Magnetic Confinement Fusion Devices Research Articles

Full-f electromagnetic gyrokinetic turbulence simulations of the edge and scrape-off layer of ASDEX Upgrade with GENE-X

Understanding and predicting turbulence in the edge and scrape-off layer (SOL) is critical for the optimization of magnetic confinement fusion devices. While there has been progress along these lines, especially with the help of fluid codes, the development of full-f electromagnetic gyrokinetic codes for the edge and SOL, in general, diverted geometries, remains crucial. In this work, we present simulations of the edge and SOL of the ASDEX Upgrade tokamak with the novel grid-based gyrokinetic (continuum) code GENE-X. The presented simulations are performed at both reduced and realistic electron-to-ion mass ratios on millisecond time scales, studying profile evolution. We compare the resulting plasma profiles to experimental measurements and to previous simulations with the Braginskii fluid code GRILLIX. Furthermore, we measure and validate the SOL power falloff length λq according to the Eich fit function. Based on the results, we discuss the influence of the ion-to-electron mass ratio and collisional effects on gyrokinetic SOL turbulence.

The Engineering Commissioning of the Magnets System On HL-2M Tokamak

HL-2M tokamak is one of largest magnetic confinement fusion devices designed and constructed by Southwestern Institute of Physics (SWIP), China National Nuclear Corporation (CNNC), the first plasma was obtained on December, 4 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> , 2020. The magnets system which plays an important role for the plasma confinement consists of 20 toroidal field (TF) coils and 18 poloidal field (PF) coils. The magnets system commissioning was carried out by two phases: self-commissioning and integrated commissioning. The first phase: self-commissioning including joint resistance commissioning, turn to turn insulation commissioning, ground insulation commissioning, deionized water sealing commissioning was accomplished during the installation and test of the magnets system. The second phase: the integrated commissioning with power supply system, ground system, measurement and data acquisition system, plasma control system, interlock and safety system was accomplished based on the results of the first stage. The total commissioning of magnets system in HL-2M tokamak for the first plasma have been accomplished successfully.

Multi-scale interactions between turbulence and magnetohydrodynamic instability driven by energetic particles

In order to realize high performance burning plasmas in magnetic-confinement fusion devices, such as tokamaks, both bulk plasma transport and that of energetic fusion alpha-particles, which result from different scale fluctuations with different free energy sources, have to be reduced simultaneously. Utilizing the advantage of global toroidal non-linear simulations covering a whole torus, here, we found a new coupling mechanism between the low-frequency micro-scale electromagnetic drift-wave fluctuations regulating the former, while the high-frequency macro-scale toroidal Alfven eigenmode (TAE) regulates the latter. This results from the dual spread of micro-scale turbulence due to the macro-scale TAE not only in wavenumber space representing local eddy size but also in configuration space with global profile variations. Consequently, a new class of turbulent state is found to be established, where the turbulence is homogenized on the poloidal cross-section with exhibiting large-scale structure, which increases fluctuation levels and then both transports, leading to deterioration in the fusion performance.

Surrogate approximation of the Grad–Shafranov free boundary problem via stochastic collocation on sparse grids

In magnetic confinement fusion devices, the equilibrium configuration of a plasma is determined by the balance between the hydrostatic pressure in the fluid and the magnetic forces generated by an array of external coils and the plasma itself. The location of the plasma is not known a priori and must be obtained as the solution to a free boundary problem. The partial differential equation that determines the behavior of the combined magnetic field depends on a set of physical parameters (location of the coils, intensity of the electric currents going through them, magnetic permeability, etc.) that are subject to uncertainty and variability. The confinement region is in turn a function of these stochastic parameters as well. In this work, we consider variations on the current intensities running through the external coils as the dominant source of uncertainty. This leads to a parameter space of dimension equal to the number of coils in the reactor. With the aid of a surrogate function built on a sparse grid in parameter space, a Monte Carlo strategy is used to explore the effect that stochasticity in the parameters has on important features of the plasma boundary such as the location of the x-point, the strike points, and shaping attributes such as triangularity and elongation. The use of the surrogate function reduces the time required for the Monte Carlo simulations by factors that range between 7 and over 30.

Numerical Analysis of Heat Transfer and Flow Field for CFETR Divertor

Nuclear fusion is now the ultimate need for energy, tokamak is one of the possible devices for controlled nuclear fusion. Based on the advanced tokamak magnetic confinement configuration and the research results of fusion physics in recent decades at home and abroad, Chinese Fusion Engineering Test Reactor (CFETR) will carry out innovative physical design to achieve the international requirements. Divertor is an essential component of the magnetic confinement fusion device, which directly sustains the strong particle flow and high heat load. The cooling system of divertor will directly affect whether divertor can function properly. In this paper, a new structure of divertor suitable for CFETR is designed and simulated, both the flow field analysis and the heat transfer analysis of the model were verified. The results show that the designed divertor structure can meet the system requirements.

On the use of recombination rate coefficients in hydrogen transport calculations

The commonly accepted picture for the uptake of hydrogen isotopes (HIs) from the gas phase across the surface into a metal with an endothermic heat of solution for HIs is that of dissociation followed by thermalisation in a chemisorbed surface state and finally overcoming a surface barrier to enter the metal bulk where the HIs occupy interstitial solute sites. To leave the metal bulk the HIs first transition to the chemisorbed surface state from which they then enter gas phase by recombining into a diatomic molecule. This model is generally attributed to the work of Pick and Sonnenberg from 1985. They clearly distinguish surface states and subsurface solute sites where the recombination flux is proportional to the square of the concentration of chemisorbed atoms due the diatomic nature of this Langmuir–Hinshelwood process. In an effort to compare their extended model with an earlier surface model by Waelbroeck, which uses an expression for the recombination flux proportional to the square of the sub-surface interstitial solute concentration, they derive an effective recombination coefficient. However, also with the so-derived Pick and Sonnenberg recombination coefficient, the Waelbroeck model is only applicable under certain conditions. But, due to its simplicity, it is often used in boundary conditions of diffusion trapping type calculations, generally ignoring whether or not these conditions are met. This paper will use the full Pick and Sonnenberg model implemented in the TESSIM-X code and in simplified algebraic approximations, to show the limits of applicability of the Waelbroeck–Ansatz in modelling hydrogen transport in metals foreseen for the first wall of magnetic confinement fusion devices.

Model based computation of electromagnetic forces in magnetic confinement toroidal devices by using magnetic measurements

In this paper we present a method to compute spatial and time evolution of electromagnetic forces in magnetic confinement fusion devices based on a combination of signal analysis of magnetic measurements with a simple electromagnetic modelling. The method allows analyzing the whole plasma discharge including fast transient phenomena such as fast terminations or disruptions. It has been tested and verified on RFX-mod, an experiment equipped with a non-continuous toroidal passive stabilizing shell, but it can be applied to cases with continuous wall such as tokamak devices, as well. An experimental plasma discharge, with a fast termination phenomenon at the end of the flat-top phase, has been analyzed and the related forces have been computed by using two different sets of data: synthetic magnetic field measurements provided by ANSYS simulation and real experimental ones. The results are in good agreement in all the phases of the plasma discharge. Moreover, because of the very low-computational requirements, it is shown that the presented method can be used as a tool for real-time monitoring of the electromagnetic forces on the shell to be integrated in the protection system of the future RFX-mod2 experiment.

A Band Rejection Filter of High Current Radio Frequency Ion Source for Neutral Beam Injector

As a promising ion source, the radio frequency (RF)–driven ion source is widely equipped in the neutral beam injector of magnetic-confinement fusion devices. In order to study the characteristics of the RF-driven ion source and plasma generation, a prototype RF negative ion source–based test facility has been developed at the Institute of Plasma Physics, Chinese Academy of Sciences. Many diagnostic tools have been developed for this test facility. The electrostatic sensors are a critical part of the diagnostic methods. Considering the electrostatic sensors to be used in the harsh RF environment, an improved RF compensation circuit [band rejection filter (BRF)] has been developed that effectively reduces RF interference and improves signal acquisition accuracy. The BRF has been successfully verified for frequency response characteristics in a magnitude plot.

Optimized cycle basis in volume integral formulations for large scale eddy-current problems

We present a Volume Integral formulation for the solution of large scale eddy-current problems coupled with low-rank approximation techniques. Two alternative approaches are introduced to map the problem unknowns into a subset of grid elements forming a base of global or mixed (global and local) cycles, respectively, and guarantee the well-posedness of the problem both in simply and multiply connected domains. The paper shows that the adoption of mixed cycles is computationally more efficient than global ones. In particular, integral formulations based on global cycles cannot be safely coupled with low-rank approximation techniques, which, however, are crucial to increase the size of the largest solvable problem, like the ones involving conducting structures in magnetic confinement fusion devices. The aim of this paper is to demonstrate how such bottleneck can be overcome by considering local and global cycles differently, on the basis of the cohomology theory. An improved, efficient, and robust algorithm for computing a base of global cycles is described in detail. In particular, the presented algorithm is able to almost minimize the cohomology basis length, i.e. the number of mesh edges forming such a basis, in order to allow an efficient solution of large scale problems. Furthermore, a novel and general method to handle global and local cycles together, in the context of low-rank approximated matrices, is shown to be efficient for the solution of large scale eddy-current problems in multiply connected domains. Along the manuscript, pseudo-codes are given, which clarify the proposed methods and help to implement them by Volume Integral Equation practitioners.

GENE-X: A full-f gyrokinetic turbulence code based on the flux-coordinate independent approach

Understanding and predicting plasma turbulence in the scrape-off layer of a magnetic confinement fusion device is a key open problem in modern plasma physics. The transitional region between the core and scrape-off layer poses a difficult problem for turbulence simulations. The poloidal magnetic field vanishes at the X-point of a fusion device, which introduces a coordinate singularity in the commonly used field-aligned coordinates. In the present work, we present a full-f gyrokinetic code based on a locally field-aligned coordinate system that is flux-coordinate independent and free of singularities. The coordinate system, as well as the equations and numerical methods are described. In addition, careful numerical and physical verifications in closed magnetic flux surfaces are included.

Study of the Design and Assembly of a High Harmonic Fast Wave Antenna for an LAPD

The simulation survey of TAE Technologies has demonstrated that high harmonic fast wave (HHFW) heating is a promising method for core electron heating of FRC plasma. This study mainly describes the HHFW antenna mechanical design and assembly on the basis of the results of electromagnetic simulations performed by Oak Ridge National Laboratory (ORNL), the available port dimensions, and antenna installation position of the LAPD. Compared to the original scheme, this antenna is also optimized in the design. It is found that the E field distribution of optimized antenna becomes even, and the maximum electric field decreases by approximately 14%. The current on the antenna box and FS is reduced after optimization, whereas the maximum J density decreases from 53.3 kA to 14.5 kA. The reflection performance of the port at 30 MHz is also improved after the structural optimization; The k// spectrum distribution is sharper at the monopole phase (0, 0, 0, 0) and dipole phase (0, π, 0, π) and (0, 90, 270,180) than other phases. The optimized antenna can obtain a maximum |k//| spectrum, which peaks about |k//| = 30 m−1 at the dipole phase (0, π, 0, π). The analysis results and assembly strategy can provide useful reference and guidance for the study of HHFW antenna design and fabrication in LAPD or other magnetic confined fusion devices.

RF wave propagation simulation for ICRF antenna in EAST

ABSTRACT The propagation performance of radiofrequency (RF) waves excited by ICRF antenna in plasma has a great influence on the coupling effect. In this paper, the propagation characteristics of fast wave (FW) and slow wave (SW) under tokamak experimental condition are calculated, and the influence of angular frequency w 0, antenna power spectrum k// and tokamak total magnetic field B 0 is illustrated based on the typical operating parameters in EAST. By applying perfect matched layer (PML) boundary processing method to simulate single-pass absorption and using finite element method (FEM) to solve Maxwell equations in frequency domain, the propagation and coupling simulation of RF waves in cold plasma for EAST ICRF antenna in B-window were carried out to obtain the polarization characteristics of fields and the relationship between the coupling impedance and the gap length from antenna to the FW cut-off layer. The analysis results in this paper provide useful reference and guidance for the study of propagation coupling of ICRF waves in EAST and other magnetic confined fusion devices.

Interaction between filaments and ICRF in the plasma edge

• Recent progresses in understanding of interaction between filaments and ICRF in the plasma edge are summarized. • RF induced convective cells can poloidally distort, stretch and break up filaments and influence the local plasma transport in the SOL. • The mutual influence of multiple filaments induces poloidally distributed and radially extended stripe structures in the wave fields and causes wave scattering. • Filaments redirect part of the RF power from the perpendicular direction to the parallel direction as a result of rectification of antenna power spectrum and mode conversion of fast wave to slow wave. Ion Cyclotron Range of Frequencies (ICRF) heating is a standard heating method for magnetic confinement fusion devices. ICRF waves can interact with filaments in the scrape-off layer (SOL) and induces a series of important phenomena. Understanding their interaction and the related physics is essential to the success of ICRF heating and plasma confinement. This paper is a review paper summarizing the recent progress in understanding of interaction between filaments and ICRF in the plasma edge and presents some key experimental and simulation results. In particular, the influence of RF convective cells on filament transport and the influence of filament on RF wave scattering, energy spectrum density, power redirection and wave mode conversion are discussed in detail. Most results shown in the paper make use of the published work, and some new results about mode conversion are also present.

Fiber-optic silicon Fabry-Perot interferometric bolometer with improved detection limit for magnetic confinement fusion.

Fiber-optic bolometers (FOBs) intended for plasma radiation measurement in magnetically confined fusion have been previously developed using a silicon pillar that functions as both a Fabry-Perot interferometer (FPI) for temperature measurement and an absorber for the radiation. We report an FOB design that can significantly improve the detection sensitivity over earlier designs by engineering the absorber of the FOB. Our design uses the fact that, compared with a silicon pillar, a gold film with the same x-ray absorption thickness will show a much higher temperature rise from a given power density of the radiation. Therefore, the responsivity of an FOB can be improved by attaching a large gold disk to the silicon FPI as the absorber. We have developed a fabrication method for FOBs of such design and obtained an FOB with a 4-µm-thick, 0.6-mm-diameter gold disk attached to a 200-µm-diameter, 100-µm-thick silicon FPI. We have characterized the noise, responsivity, response time, and noise-equivalent power density (NEPD) and compared these with the earlier design where the absorber is mainly the silicon FPI itself. The experimental result suggests that the FOB with the gold disk achieves a responsivity of ∼2.8 mK/(W/m2) and a noise-equivalent-power-density of 0.13 W/m2, which are, respectively, more than nine times larger and six times smaller compared to the FOB using a previous design. Improved NEPD and good absorption over a broad frequency range will make the FOB more attractive for applications in magnetic-confinement fusion devices.

Fast Fourier transform-volume integral: a smart approach for the electromagnetic design of complex systems in large fusion devices

A volume integral (VI) equation method based on the fast Fourier transform (FFT) is proposed for the electromagnetic simulation of complex systems in large magnetic confinement fusion (MCF) devices. Such FFT-VI allows for dramatically reducing the time and the human effort required to adapt the CAD input files to models suited for electromagnetic simulations. Moreover, with FFT-VI the mesh generation is performed automatically and is very efficient and robust. The adoption of the FFT allows for significantly reducing the computation time and the memory requirements, therefore making simulations with several millions of unknowns feasible. Numerical results show that, in terms of numerical performances, the FFT-VI method applied to challenging electromagnetic problems in representative MCF devices is markedly superior with respect to the state of the art VI techniques. A sample implementation of FFT-VI is made publicly available.

Tailored tungsten lattice structures for plasma-facing components in magnetic confinement fusion devices

Tailored tungsten lattice structures for plasma-facing components in magnetic confinement fusion devices

Destabilization of Alfvénic activity by non-axisymmetric magnetic field induced rotation braking in KSTAR tokamak

We report an experimental observation of destabilization of the Alfvénic activity by non-axisymmetric (3D) magnetic field induced toroidal rotation braking in the KSTAR tokamak. The toroidicity-induced Alfvén eigenmodes (TAEs) are destabilized when toroidal plasma rotation is reduced down to the minimum to approach near-zero rotational shear by 3D magnetic braking. This observation indicates that the stability of the TAEs is strongly correlated to the plasma rotation that modifies Alfvén continuum. The TAE frequency predicted by stability calculation with the reduced rotation is in a good agreement with the experimental measurement. It is suggested that the impact of 3D magnetic field and rotation on the Alfvénic modes and associated fast ion confinement should be considered for the operation of magnetic confinement fusion devices.

Summary of the 16th IAEA technical meeting on energetic particles in magnetic confinement systems—theory of plasma instabilities

This report summarizes the contributions presented at the 16th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems—Theory of Plasma Instabilities, held in Shizuoka, Japan, 3–6 September 2019. The meeting brought together about 100 experts from nuclear fusion research sites worldwide to discuss the physics of energetic particles and plasma instabilities, at the first joint meeting from the two scientific disciplines. The main topics of the meeting were: alpha particles physics; transport of energetic particles; effects of energetic particles in magnetic confinement fusion devices; collective phenomena such as Alfvén eigenmodes, energetic particle modes and others; runaway electrons and disruptions; diagnostics for energetic particles; control of energetic particles confinement; multiscale physics and instabilities in burning plasmas.

Thermal-Hydraulic Analysis on Quench Behavior of Indium-Tin Soldered REBCO Composite Conductor

The second-generation high-temperature superconductor REBCO is one of the most valuable candidates that meet the requirements of future magnetic confinement fusion device. Quench behavior of an improved high-current REBCO composite conductor is simulated by a 1-D finite element method. Indium-tin alloy foil placed in the gap between stabilizer, jacket, and superconducting tape stacks can significantly reduce thermal contact resistance, which leads to an increase on the minimum quench energy of composite conductor. Short perturbations have a better response to this improvement than long perturbations, due to the low normal zone propagation velocity (NZPV) of REBCO conductor. For short perturbation (perturbation length (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 0.1 m), the peak temperature in cable and helium drops 12.76 and 17.99%, respectively, after soldered. Due to the change of heat transfer in a composite conductor caused by helium wave propagation, the development and maximum length of normal zone significantly decrease from 13.06 to 4.78 m under long perturbation (l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 10 m) circumstance. High current REBCO composite conductor with proper indium-tin alloy soldered can enhance the stability and safety of composite conductors under severe working conditions, but it also slows down the NZPV and hinders the quench detection of high-temperature superconducting magnet.

Neutronics Assessment of a Compact D-D Neutron Generator as a Neutron Source for the Neutron Calibration in Magnetic Confinement Fusion Devices

Neutronics Assessment of a Compact D-D Neutron Generator as a Neutron Source for the Neutron Calibration in Magnetic Confinement Fusion Devices