Nuclear Fusion Devices Research Articles

Automatic recognition of anomalous patterns in discharges by recurrent neural networks

Anomaly detection refers to the problem of finding patterns in data that do not conform to expected behavior. These off-normal patterns are often referred to as anomalies, outliers, discordant observations, or exceptions in different application domains. The importance of anomaly detection is due to the fact that anomalies in data frequently involve significant and critical information in many application domains. In the particular case of nuclear fusion, there are a wide variety of anomalies that could be related to particular plasma behaviors, such as disruptions or L-H transitions. In the case of unknown anomalies, they probably represent the major proportion with respect to the total anomalies that can be found in fusion. Whether the anomaly is known or not, all the anomalies in a nuclear fusion device should be detected by using the same approach, i.e. the physical state of the plasma during a shot should be reflected in some of the thousands acquired signals. In this article, we study the application of Deep Learning and a particular recurrent neural network called LSTM to detect anomalies by forecasting in a discharge.

Design of Laser Speckle Interferometry Experiment Control Platform Based on LabVIEW

The online diagnosis of the material morphology of the first wall of the nuclear fusion device is of great significance for the safe operation of the fusion device. Laser speckle interferometry is one of the most promising means to achieve real-time monitoring of wall material erosion and redeposition. Based on LabVIEW, this paper establishes a laser speckle interferometry experimental platform for precise automatic timing control and automatic image acquisition. It realizes independent communication and remote control for functional experimental equipment, including laser, piezoelectric ceramic and CCD camera. The MATLAB program is called to perform real-time inversion of the morphology of the sample to be tested. The experimental results show that the laser speckle interference experiment control platform meets the experimental operation requirements, realizes the precise control of the timing and reduces the experimental error. The experimental platform has good expansibility and has strong reference and transportability for the construction of related platforms.

Highly depth-resolved characterization of fusion-related tungsten material based on picosecond laser-induced breakdown spectroscopy

The objective of the present study has been to evaluate the potential applications of picosecond laser-induced breakdown spectroscopy (ps-LIBS) in nuclear fusion devices.

The calculation of the dissociation of hydrogen from helium clusters in tungsten

The synergistic effects of H and He interactions in W are important issues in the development of nuclear fusion devices. In this work, a series of molecular static and molecular dynamic (MD) simulations were conducted to investigate the H dissociation from interstitial HemH clusters. The molecular static simulations show that Hem clusters and HemH clusters have several energy states at 0 K, suggesting the complexity of the dissociation process where various dissociation paths with differing energy barriers exist. Thus, in the molecular dynamics phase, we propose a linearly increasing temperature technique that is an emulation of thermal desorption experiments to calculate the coarse-grained dissociation rate of H atoms from HemH clusters. Despite the complexity of the H dissociation process, our results showed that a single coarse-grained activation energy Ea can be extracted to characterise the complicated process that can be considered a thermal activated event based on our theory. To demonstrate the application of the results, we calculated and compared the H dissociation rate and the rate for Hem clusters colliding with H atoms. The comparison at 500 K shows that the rate for the Hem clusters colliding with the H atoms is much faster than H atoms dissociating from HemH clusters, suggesting that HemH clusters at this temperature should be used in some long-term models. The proposed method can also be extended to calculate the coarse-grained rate coefficients of other types of clusters.

Recent Developments in Dual-Laser Digital Holography for Plasma-Facing Surface Characterization

A digital holography device is currently undergoing development at the Oak Ridge National Laboratory for the purpose of measuring surface topography, with the goal of deployment as a real-time plasma-facing component diagnostic for the study of materials that could be utilized in a nuclear fusion device. The holography system utilizes one or two lasers depending on the scale of surface features under measurement. Measurements of surface roughness were performed in a single-laser mode and compared with the data from profilometry, with a linear correlation of increased holographic measurement fidelity as surfaces became smoother. Characterization of the dual-laser operating mode has been performed via surface measurement of stainless steel targets with “stair-step” features in various sizes. Results demonstrated that surface features with known sizes as small as 25.4 μm could be resolved. Measurements were within ~55 μm or less deviation from the actual sizes, and measurement accuracy was improved as feature size was increased, corresponding to the effect of noise becoming less pronounced. A target exposed to plasma generated by an electrothermal (ET) arc source was analyzed with flatfield correction and averaging of sequential image frames to demonstrate the improved measurement quality in preparation for future use of holography on ET arc-exposed targets.

Radiation Protection Design and Licensing for an Experimental Fusion Facility: The Italian and European Approaches

An experimental nuclear fusion device could be seen as a step toward the development of the future nuclear fusion power plant. If compared with other possible solutions to the energy problem, nuclear fusion has advantages that ensure sustainability and security. In particular, considering the radioactivity and the radioactive waste produced in a nuclear fusion plant, the component materials for the plant could be selected in order to limit the decay period, making recycling possible in a new reactor after about 100 yr from the beginning of decommissioning. To achieve this and other pertinent goals, many experimental machines have been developed and operated worldwide in the last decades, underlining that radiation protection and worker exposure are critical aspects of these facilities due to the high-flux, high-energy neutrons produced in the fusion reactions. Direct radiation, material activation, tritium diffusion, and other related issues pose a real challenge to demonstrating that these devices are safer than nuclear fission facilities. In Italy, for the past 30 yr, a limited number of fusion facilities have been constructed and operated, mainly at the ENEA Frascati Center, where a new one, the Italian Divertor Tokamak Test Facility (DTT), is now under development. The radiation protection approach, addressed by national licensing requirements, shows that respecting the constraints for worker exposure to ionizing radiation is not always straightforward. In the current analysis the main radiation protection issues encountered in the Italian fusion facilities are considered and discussed, and the technical and legal requirements are described. The licensing process for this kind of device is outlined and compared with that of other European countries.The following aspects are considered throughout the current study: description of the installation, plant, and systems; suitability of the area; buildings and structures; radioprotection structures and organization; exposure of personnel; accident analysis and relevant radiological consequences; and radioactive waste assessment and management.In conclusion, the analysis points out the need for special attention to the radiological exposure of workers in order to demonstrate at least the same level of safety as that reached at nuclear fission facilities.

Numerical analyses and tests for optimized and enhanced heat transfer solutions in DEMO

A set of novel design solutions for high performance cooling systems have been developed and tested by Consorzio RFX, achieving, with experimental tests, the challenging heat transfer conditions foreseen for Heating and Current Drive Systems of present and future nuclear fusion devices. The project, called Multi-design Innovative Cooling Research & Optimization (MICRO), has the triple objective of:•Verifying the full qualification of the manufacturing process and assess the cooling performance of the present solutions applied on the acceleration grids designed for ITER Heating Neutral Beam Injectors and MITICA experiment, which have similar heat loads as for DEMO1.•Identifying alternative designs realizable with presently qualified manufacturing processes for improving heat transfer mechanisms with limited pressure drops.•Developing further optimized solutions by means of the investigation of the interrelated effects of geometric parameters and thermofluid dynamic properties of the coolant fluid.The main aim of present work is to extend the fatigue life-cycle of high thermal stress components. In the first part of the paper the design description and constraints are presented together with the results obtained from numerical analyses carried out on the full-scaled model of the electrostatic grids; the outcomes of the experimental campaign, together with the characterization of the properties and performance of a set of alternative dielectric fluids, are shown in the second one.

Effects of transmutation elements in tungsten

Tungsten (W) is widely considered as the most promising plasma facing material (PFM), which will be used in nuclear fusion devices. Due to the transmutation reaction caused by the fusion neutron irradiation, transmutation elements (such as Re, Os, and Ta) are generated in the W-based PFM during the operation of nuclear fusion devices. In this paper, transmutation elements effects on mechanical properties of the W and the behavior of hydrogen/helium (H/He) atom in the W were investigated by using the first-principles calculation method. The results show that the ductility of the W is enhanced by transmutation elements if dislocation and other defects were ignored, while mechanical properties of the W incompletely depend on transmutation elements content. Compared with the pure W, the formation energy of the H/He in the W is reduced by transmutation elements, but the most favorable sites of the H/He in the W is not changed. Except for a repulsion between Ta and He in the W, the H/He in the W is attracted by transmutation elements. In addition, transmutation elements can change the best diffusion path of the H/He in the W and increase the diffusion rate of the H/He in W. This study provides a theoretical basis for the Tungsten (W) application, and further study on the effects of transmutation elements in the W will be needed.

Study of Kinetic Parameters of Globus-M Plasma by Thomson Scattering Using an Advanced Digital Polychromator

The local measurements of electron density and temperature by Thomson scattering (TS) diagnostics are essential for research of energy confinement, plasma heating, and control at nuclear fusion devices. This paper presents a digital filter polychromator designed for TS spectrum registration. A distinguishing feature of such a digital polychromator is the use of an analog-to-digital converter (ADC) having analog memory with discretization frequency of 5 GHz and 12-bit sampling. The low heat dissipation and compact design of the ADC allow us to integrate the recording and data handling systems in the spectral unit to obtain a standalone device that is totally isolated from other systems and equipment in the voltaic sense. The scattered laser radiation received from the plasma comes to the entrance of the polychromator through a fiber-optic cable, and the processed scattering signals and calculated electron density and temperature are available on a digital interface. The polychromator was tested within the Thomson scattering system at the Globus-M tokamak using two probing lasers: Nd:YAG 1064 nm and Nd:Glass 1055 nm operating with pulse energy up to 2J and FWHM of 4 and 40 ns, respectively. The employed ADCs allowed us to perform superfast signal digitizing in the oscilloscope mode and to demonstrate the possibility of stray signal time separation despite the modest size (~1 m) of the vacuum chamber. The results of measurements performed during the plasma experiment at the Globus-M tokamak are presented, and their comparative analysis with the data obtained at the operating TS system is made.

Theory of the Drift-Wave Instability at Arbitrary Collisionality

A numerically efficient framework that takes into account the effect of the Coulomb collision operator at arbitrary collisionalities is introduced. Such a model is based on the expansion of the distribution function on a Hermite-Laguerre polynomial basis to study the effects of collisions on magnetized plasma instabilities at arbitrary mean-free path. Focusing on the drift-wave instability, we show that our framework allows retrieving established collisional and collisionless limits. At the intermediate collisionalities relevant for present and future magnetic nuclear fusion devices, deviations with respect to collision operators used in state-of-the-art turbulence simulation codes show the need for retaining the full Coulomb operator in order to obtain both the correct instability growth rate and eigenmode spectrum, which, for example, may significantly impact quantitative predictions of transport. The exponential convergence of the spectral representation that we propose makes the representation of the velocity space dependence, including the full collision operator, more efficient than standard finite difference methods.

Effect of the magnetic field and current orientation on the splashing of liquid metal free surface of fusion reactor PFCs

In future tokamak nuclear fusion devices, the development of suitable plasma facing materials is considered to be one of the great challenges. Liquid metal, instead of conventional solid materials, has been proposed as a potential solution to plasma facing components (PFCs) for future nuclear fusion reactors, and it can solve the problems that most fusion reactors are facing. The splashing of liquid metal into the fusion plasma is a big problem because of the implementation of liquid metal PFCs in fusion reactors. An experiment involving the splashing of liquid metal has been developed under a strong magnetic field and electric field to imitate the interaction between plasma electric current and PFCs of liquid metal in fusion reactors. The current direction is perpendicular or parallel to the magnetic field direction. The influences of Kelvin–Helmholtz (KH) instability are experimentally observed and quantitatively analyzed on the magneto-electricity-driven liquid metal free surface. Processes of liquid metal splashing in the rectangular chamber are recorded by a high-speed camera. When the Lorentz force is upward and the external magnetic field and liquid thickness are constant, the liquid metal splashing from the liquid metal free surface when the external current exceeds the critical value. Critical current presents as almost inverse proportional functions with the magnetic field variation for different liquid thicknesses. In the experiment, the instability is characterized by the dimensionless numbers Bond number, Bd, determined from the importance of gravitational forces compared to surface tension forces, and the Hartmann number, Ha, determined from the intensities of the imposed magnetic field. Multiple linear regression models of the critical current density are summarized which indicates that the magnetic field and current’s influence determine the splashing critical point of liquid metal under the condition of multi-field coupling.

Finite element methods on composite meshes for tuning plasma equilibria in tokamaks

The fundamental concept of magnetically confined nuclear fusion devices is the magnetohydrodynamic (MHD) equilibrium: the pressure gradient due to highly energetic charged particles is balanced by the Lorenz force due to strong magnetic fields. Hence, numerical methods for MHD equilibria are also fundamental for fusion engineering applications. We rely here on a finite element method on composite meshes for the simulation of axisymmetric equilibria in tokamaks, torus-shaped nuclear fusion devices. One mesh with Cartesian quadrilaterals covers the domain accessible by the plasma and one mesh with triangles discretizes the region outside the chamber. The two meshes overlap in a narrow region. This approach gives the flexibility to achieve easily and at low cost higher order regularity for the approximation of the principal unknown, the poloidal magnetic flux, while preserving accurate meshing of the geometric details in the exterior. We show that higher order regularity allows to formulate appropriate optimal control problems that help to find a special type of equilibria, called snowflake equilibria, that are a very promising concept to mitigate high heat loads due to plasma escaping particles.

Laser induced ablation spectroscopy for in situ characterization of the first wall on EAST tokamak

Plasma-wall interaction (PWI) is a crucial issue for high-performance operation and first wall lifetime of future fusion reactors. An in situ measurement and the control of fuel retention in the vacuum-vessel will be an important aspect for the safe operation of nuclear fusion devices. Laser-Induced Ablation Spectroscopy (LIAS) is an attractive option with promising capabilities for in situ characterization of fuel retention and co-deposition during plasma discharges. In this work, an in situ LIAS wall diagnostic system is developed for real-time during discharge diagnosis of the first wall on the EAST superconducting tokamak. A dedicated timing system, which offers the function of multiple temporal settings with high time resolution for LIAS measurement during long-pulse discharges, is successfully commissioned. The experimental setup of the LIAS system for the characterization of the first wall on EAST is presented in detail. First LIAS results obtained in EAST during high power discharges are presented. The results indicate that in situ measurements of deposition, co-deposition and dynamic retention are possible.

Finite element discretization of a Stokes-like model arising in plasma physics

We consider a time-dependent diffusion–reaction model for two vector unknowns, satisfying a divergence-free constraint, and the associated scalar Lagrange multiplier. The motivation for studying such a model is provided by a plasma physics problem arising in the modeling of nuclear fusion devices (Braginskii equations), where the two vector unknowns represent ion and electron velocities, the scalar unknown is the electrostatic potential and the divergence-free constraint reflects the physical assumption of quasi-neutrality. We first recast the problem in a form reminiscent of the standard Stokes problem, which allows us to recognize the importance of using a compatible discretization for the vector and scalar unknowns, then propose and analyze a stable finite element formulation. Following this, we address some peculiar geometrical aspects of the model, showing how they can be naturally dealt with within our formulation, and finally discuss a solution procedure for the resulting linear system based on the classical Uzawa algorithm. Some numerical experiments complete the paper.

Possible Applications of Nanomaterials for Nuclear Fusion Devices

Abstract Conditions of nuclear fusion and nuclear fusion devices were described, and some possible applications of nanomaterials for nuclear fusion devices were presented in the present article. Muon-catalyzed fusion is one of methods for nuclear fusion to cause even at room temperature or lower, and protons or heavy ions with huge energy are irradiated to metals such as beryllium or copper, which results in emission of negative or positive charged muons from the metals. An experiment using a pyroelectric power source using lithium tantalite crystal was also reported to achieve nuclear fusion in a desktop-like device. Hydrogen storage is also important for the fusion devices, and the possibility of hydrogen storage in hydrogen storage metallic alloys was studied by diffusion calculation and potential calculation of deuterium fusion. Enhancement of deuterium diffusion in the Pd alloys would be one of the key points for energy materials. Carbon(C)/copper(Cu)-based composite materials with high thermal conductivity and good stability at high temperatures were also developed by adding a small amount of titanium, which has a low enthalpy of alloy formation with C and Cu. These carbon-based materials could be a candidate material for the plasma facing components of fusion devices.

Textile preforms for tungsten fibre-reinforced composites

Demanding high heat flux applications, as for example plasma-facing components of future nuclear fusion devices, ask for the development of advanced materials. For such components, copper alloys are currently regarded as heat sink materials while monolithic tungsten is foreseen as directly plasma-facing material. However, the combination of these materials in one component is problematic since they exhibit different thermomechanical characteristics and their optimum operating temperatures do not overlap. In this context, an improvement can be achieved by applying composite materials that make use of drawn tungsten fibres as reinforcement. For the manufacturing processes of these composites, suitable tungsten fibre preform production methods are needed. In the following, we will show that tungsten fibres can be processed to suitable preforms by means of well-established textile techniques as studies regarding the production of planar weavings (wire distances of 90–271 µm), circular braidings (multilayered braidings with braiding angle of 60° and 12°) as well as multifilamentary yarns (15 tungsten filaments with 16 µm diameter) are presented. With such different textile preforms tungsten fibre-reinforced tungsten (W f/W) with a density of over 99% and pore-free tungsten fibre-reinforced copper W f/Cu composites were produced which proves their applicability with respect to a composite material production processes.

Molecular dynamics simulations of tungsten bombardments on tungsten nanoparticles

The bombardment process of a single tungsten atom on a tungsten nanoparticle was simulated by molecular dynamics. Projectile tungsten atoms ranging from 50 eV to 50 keV were injected into tungsten nanoparticles with diameters ranging from 2 to 12 nm. The sputtering of the nanoparticles and the retention of projectiles were found to be relative to the nanoparticle size and projectile energy. The axial direction of the nanoparticles also influenced the sputtering to some degree. The results are useful in understanding the behavior of tungsten nanoparticles generated in nuclear fusion devices.

First-principles study of solvent-solute mixed dumbbells in body-centered-cubic tungsten crystals

Tungsten (W) is considered as a promising candidate for plasma-facing materials for future nuclear fusion devices, and selecting optimal alloying constituents is a critical issue to improve radiation resistance of the W alloys as well as to improve their mechanical properties. We conducted in the current study a series of first-principles calculations for investigating solvent-solute mixed dumbbells in W crystals. The results suggested that titanium (Ti), vanadium (V), and chromium (Cr) are favorable as solutes for W alloys from irradiation-effect perspectives because these elements are expected to promote vacancy-interstitial recombination without causing radiation-induced precipitation that reduces ductility of irradiated materials.

Combination of Flywheel Energy Storage System and Boosting Modular Multilevel Cascade Converter

This paper deals with a pulsed power supply system combining flywheel energy storage system (FESS) and a modular multilevel cascade converter (MMCC) for power compensation in performing rapid excitation of highly inductive and pulsed heavy loads. The FESS system consists of an induction motor with a flywheel to store kinetic energy. Parallelly connected capacitors cause self-excitation phenomena in an induction motor, and it works as an induction generator. Furthermore, the induction generator generates more than twice of electric power of its rated value for a short time. We can apply the proposed FESS to particle accelerators for physics experiments or medical use, and plasma shape and position control in pulsed nuclear fusion devices that require pulsed highpower. In addition, the coils in these applications that generate magnetic fields have large inductances. In plasma control and repetitive operation applications, it is necessary to change current rapidly. We can realize high-speed current control by using the proposed MMCC that can output a voltage higher than the input voltage. By combining the FESS and MMCC, a power supply may be realized with the ability to implement rapid current control while compensating for large power consumption and without significant load disturbance on the power grid.

A Fast Reconstruction Approach for the Assessment of Magnetic Diagnostic Systems in Nuclear Fusion Devices

The information coming from the diagnostic system is of vital importance for the correct functioning of many electromagnetic devices. In particular, for nuclear fusion devices, the magnetic measurements are used for a number of fundamental identification tasks such as equilibrium analysis and plasma boundary reconstruction. The quality and the reliability of the measurements strongly impact the final results; therefore, proper techniques able to detect faulty probes and to assess their effectiveness are very beneficial. When calibration and other self-consistency tools fail to detect loss of reliability, numerical approaches can help in detecting and eventually correcting information from the malfunctioning channels. This paper proposes a general approach for the fast identification of broken probes based on suitable reconstruction procedures. The approach is tested in Tokamak reactors, but it is quite general and it can be easily extended to other application fields.