited layers with significant amounts of He and D were measured on W samples exposed to ICWC plasmas at the low-field side (outer) midplane. This is a sign of local migration and accumulation of materials and residual fuel in regions shadowed from direct plasma exposure albeit globally D was removed from the vessel. When exposing W samples to ELMy H-mode helium plasmas in the outer strike-point region, no net erosion was observed but the surfaces had been covered with co-deposited layers mainly consisting of W, B, C, and D and being the thickest on rough and modified surfaces. This is different from the typical erosion-deposition patterns in D plasmas, where usually sharp net-erosion peaks surrounded by prominent net-deposition maxima for W are observed close to the strike point. Moreover, no clear signs of W nanostructure growth or destruction could be seen. The growth of deposited layers may impact the operation of future fusion reactors and is attributed to strong sources in the main chamber that under suitable conditions may switch the balance from net erosion into net deposition, even close to the strike points. In addition, the absence of noticeable chemical erosion in helium plasmas may have affected the thickness of the deposited layers. Retention of He, for its part, remained small and uniform throughout the strike-point region although our results indicate that samples with smooth surfaces can contain an order of magnitude less He than their rough counterparts.

A LIBS method for simultaneous monitoring of the impurities and the hydrogenic composition present in the wall of the TJ-II stellarator.

The study of plasma-wall interactions and impurity transport in the plasma fusion devices is critical for the development of future fusion reactors. An experiment to perform laser induced breakdown spectroscopy, using minor modifications of our existing laser blow-off impurity injection system, has been set up thus making both experiments compatible. The radiation produced by the laser pulse focused at the TJ-II wall evaporates a surface layer of deposited impurities and the subsequent radiation produced by the laser-produced plasma is collected by two separate lens and fiber combinations into two spectrometers. The first spectrometer, with low spectral resolution, records a spectrum from 200 to 900 nm to give a survey of impurities present in the wall. The second one, with high resolution, is tuned to the wavelengths of the Hα and Dα lines in order to resolve them and quantify the hydrogen isotopic ratio present on the surface of the wall. The alignment, calibration, and spectral analysis method will be described in detail. First experimental results obtained with this setup will be shown and its relevance for the TJ-II experimental program discussed.

Fusion related research with laser-induced-breakdown-spectroscopy on metallic samples at the ENEA-Frascati laboratory.

The study of plasma-wall interactions is of paramount importance for continuous and fault free operations in thermonuclear fusion research to monitor the damages of plasma facing components (PFCs), plasma pollution from impurities and wall retention of hydrogen isotopes, like tritium. These needs make laser-induced-breakdown-spectroscopy (LIBS) a suitable candidate for a real time monitoring of PFCs in the current and next generation fusion devices, like ITER. It is also worthwhile for the quantitative analysis of surfaces, with micro-destructivity of the sample and depth profiling capabilities with sub-micrometric sensitivity. In this paper LIBS spectroscopy is exploited as a valid diagnostic tool for PFCs at the ENEA Research Center in Frascati (Italy) and at the Institute of Plasma Physics and Laser Microfusion (IPPLM) of Warsaw (Poland). The activities have been focused on LIBS characterization of samples simulating PFCs surfaces eroded/redeposited or contaminated from nuclear fuel after or during the normal operation of the reactor.

An Eulerian Vlasov code for plasma-wall interactions

In this work we present a new Eulerian kinetic 1D3V code devoted to the study of plasma-wall interactions and sheath structure. The code solves the Vlasov-Poisson system for two or more kinetic species on a one-dimensional spatial grid between two limiting plates, in the presence of a uniform magnetic field tilted with respect to the normal to the plates. A source-sink term in the Vlasov equation leads to a stationary solution comprising a magnetic and a collisional presheath, as well as the usual Debye sheath in front of the wall. Thanks to a nonuniform spatial grid the sheath structure can be resolved accurately. Several advection schemes are implemented and the code is parallelized. Here, the code is used to illustrate the problems of a stationary sheath-presheath structure in front of a negatively biased wall, with and without a tilted magnetic field.

GAMMA 10/PDX Project Status and Future

GAMMA 10/PDX project is the development of the mirror devices to aim the fusion-reactor relevant research using the potential control and the end loss as the particle and heat fluxes for transport, divertor plasma physics and Plasma Wall Interaction (PWI) studies. It was obtained that more than 10 MW/m2 heat flux density for PWI studies. The closed divertor-simulator module has been newly installed. The first studies of this module have been done and obtained the expected plasma flow. We will strengthen the module functions and diagnostic capability and heating systems to enable to be more reactor-relevant simulator. In the core transport control by the ECH, correlation studies of the radial electric field profile and the fluctuations are made and the high coherency is confirmed by using the new two-point potential detector of the Gold Neutral Beam Probe(GNBP). The maximum electron temperature obtained so far is - 150 eV during the ECH by Thomson Scattering, but we need more studies for the high power ECH optimization. As one of the key tools for the new GAMMA 10/PDX project, 2 MW gyrotron development at 28 GHz has been started, based on the success of 77 GHz-1.9 MW output. The maximum outputs of 1 MW at 28 GHz in short pulse and 540kW for 2 s are obtained.

Design and construction status of the TPM-1U tokamak

TPM-1U is a small size tokamak, with a vacuum vessel of 0.15 m minor radius and 0.4 m major radius, with an expected magnetic field in the torus centre of 0.5 T and a plasma current above 150 kA. One of the goals of the TPM-1U is to establish the scientific and technological base for the study of magnetically confined high temperature plasmas in Mexico, and help bring cohesion to the efforts in this field that have been scattered to date. Another goal is to contribute to the generation of high level human resources that can help the community grow in size. In addition to these two strategic goals, the machine will also have technical and scientific goals such as plasma-wall interaction studies, development of new diagnostics and exploration of new magnetic topologies. In this work, the design for the pulsed power system that will be used to feed the set of toroidal coils and design considerations for the central solenoid will be presented. Calculations for the dimensioning of the central solenoid used for ohmic heating are also presented, as well as preliminary considerations for the implementation of microwave power injection into the plasma.

Plasma-wall interaction studies with optimized laser-produced jets

The production of the laser-produced plasma jets at burnt-through low-Z foils was optimized by using three-frame interferometry. When striking secondary targets, these jets of energetic particles represent an efficient tool for the investigation of transient phenomena at surfaces of the plasma-exposed solids. Two sets of precisely measured x-ray spectroscopic data demonstrate diagnostic potential of the collimated jets in the plasma-wall interaction studies: Blue Doppler shifts of the Al jet self-emission visualize ion deceleration in the near-wall region. Local depressions found in Al Lyγ profiles emitted from Al/Si(PMMA) targets indicate charge exchange between the Al XIII and fully stripped C ions.

Tritium plasma experiment: Parameters and potentials for fusion plasma-wall interaction studies

The tritium plasma experiment (TPE) is a unique facility devoted to experiments on the behavior of deuterium/tritium in toxic (e.g., beryllium) and radioactive materials for fusion plasma-wall interaction studies. A Langmuir probe was added to the system to characterize the plasma conditions in TPE. With this new diagnostic, we found the achievable electron temperature ranged from 5.0 to 10.0 eV, the electron density varied from 5.0 × 10(16) to 2.5 × 10(18) m(-3), and the ion flux density varied between 5.0 × 10(20) to 2.5 × 10(22) m(-2) s(-1) along the centerline of the plasma. A comparison of these plasma parameters with the conditions expected for the plasma facing components (PFCs) in ITER shows that TPE is capable of achieving most (∼800 m(2) of 850 m(2) total PFCs area) of the expected ion flux density and electron density conditions.

Summary of the 19th International Atomic Energy Agency Technical Meeting on ‘Research Using Small Fusion Devices’

This paper presents a summary of recent results reported on several topics on magnetic confinement, dense magnetized plasmas, innovative fusion technology and applications, diagnostic systems and control and data acquisition systems. The main topics covered on the magnetic confinement devices, diagnostics and data acquisition concern the tokamak KTM (Kazakhstan Tokamak for Material testing) for materials research and testing, and IAEA Joint Experiments on small tokamaks. For the dense magnetized plasmas results on development and commissioning of plasma focus devices were reported. The plasmatron VISION I for innovative plasma–wall interaction studies, a lithium divertor for KTM and compact fusion reactors as neutron sources were presented.