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It is expected that fusion energy development, which is at a critical stage of breaking through energy break-even in the engineering sense, has the potential to reshape the future pattern of energy development with both opportunities and challenges. This study reviews the main progress of fusion energy research and summarizes the major challenges regarding energy balance, tritium selfsustainability, high availability, development of irradiation-resistant materials, and economical efficiency in general sense. On the basis of sorting out the common basic technology breakthroughs of the International Thermonuclear Experimental Reactor (ITER) program and the supporting research activities of its member states, the study summarizes the progress of China’s magnetic confinement fusion research in terms of overall planning, self-developed projects, and technical tracking. Furthermore, focusing on the Z-pinch-driven fusion-fission hybrid reactor (Z-FFR) concept, which is independently proposed by China, this study elaborates on the basic principles, application advantages, and series of progresses, and proposes a development plan toward commercial energy supply by 2040, covering the task objectives in the stages of key technology campaign, engineering demonstration, and promotion of commercial power generation. To comprehensively promote China’s fusion energy development, several suggestions are given. In the magnetic confinement fusion area, it is suggest that China deeply participate in the ITER program and relevant international cooperation, overcome key physics and engineering technologies of commercial fusion reactors, promote the research and development of key components of China Fusion Engineering Test Reactor (CFETR) device, and construct and operate CFETR in due course. In the Z-FFR area, it is of first priority to start the construction of electromagnetic-driven large-scale scientific devices the soonest possible, develop key technologies for fusion energy, and promote both engineering demonstrations and commercial projects in proper time.

The objectives of NSTX-U research are to reinforce the advantages of STs while addressing the challenges. To extend confinement physics of low- A, high beta plasmas to lower collisionality levels, understanding of the transport mechanisms that set confinement performance and pedestal profiles is being advanced through gyrokinetic simulations, reduced model development, and comparison to NSTX experiment, as well as improved simulation of RF heating. To develop stable non-inductive scenarios needed for steady-state operation, various performance-limiting modes of instability were studied, including MHD, tearing modes, and energetic particle instabilities. Predictive tools were developed, covering disruptions, runaway electrons, equilibrium reconstruction, and control tools. To develop power and particle handling techniques to optimize plasma exhaust in high performance scenarios, innovative lithium-based solutions are being developed to handle the very high heat flux levels that the increased heating power and compact geometry of NSTX-U will produce, and will be seen in future STs. Predictive capabilities accounting for plasma phenomena, like edge harmonic oscillations, ELMs, and blobs, are being tested and improved. In these ways, NSTX-U researchers are advancing the physics understanding of ST plasmas to maximize the benefit that will be gained from further NSTX-U experiments and to increase confidence in projections to future devices. [ABSTRACT FROM AUTHOR]

This theme issue collects together papers summarising the conceptual design of the Spherical Tokamak for Energy Production (STEP). In 2019, the UK government funded the first design stages of a prototype fusion powerplant based on a compact toroidal geometry, called STEP. The primary technical aims of STEP are to produce net energy, to be self-sufficient in tritium fuel and to demonstrate a maintenance regime that would extrapolate to appropriate availability for commercial powerplants. After 5 years and over 1000 person-years of detailed scientific and engineering conceptual design, this theme issue acts as a compendium of the current design basis for STEP, noting that this is a snapshot in time and that the design will continue to evolve. This article is part of the theme issue 'Delivering Fusion Energy – The Spherical Tokamak for Energy Production (STEP)'. [ABSTRACT FROM AUTHOR]

We consider a visco-resistive magnetohydrodynamic modelling of a steady-state incompressible tokamak plasma with a prescribed toroidal current drive, featuring constant resistivity η and viscosity ν. It is shown that the plasma velocity root-mean-square behaves as ηf(H) as long as the inertial term remains negligible, where H stands for the Hartmann number H≡(ην)−1/2, and that f(H) exhibits power-law behaviours in the limits H≪1 and H≫1. In the latter limit, we establish that f(H) scales as H1/4, which is consistent with numerical results.

Highly accurate simulation of plasma transport is needed to drive the successful design and operation of magnetically confined fusion reactors. Unfortunately, the extreme anisotropy present in magnetized plasmas results in thin boundary layers that are expensive to resolve. This work investigates how various mesh refinement strategies might reduce that expense to allow for more efficient simulation by comparing standard variable refinement approaches that use a field quantity to an adaptive approach that uses an error estimator. It is first verified that higher order discretization only realizes the proper rate of convergence once the mesh resolves the thin boundary layer, therefore motivating the focusing of refinement on the boundary layer. For three two-dimensional test cases that contain characteristic features of tokamak magnetic fields, an exponential refinement strategy based on the magnetic flux function, which is the standard refinement approach in the field, is compared to an adaptive strategy utilizing the established Zienwiekicz and Zhu error estimator. The adaptive mesh refinement strategy consistently achieves the same accuracy using orders of magnitude less degrees of freedom than either exponential or uniform refinement. This result makes the adaptive refinement strategy more efficient than the exponential refinement strategy while also being more generalizable to problems with complex magnetic geometries. Scaling laws are derived that quantify the improvement in cost of the adaptive refinement strategy over other refinement approaches in 2D and 3D. [ABSTRACT FROM AUTHOR]

Superconducting tokamaks have garnered significant research and interest in the quest for harnessing nuclear fusion energy. They are considered one of the most promising devices for achieving this goal. After more than half a century of dedicated efforts, a group of international researchers has successfully constructed and operated a batch of tokamak devices, resulting in significant achievements in magnetic confinement fusion research. The purpose of this paper is to provide a comprehensive review of the development of tokamak devices and magnetic confinement fusion. Firstly, the principles, main research directions and prospects of magnetic confinement fusion are introduced. It then reviews the construction and operation of major devices in the international context, including the International Thermonuclear Experimental Reactor (ITER), and describes the purpose and main contributions of their construction, and analyses the latest research results and development trends in this field from the perspective of experimental research progress. The current challenges and future directions of superconducting tokamak devices are also discussed. The review will provide a comprehensive and systematic introduction and reference for researchers and engineers in related fields. [ABSTRACT FROM AUTHOR]

This paper presents an efficient numerical tool for solving the problem of transient nonlinear evolution of plasma equilibrium in presence of eddy currents. The code, named FRIDA-TD (FRIDA- Time Domain), and based on the existing FRIDA (FRee-boundary Integro-Differential Axisymmetric) code, exploits two different approaches for solving the elated nonlinear system of Partial Differential Equations (PDE): (i) Finite Element Method - Boundary Element Method (FEM-BEM) for plasma equilibrium, and (ii) Axisymmetric Volume Integral (AVI) formulation for eddy currents. The resulting FRIDA-TD is a fast and flexible tool suitable for engineering-oriented purposes, such as the design of Tokamak devices, set up of plasma operations, prediction of performance scenarios, and design of feedback control systems. FRIDA-TD is successfully validated at first with the well-established CarMa0NL code, and then against experimental data of the RFX-mod device.

Plasma β dependence of ion temperature gradient (ITG) driven turbulence is investigated using gyrokinetic simulations, where β is the normalized pressure. In our β scan, self-consistent magnetohydrodynamic (MHD) equilibrium state is numerically calculated for each value of β. It is found that the influence of the Shafranov shift cancels out the electromagnetic stabilizing effect on the ITG mode, and the growth rate of the ITG mode is accordingly unchanged as β increases. As a result, the turbulent energy transport does not decrease with β as suggested by the s − α model (Ishizawa et al 2019 Phys. Rev. Lett. 123 025003). A significant difference from the s − α model is the increase of the energy transport with β. It is also found that the critical onset β value for the kinetic ballooning mode is significantly increased by the influence of the Shafranov shift. The cancellation of the electromagnetic stabilization by the Shafranov shift is explained by the decrease of magnetic drift frequency in the dispersion relation of electromagnetic ITG modes obtained by using a fluid approximation. [ABSTRACT FROM AUTHOR]

The Princeton Field-Reversed Configuration (PFRC) nuclear fusion reactor concept is an innovative approach to fusion power generation prioritizing low neutron production and small size. A combination of analytical modeling and numerical simulation shows that the novel heating approach generates an FRC with closed field lines. Simulation data from a single-particle Hamiltonian code predicts ms-scale plasma heating in reactor-scale conditions while PIC codes predict formation of warm FRC plasmas from initial mirror fields. The PFRC-1 and PFRC-2 experiments have heated electrons to energies well in excess of 100 eV and plasma durations to 300 ms, more than 10 4 times longer than the predicted tilt instability growth time. From these data, we have created a development plan and anticipated performance metrics for a fusion reactor based on the PFRC concept. The resulting 1–10 MW PFRC reactors would be suitable for diverse applications, from submarines to urban environments to space propulsion. PFRC is a steady-state, driven magnetic confinement device. Plasma, inside a cylindrical array of coils, is confined and heated by external RF antennae. PFRC would be ultra-low radiation due to both its fuel and small size. The choice of advanced fuels, deuterium and helium-3 (D–3He), may be enabled by the high- β FRC configuration. The small size of the reactor would enable rapid exhaust of the dangerous tritium ash. Low radiation would make the reactor safer to operate and, in combination with simple geometry and small size, dramatically lowers development and maintenance costs. This review paper gives an introduction to the physics of the PFRC and a summary of the PFRC-2 experiment results to date. It then discusses the future program plan and how PFRC reactors would be commercialized. [ABSTRACT FROM AUTHOR]

The self-organization of structures in a tokamak plasma as it undergoes an H-mode transition shows properties similar to simpler shear flow configurations. We will describe recent dynamical studies of plasma shear flows, including the idea of tracking the edge of chaos that separates two bistable states, computing the nonlinear minimal seed that can lead to turbulence, finding the attractor solution on the edge and seeing how starting from this solution we can understand the stability of relative period orbits that permeate the turbulent basin of attraction. We present a modus operandi developed for these simple configurations that can be adapted to understand the H-mode transition. [ABSTRACT FROM AUTHOR]

Magnetic confinement fusion is believed to be one of the promising paths that provides us with an infinite supply of an environment-friendly energy source, naturally contributing to a green economy and low-carbon development. Nevertheless, the major disruption of high temperature plasmas, a big threat to fusion devices, is still in the way of mankind accessing to fusion energy. Although a bunch of individual techniques have been proved to be feasible for the control, mitigation, and prediction of disruptions, complicated experimental environments make it hard to decide on specific control strategies. The traditional control approach, designing a series of independent controllers in a nested structure, cannot meet the needs of real-time complicated plasma control, which requires extended engineering expertise and complicated evaluation of system states referring to multiple plasma parameters. Fortunately, artificial intelligence (AI) offers potential solutions towards entirely resolving this troublesome issue. To simplify the control system, a radically novel idea for designing controllers via AI is brought forward in this work. Envisioned intelligent controllers should be developed to replace the traditional nested structure. The successful development of intelligent control is expected to effectively predict and mitigate major disruptions, which would definitely enhance fusion performance, and thus offers inspiring odds to improve the accessibility of sustainable fusion energy.

The initial operation of neutral beams in Wendelstein 7-X enabled the first measurements of carbon (C6+) temperature and rotation with a multi-chord charge exchange recombination spectroscopy system. In this article we analyse the velocity measurements obtained with this system to infer the profiles of radial electric field, E r, and net parallel velocity of fully stripped carbon ions, V ‖. We include a general treatment of the flow geometry and the velocity corrections due to the energy dependence of the charge exchange cross section. The inverted profiles are compared with neoclassical calculations for various plasma conditions and two magnetic configurations. It is found that both the E r and V ‖ experimental profiles agree in sign and order of magnitude with the neoclassical expectations and reproduce the expected dependencies on plasma parameters and magnetic configuration, within the reduced dataset presented. However, quantitative differences with respect to the neoclassical profiles are observed above the estimated errors in various conditions. [ABSTRACT FROM AUTHOR]

The fusion reactions involving deuterium are of great interest for the exploitation of the fusion energy via magnetic-confinement devices. In classical thermodynamics, the spontaneity of a process is established through the assessment of the change in Gibbs free energy. So far, the feasibility of nuclear reactions has been characterized in terms of cross section and Q-value while the entropic term (T ΔS) has been neglected. Such an assumption is always justified for fission reactions where the term ΔS is positive. In the case of fusion reactions that operate at very high temperatures (106–107 K) and where ΔS is negative, the change in Gibbs free energy may be positive, making the reaction non-spontaneous. This paper proposes a classical thermodynamic analysis of D-based reactions of interest for the magnetic-confinement fusion applications. The entropy contribution was evaluated via the Sackur–Tetrode equation while the change in enthalpy was considered constant and as corresponding to the Q-value of the fusion reaction. The results of the thermodynamic analysis are compared with nuclear reaction feasibility criteria based on the reaction reactivity. The DT and D3He reactions show a high degree of spontaneity although the second one presents a lower reactivity. An increase in temperature could enhance the reactivity of the D3He reaction at the cost of decreasing its thermodynamic spontaneity. Both branches of the DD reaction are characterized by a much lower thermodynamic spontaneity than that of the DT and D3He reactions. Furthermore, at the temperature of their maximum cross section, the DD reactions exhibit a largely positive change in Gibbs free energy and, therefore, are not spontaneous. At the temperature of magnetic-confinement fusion machines (1.5 × 108 K), among the D-based reactions studied, the DT one exhibits the highest degrees of spontaneity and reactivity.

A new scintillator based fast-ion loss detector (FILD) system has been designed for the Wendelstein 7-X (W7-X) stellarator. The mechanical design of the system is presented here along with engineering analyses of the system. This includes an assessment of the structural loads which also considers electromagnetic forces calculated for possible disruption scenarios as well as a mode frequency analysis of the system. Furthermore, an analysis of the thermal characteristics of the actively cooled probe head under prescribed steady state and transient heat loads is presented. Finally, the optical relay system is described and its properties used to forward model synthetic camera signals using the FILDSIM code and based on theoretical fast-ion losses incident on the probe head calculated using the ASCOT5 code. [ABSTRACT FROM AUTHOR]

A first-of-a-kind optimized stellarator for confining plasma has been designed and is being constructed with planar circular coils and permanent magnets composed of identical elements. The equilibrium is optimized to be quasi-axisymmetric for good particle confinement. The combination of permanent magnets and planar coils is significantly simpler to construct than fabricating three-dimensionally shaped coils, yet they are able to produce lower helical magnetic ripple than existing devices by two orders of magnitude in ϵ eff 3 / 2 , a characteristic neoclassical transport metric. [ABSTRACT FROM AUTHOR]

Ion orbit loss is considered important for generating the radially inward electric field E r in a tokamak edge plasma. In particular, this effect is emphasized in diverted tokamaks with a magnetic X point. In neoclassical equilibria, Coulomb collisions can scatter ions onto loss orbits and generate a radially outward current, which in steady state is balanced by the radially inward current from viscosity. To quantitatively measure this loss-orbit current in an edge pedestal, an ion-orbit-flux diagnostic has been implemented in the axisymmetric version of the gyrokinetic particle-in-cell code XGC. As the first application of this diagnostic, a neoclassical DIII-D H-mode plasma is studied using gyrokinetic ions and adiabatic electrons. The validity of the diagnostic is demonstrated by studying the collisional relaxation of E r in the core. After this demonstration, the loss-orbit current is numerically measured in the edge pedestal in quasisteady state. In this plasma, it is found that the radial electric force on ions from E r approximately balances the ion radial pressure gradient in the edge pedestal, with the radial force from the plasma flow term being a minor component. The effect of orbit loss on E r is found to be only mild. [ABSTRACT FROM AUTHOR]

The nuclear fusion reaction using deuterium and tritium fuel produces a large number of neutrons, γ rays and activation products, which have an impact on the radiation safety of people and the environment. In order to reduce the impact of ionizing radiation, it is necessary to know accurately well the time and space distribution information of nuclear radiation field intensity in fusion device. The magnetic confinement fusion devices built in the world have established a complete nuclear radiation monitoring system according to their own operating conditions to deal with the potential impact of ionizing radiation. By monitoring the radiation dose during the operation and maintenance of the magnetic confinement fusion device, the ionizing radiation and radionuclide data of the experimental site and the surrounding environment are obtained, which provides data support for radiation safety protection management. Based on the investigation of radiation monitoring systems of magnetic confinement fusion device at home and abroad, the main ionizing radiation source terms and monitoring system architecture are reviewed in this paper, and the measuring methods and common detectors of neutron and γ radiation dose in magnetic confinement fusion are introduced. Finally, the research status of radiation monitoring system for nuclear fusion devices at home and abroad is summarized, and the development trend and goal of nuclear radiation monitoring system in the future are prospected.

The fusion reactions involving deuterium are of great interest for the exploitation of the fusion energy via magnetic-confinement devices. In classical thermodynamics, the spontaneity of a process is established through the assessment of the change in Gibbs free energy. So far, the feasibility of nuclear reactions has been characterized in terms of cross section and Q-value while the entropic term (T ΔS) has been neglected. Such an assumption is always justified for fission reactions where the term ΔS is positive. In the case of fusion reactions that operate at very high temperatures (106–107 K) and where ΔS is negative, the change in Gibbs free energy may be positive, making the reaction non-spontaneous. This paper proposes a classical thermodynamic analysis of D-based reactions of interest for the magnetic-confinement fusion applications. The entropy contribution was evaluated via the Sackur–Tetrode equation while the change in enthalpy was considered constant and as corresponding to the Q-value of the fusion reaction. The results of the thermodynamic analysis are compared with nuclear reaction feasibility criteria based on the reaction reactivity. The DT and D3He reactions show a high degree of spontaneity although the second one presents a lower reactivity. An increase in temperature could enhance the reactivity of the D3He reaction at the cost of decreasing its thermodynamic spontaneity. Both branches of the DD reaction are characterized by a much lower thermodynamic spontaneity than that of the DT and D3He reactions. Furthermore, at the temperature of their maximum cross section, the DD reactions exhibit a largely positive change in Gibbs free energy and, therefore, are not spontaneous. At the temperature of magnetic-confinement fusion machines (1.5 × 108 K), among the D-based reactions studied, the DT one exhibits the highest degrees of spontaneity and reactivity. [ABSTRACT FROM AUTHOR]

Recycling coefficient is an important parameter for modeling of particle balance in the Tokamak. In this paper, recycling coefficient was considered as a function of time and energy to demonstrate its effect on density of plasma and wall of the Tokamak. Results showed that energy of incident particle plays a major part in plasma particles of the Tokamak. In addition, other calculations were performed to investigate the effects of different fuel injection rates on particle balance. [ABSTRACT FROM AUTHOR]

The dynamic behaviour of the ion and electron energy, particle and momentum transport measured during type-I edge localized mode (ELM) cycles at ASDEX Upgrade is presented. Fast measurements of the ion and electron temperature profiles revelead that the ion and electron energy transport recover on different timescales, with the electrons recovering on a slower timescale (Cavedon et al 2017 Plasma Phys. Control. Fusion 59 105007). The dominant mechanism for the additional energy transport in the electron channel that could cause the delay in the electron temperature gradient (ΔTe) recovery is attributed to the depletion of energy caused by the ELM. The local sources and sinks for the electron channel in the steep gradient region are much smaller compared to the energy flux arriving from the pedestal top, indicating that the core plasma may dictate the local dynamics of the ΔTe recovery during the ELM cycle. A model for the edge momentum transport based on toroidal torque balance that takes into account the existence of poloidal impurity asymmetries has been developed. The analysis of the profile evolution during the ELM cycle shows that the model captures the dynamics of the rotation both before the ELM crash and during the recovery phase. [ABSTRACT FROM AUTHOR]

Over the past 20 years, advances in tokamak physics and technology have prepared the field of magnetic confinement fusion research for the next step toward a steady-state burning plasma., (© The Author(s) 2023. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.)

A mathematical model for the compression process of a cylindrically symmetric thermonuclear target is presented. A numerical method of an increased accuracy order is considered, which is used to calculate the basic physical processes that occur in target plasma exposed to an external magnetic field. [ABSTRACT FROM AUTHOR]