Tokamak Wall Research Articles

Observation of magnetic islands in tokamak plasmas during the suppression of edge-localized modes

AbstractIn tokamaks, a leading platform for fusion energy, periodic filamentary plasma eruptions known as edge-localized modes occur in plasmas with high-energy confinement and steep pressure profiles at the plasma edge. These edge-localized modes could damage the tokamak wall but can be suppressed using small three-dimensional magnetic perturbations. Here we demonstrate that these magnetic perturbations can change the magnetic topology just inside the steep gradient region of the plasma edge. We identify signatures of a magnetic island, and their observation is linked to the suppression of edge-localized modes. We compare high-resolution measurements of perturbed magnetic surfaces with predictions from ideal magnetohydrodynamic theory where the magnetic topology is preserved. Although ideal magnetohydrodynamics adequately describes the measurements in plasmas exhibiting edge-localized modes, it proves insufficient for plasmas where these modes are suppressed. Nonlinear resistive magnetohydrodynamic modelling supports this observation. Our study experimentally confirms the predicted role of magnetic islands in inhibiting the occurrence of edge-localized modes. This will be beneficial for physics-based predictions in future fusion devices to control these modes.

The transient thermal ageing of Eurofer 97 by mitigated plasma disruptions

Plasma disruptions in a commercial-scale tokamak will impose high magnitude, short-duration thermal loads on its plasma-facing first wall. Despite a plethora of mitigation measures, these severe, off-normal events are likely to briefly expose the structural materials of the first wall to significant temperature excursions. Eurofer 97, the reference structural material for the plasma-facing first wall of the EU DEMO tokamak, is a reduced activation 9Cr steel with a normalised and tempered ferritic/martensitic microstructure. Repeated exposure to the thermal effects of disruptions over the operating lifetime of a reactor may promote a cumulative evolution of Eurofer 97′s microstructure, affecting key material properties crucial to first wall performance. This novel transient thermal degradation mechanism has been explored via a laser-based transient heating experiment, supported by time-dependent finite element thermal analysis studies of DEMO’s water-cooled lithium–lead first wall during a mitigated plasma disruption. Transient-affected samples of Eurofer 97 were characterised via scanning and transmission electron microscopy techniques (SEM/TEM), including electron backscatter diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDX), and selective area electron diffraction (SAED). Microhardness and magnetisation testing data are also presented. A single 700 °C thermal transient was found sufficient to coarsen and partially recrystallise Eurofer 97′s tempered martensite sub-grains at the tungsten-Eurofer 97 interface. Further transient exposure at 700 °C resulted in the significant growth of equiaxed grains, the nucleation of intergranular Cr-rich M7C3 and M23C6 carbides, and the coarsening of V-rich and intra-granular Ta-rich MX precipitates. The microstructural effects of 850 °C transients are also reported. Notably, after 1,000 transients at 850 °C the hardness of Eurofer 97 was found to have decreased by 32 %.

Laser Diagnostics of Content of Hydrogen Isotopes in the Globus-M2 Tokamak Wall

Laser Diagnostics of Content of Hydrogen Isotopes in the Globus-M2 Tokamak Wall

Modeling of tungsten impurity transport and distribution in EAST based on multi-fluid and kinetic Monte Carlo simulations

In recent years, the materials of plasma facing components, such as divertor target plates, domes, and outer walls of tokamaks, such as ASDEX Upgrade, WEST, JET, EAST, and ITER, have been changed from carbon to tungsten because of its lower erosion and tritium retention rates. Impurities are produced by interactions between the plasma and the first wall. This study provides an investigation to simulate the transport and distribution of tungsten impurities in the edge plasma on EAST. The 2D multi-fluid edge plasma transport code SOLPS-ITER and 2D kinetic Monte Carlo impurity transport code DIVIMP were used in the simulations. The multi-fluid model in SOLPS-ITER and the kinetic Monte Carlo model in DIVIMP were employed to treat tungsten impurity ions. The 2D density contour distributions in the computational region and the 1D density radial profiles at the inner and outer midplanes of tungsten impurity particles with ionization states (W0–W+74) and the total tungsten particles with all charge states were obtained. With the heating power 1.5 MW and the line-averaged plasma density 2 × 1019 m−3, the results from SOLPS-ITER and DIVIMP show that the maximum density of tungsten ion with single ionization state is about 1014 m−3 and the total density of tungsten impurities with all charge states is about 1015 m−3 at the core boundary. To the best of our knowledge, the simulation results from SOLPS-ITER and DIVIMP are compared for the first time to benchmark SOLPS-ITER with the multi-fluid mode and DIVIMP with the kinetic model for tungsten impurity transport. The density distributions of tungsten impurities with different ionization states from SOLPS-ITER and DIVIMP are highly similar, and good agreement can be found under similar conditions involved in the calculation. From the comparison benchmark between SOLPS-ITER and DIVIMP for tungsten impurity transport, it can be concluded that the impurity transport approximation used by DIVIMP is good.

Probabilistic locked mode predictor in the presence of a resistive wall and finite island saturation in tokamaks

We present a framework for estimating the probability of locking to an error field in a rotating tokamak plasma. This leverages machine learning methods trained on data from a mode-locking model, including an error field, resistive magnetohydrodynamics modeling of the plasma, a resistive wall, and an external vacuum region, leading to a fifth-order ordinary differential equation (ODE) system. It is an extension of the model without a resistive wall introduced by Akçay et al. [Phys. Plasmas 28, 082106 (2021)]. Tearing mode saturation by a finite island width is also modeled. We vary three pairs of control parameters in our studies: the momentum source plus either the error field, the tearing stability index, or the island saturation term. The order parameters are the time-asymptotic values of the five ODE variables. Normalization of them reduces the system to 2D and facilitates the classification into locked (L) or unlocked (U) states, as illustrated by Akçay et al., [Phys. Plasmas 28, 082106 (2021)]. This classification splits the control space into three regions: L̂, with only L states; Û, with only U states; and a hysteresis (hysteretic) region Ĥ, with both L and U states. In regions L̂ and Û, the cubic equation of torque balance yields one real root. Region Ĥ has three roots, allowing bifurcations between the L and U states. The classification of the ODE solutions into L/U is used to estimate the locking probability, conditional on the pair of the control parameters, using a neural network. We also explore estimating the locking probability for a sparse dataset, using a transfer learning method based on a dense model dataset.

High heat flux cooling channel design based on porous copper structure

A cooling channel design based on porous copper material is proposed for high heat flux cooling of the first wall in tokamaks. The porous copper structure is examined by optical and electron microscopy, material analysis provides input values for empirical equations defining the pressure drop properties of the porous zone. The properties of the porous zone are applied to the simulation of ANSYS Fluent 2023 Release 1. The experimental samples of the porous copper channel and the plain channel are manufactured. A series of experimental measurements are conducted to compare both samples and to verify the results of numerical simulations. Critical heat flux is significantly increased in the case of a porous channel design.

Study of the layer thickness of multilayer sample by the LIBS method based on ablation rate correction

As a remote and in situ diagnostic technique for the first wall of tokamaks, laser-induced breakdown spectroscopy (LIBS) has shown promising potential for depth profile analysis of deposition layers on plasma-facing components (PFCs).

Runaway electrons and their interaction with tungsten wall: a comprehensive study of effects

Runaway electrons are a notable phenomenon occurring during the operation of a tokamak. Proper material selection for the tokamak's first wall structure and plasma facing components, particularly in large sizes tokamaks like ITER and DEMO, is crucial due to the energy deposition of runaway electrons on plasma facing components during collision events, resulting in severe heat transfer and material damage in the form of melting, corrosion, and fracture. These runaway electrons also contribute to the production of photoneutrons through (γ, n) nuclear reactions, lead to material activation and require remote handling. In this study, using a Monte Carlo code and simulating the collision of runaway electrons with a tungsten target exposed to their radiation, the electron transport is investigated, and the energy deposition spectrum resulting from these collisions on the target is analyzed. The influence of incident angle and magnetic field on the energy deposition spectrum and the energy deposition per particle in the target is examined. With an increase in the incident angle of incoming electrons, the amount of energy deposited in the target rises and the energy deposition spectrum broadens. Moreover, applying a magnetic field, results the most significant increase in energy deposition for electrons with energies below 1 MeV in the tangential radiation case. The energy deposition spectrum resulting from each collision event in these interactions is determined. For electrons with energies below 5 MeV, multiple scattering and ionization processes are the primary contributors to energy deposition in the target. However, as the incident electron energy increases, the significance of multiple scattering and ionization diminishes, and the bremsstrahlung process becomes the most effective reaction in energy deposition. The energy deposition profile of electrons in the tungsten target indicates that higher incident electron energies lead to a shift of the maximum energy deposition location towards the inner layers of the target, and the energy deposition peak broadens. Analyzing the electrons transport inside the tungsten target reveals that a substantial portion of electrons with energies of 50–100 MeV passes through the wall and may exit from the back surface, potentially causing damage to equipment behind the tungsten wall. Additionally, secondary products of the reaction, such as photons, secondary electrons, and neutrons and their energy profiles are thoroughly studied. These secondary products can penetrate the target and activate materials in the equipment behind the plasma-facing components. For primary electrons below 1 MeV hitting tungsten, reflection process is significant. Analysis of primary and secondary runaway electrons in the tokamak's tungsten wall shows that electrons with energies of 0.1, 0.2, and 0.5 MeV predominantly interact within a first 0.1 mm layer, without passing through it. The secondary electrons can escape the tungsten target and impact other components, which making them an important consideration in runaway electron collisions with the tokamak wall. Produced photons, as one of the secondary products, also linearly increase with the rising energy of primary electrons. Also, the photoneutrons are produced only when runaway electrons with energies of 10 MeV and above collide with the target. These secondary products can penetrate the target and activate materials in the equipment behind the plasma-facing components.

Heat Load onto the Globus-M2 Tokamak Wall due to Fast Ion Loss during Development of Toroidal Alfvén Eigenmodes

The results of experiments are described, which were performed at the Globus-M2 tokamak andaimed at studying the fast ion loss at the outer tokamak wall due to fast ions interaction with the toroidalAlfvén eigenmodes. The local heating of carbon tiles was experimentally measured, and the correspondingheat flux was calculated. It was shown how simulations of the lost particle orbits can explain the characteristicfeatures of the spatial map of wall heating. The flux of lost fast particles onto the wall was studied as a functionof the instability amplitude. It has been demonstrated that the simulations predict similar dependence of thefast ion flux on the instability amplitude and also correlate its nature to the peculiarities of fast ions spatialdistribution.

Measurement of the Characteristics of Fine-Grained Graphite Used as a Material for the First Wall of the T-15MD Tokamak

Measurement of the Characteristics of Fine-Grained Graphite Used as a Material for the First Wall of the T-15MD Tokamak

Heat Load onto the Globus-M2 Tokamak Wall due to Fast Ion Loss during Development of Toroidal Alfvén Eigenmodes

Heat Load onto the Globus-M2 Tokamak Wall due to Fast Ion Loss during Development of Toroidal Alfvén Eigenmodes

Modeling and meshing for tokamak edge plasma simulations

The accurate representation of a problem domain in terms of a geometric model and its effective discretization into a mesh plays an important role in achieving higher-quality simulation results and better computational efficiency. In this work, recent developments to extend an automated tokamak modeling and meshing infrastructure to better support the near flux field following meshing requirements of the XGC Gyro-kinetic Code are presented. The first extension is a procedure to effectively represent a general set of O-point and X-points configurations. The second extension is a procedure to decompose the geometric model that includes the tokamak wall, selected flux curves and the separatrix curves in a manner such that an appropriate set of mesh generation procedures can be applied to each subregion. The third extension is the application of appropriate mesh generation procedures in each subregion to create the desired final mesh.

Development of a portable laser-induced breakdown spectroscopy device for in situ and rapid characterization of wall composition on the first wall of EAST tokamak

During the long-pulse operation of nuclear fusion devices, plasma-material interaction processes such as material erosion, transportation of eroded materials, and impurity redeposition can deteriorate the performance of plasma-facing components (PFCs). Accurately identifying impurities and their spatial distributions on the first wall is crucial to comprehensively and deep understanding these processes. Laser-induced breakdown spectroscopy (LIBS) is a powerful spectral analytical technique that offers rapid, in situ, multi-elemental, and portable instrumentation advantages over the other techniques. Due to its unique and distinctive features, LIBS is a practical technique for composition and profilometry analysis in nuclear fusion devices. In this study, we report the development of a novel, compact, and portable LIBS device for in situ, real-time measurement of impurity distributions in nuclear fusion devices. This device is equipped with a mini-sized laser that can deliver 10 mJ per pulse, a main unit enclosing a power supply module, spectrometer modules, and customized software. The system was employed to characterize impurities on the first wall of the EAST tokamak under atmospheric conditions after the 2021 EAST spring-summer campaign. The successful in situ measurement demonstrates the feasibility of using the portable LIBS system integrated into a robotic arm in nuclear fusion device. The implement of this portable device can facilitate the operation of the nuclear fusion device by enabling real-time and continuous monitoring of impurities as well as the fuel retention.

De-noising of microwave reflectometry signal using maximal overlap discrete wavelet packet transform for plasma density measurement

For microwave reflectometry-based fusion plasma diagnostics, the acquired data is contaminated by the surrounding noise environment, making it difficult to measure the beat frequency accurately. Discrete wavelet and wavelet packet transforms are unsuitable as the former cannot decompose detailed coefficients, and the latter is a time-variant transform. This paper presents a correlation-based Maximal Overlap Discrete Wavelet Packet Transform (MODWPT), which decomposes the signal using different mother wavelet families. The suitable mother wavelet is chosen based on the metrics like signal-to-noise ratio (SNR), peak signal-to-noise ratio (PSNR), mean squared error (MSE), and mean absolute error (MAE). The data used for analysis is the stationary tokamak wall, for which the beat frequency and distance are measured. After rigorous analysis, it is found that sym12 outperforms with the following values 0.0493 (MSE), 0.1736 (MAE), 4.3334(SNR(dB)), 13.0634(PSNR(dB)) respectively. This technique can be extended to plasma scenarios for accurately measuring beat frequency and density profile.

Energetics of silicon in the bulk and near surfaces of tungsten: a first-principles study

Siliconization of the tokamak walls is a candidate method to improve plasma confinement in fusion tokamaks containing tungsten plasma facing components (W PFCs). To understand the interactions of silicon (Si) with W, the Si behavior in bulk W, and near three low-index W surfaces ((100), (110) and (111)) has been investigated using first-principles density functional theory. In bulk W, Si interstitial atoms have a low solution ability and high mobility, and Si atoms can be strongly trapped by W vacancies. The interaction between two Si adatoms is responsible for the stability of adatom superstructures on W surfaces, consistent with previous experimental observation (Tsong and Casanova 1981 Phys. Rev. Lett. 47 113). Although the coverage dependence of Si adsorption and diffusion energetics on surfaces is related to surface orientation, the W(110) surface has lower Si adsorption affinity and higher Si diffusivity than either the W(111) or W(100) surfaces. The most stable Si adatom superstructure on W surfaces is: square c(2 × 2) pattern on W(100) covered with 0.5 ML Si; rectangular c(4 × 2) pattern on W(110) with 0.25 ML Si; and rhombus p(1 × 1) pattern on W(111) with 1 ML Si. The coverage dependence of Si mobility on/toward W surfaces is generally related to the stability of the Si superstructures as a function of coverage on each surface. Interestingly, Si adatoms prefer to transport below the surface and into W subsurface by an exchange mechanism with W atoms, indicating the likelihood of epitaxial growth of W silicide layers on W surfaces during the operation of W PFCs.

Patterned Adhesion Layer Enables Rugged Pd-MIS Hydrogen Sensors.

Rugged Pd-metal-insulator-semiconductor (Pd-MIS) hydrogen sensors for detecting charge-exchange particles in fusion reactors have been constructed by utilizing a novel patterned adhesion layer. Poor adhesion at the interface between Pd and SiO2 is a common failure mode for Pd-MIS devices, severely limiting the Pd thickness and their usefulness as hydrogen sensors. The mechanical integrity of the Pd coatings is of particular importance in magnetic fusion energy research where the Pd-MIS diodes are used to measure hydrogen charge-exchange neutral fluence at the wall in tokamaks. In this application, particularly thick Pd contacts are desirable to prevent damage caused by high-energy particles; however, such thick Pd coatings are prone to mechanical failure due to blistering and wire bond detachment during construction or operation. A continuous Ti or Cr adhesion layer is not possible for this application since it would interfere with H uptake at the SiO2 interface, which is essential for the device to generate a response. In this work, we demonstrate that a patterned Cr interlayer substantially improves adhesion while still providing access for hydrogen to reach the SiO2-Pd interface.

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.

Commissioning of the inductive heater for hypervapotron testing

The paper describes commissioning of the inductive heater based on numerical simulation of inductive heating in the hypervapotron sample and test channel. The HEFEL loop is being developed to research a high heat flux cooling channels applicable in fusion devices. The Box Scraper hypervapotron channel geometry represents a primary focus of research. Conditions in the first wall of tokamak are simulated by induction heating of the tested hypervapotron channel. The exact heat flux of tested sample is difficult to measure, thus the numerical simulation of induction heating and thermohydraulic model of the cooling channel are an important part of HEFEL loop commissioning. ANSYS Maxwell 3D and ANSYS Fluent coupling is used for optimalization of the induction heating to achieve the highest heat fluxes using selected induction heating source.

Transport and acceleration mechanism of fast ions during edge localized modes in ASDEX Upgrade

Observations of enhanced fast-ion losses during edge localized modes (ELMs) have been reported in the ASDEX Upgrade tokamak, revealing losses above the injection energy. This suggests that fast ions can be accelerated and lost due to the ELMs. Recent analysis of the ELM-induced losses suggests that the fast ions are lost due to a resonant interaction with the electromagnetic perturbation during the ELM crash. The fast-ion transport and acceleration during ELMs is modelled using electromagnetic fields computed using the hybrid kinetic-MHD code MEGA, while fast-ion full orbits are tracked with the ASCOT code. Time-evolving 3D electromagnetic fields have been implemented in ASCOT to compute fast-ion orbits in the presence of fast MHD events such as ELMs. The simulations successfully reproduce a field-aligned pattern of the losses on the tokamak wall and the formation of an accelerated population in the lost fast-ion distribution, while they predict an accelerated population in the confined distribution. A parametric study of the fast-ion constants of motion suggests a resonant interaction between the fast-ions and the electromagnetic fields arising during the ELM crash. In the case of fast-ion acceleration, the perpendicular electric perturbation, with scales smaller than the fast-ion gyroradius, breaks magnetic moment conservation and resonantly modifies the fast-ion energy.

MOLECULAR DYNAMIC SIMULATION OF PLASMA MATERIAL INTERACTION TOCALCULATE THEORETICAL SPUTTERING YIELD

In a fusion reaction two light nuclei, Deuterium and Tritium merge to form a single heavier nucleus Helium. However, two positive nuclei repel each other. In order to merge two nuclei they need to have very high velocities. High speed means, high temperature. For the reaction it is significant for a nuclei to keep at 100 million °C temperature. At this temperature D and T atoms form a plasma. In order the reaction to take place, the plasma temperature must be conserved or plasma should not be cooled. Tokamak reactors are designed to confine the plasma in a magnetic field. Thus, the cooling of the plasma is prevented by hitting the reactor walls. Plasma density and temperature must be at a certain level in order to initiate the reaction and to ensure continuity. During the reaction process, positive and negative ions escaping from the magnetic field environment interact with Tokamak walls and cause deformation. This causes the plasma wall to deteriorate over time and the release of neutrons to the environment. Plasma-Wall interaction is one of the most important problems that cause interruption of fusion in Tokamak rectors. The materials which most resistant to ion corrosion in the plasma wall are graphite, beryllium, aluminium and tungsten. In this work, plasma-material interaction is studied theoretically physical and chemical erosion caused by the plasma interactions of different wall material samples (graphite, aluminium and Tungsten) used in the fusion reactor and investigated with the Monte Carlo method with molecular dynamics.