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A new toroidal capacitive discharge plasma (TCDP) system is designed and constructed for the first time to our knowledge. This device has been created for the exploration of plasma-material interactions including metal and semiconductor samples for different pressure rates. By using two materials, namely copper (Cu) and tungsten (W), initially, electrode couples have been tested for high-voltage DC excitations. The reactions of electron material to the low-pressure helium (He) have been observed. Then, Paschen’s curve exploration has been performed to find out the gas discharge threshold electrical potentials for various pressure (P) rates. The simulations of TCDP device have been performed in a particle in cell (PIC) analyzed 3D structure in the CST Studio medium for various excitation frequencies and toroidal/poloidal field values. It is observed that the breakdown voltage for He is UB = 500 V. The DC excitation via the electrodes reveals that the torus-structured plasma gas media produces a stable torus-shaped media for plasma formation. Besides, toroidal and poloidal fields affect the plasma stabilization in the TCDP. Especially toroidal field coils in TCDP assist in keeping the plasma in the middle of the torus channel. According to the experiments, the poloidal fields increase the electric field strength in the angular direction and improve the electrical stability of the plasma between the electrodes. [ABSTRACT FROM AUTHOR]

The CN-H1 stellarator is a HELIAC, and re-build with partly renewed coils at the University of South China. We calculate a set of equilibria varying the helical current and compute their Alfvén and sound excitation spectrum. With a plot of the density of frequencies, we provide a proxy for the continuum damping and for the visibility of excitations in the diagnostics. Furthermore, we explore the importance of magnetic islands for the excitation spectrum of the device. Thereby we expand a numerical tool to include the modifications due to islands along with the so called slow-sound approximation. We find that the frequency of the geodesic acoustic mode vs. radius changes considerably even for a moderate island size which may have consequences for the zonal flow dynamics.

Theoretical studies have suggested that nonlinear effects can lead to ‘radio frequency (RF) condensation’, where an initially broad current profile can coalesce in islands when they reach sufficient width. In suitable conditions, RF condensation can ‘self-focus’ the driven current to the center of an island, improving stabilization efficiency and reducing control complexity. In unsuitable conditions, the effect can prematurely deplete the RF energy before it reaches the island center, impairing stabilization. It is predicted that the RF condensation effect can significantly impact reactor-scale tokamaks. This paper presents a set of simulations investigating the conditions under which RF condensation might be encountered in present-day tokamaks. For concreteness, the calculations use equilibrium reconstructions for two shots from DIII-D and AUG. The Current Condensation Amid Magnetic Islands (OCCAMI) simulation code has been used for this investigation. The code takes as its input a numerically specified axisymmetric EFIT equilibrium solution, and it perturbatively constructs a 3D field with an island embedded at the appropriate rational surface. In the OCCAMI code, the GENRAY code is used for ray tracing and for calculating the power deposition along a ray trajectory, and GENRAY is coupled self-consistently to a solution of the thermal diffusion equation in the island. The simulation results described in the paper illuminate the conditions required for experimental validation of the theory of RF condensation. The simulations also provide an explanation of why the effect was not noticed in experiments prior to the publication of theoretical papers on the subject.

This paper reviews the recent advancements of the algorithm and application to fusion plasma instability study of the fully kinetic Particle-in-Cell (PIC) method. The strengths and limitations of both explicit and implicit PIC methods are described and compared. Additionally, the semi-implicit PIC method and the code ECsim used in our research are introduced. Furthermore, the application of PIC methods in fusion plasma instabilities is delved into. A detailed account of the recent progress achieved in the realm of tokamak plasma simulation through fully kinetic PIC simulations is also provided. Finally the prospective future development and application of PIC methods are discussed as well. [ABSTRACT FROM AUTHOR]

This paper reviews the recent advancements of the algorithm and application to fusion plasma instability study of the fully kinetic Particle-in-Cell (PIC) method. The strengths and limitations of both explicit and implicit PIC methods are described and compared. Additionally, the semi-implicit PIC method and the code ECsim used in our research are introduced. Furthermore, the application of PIC methods in fusion plasma instabilities is delved into. A detailed account of the recent progress achieved in the realm of tokamak plasma simulation through fully kinetic PIC simulations is also provided. Finally the prospective future development and application of PIC methods are discussed as well.

MMS mission by NASA has observed lower hybrid waves (LHWs) and associated turbulence near the reconnection regions at Earth's magnetopause. The presented work studies LHWs (electromagnetic mode) and turbulence in the presence of magnetic islands (at the reconnection sites) in magnetopause. A nonlinear model representing electromagnetic 3D LHW has been developed. We have considered that nonlinear effects are due to the ponderomotive force. The dynamical equation, thus obtained, is solved using numerical methods and computer simulation. For spatial integration, we have used the pseudo‐spectral method and the finite difference method is used to study temporal evolution. The simulation results show the spatiotemporal development of the LHW localized structures and current sheets and confirm the presence of turbulence. Using a semi‐analytic model, we have determined the scale size of current sheets and localized structures and shown that these scale sizes depend upon the lower hybrid power. The turbulent power spectra have also been studied. The power scaling of k−2.8 (approximately) and k−2.1 (approximately) are observed along perpendicular and parallel directions, respectively. The power law scaling of turbulence generation has been used to study the formation of the thermal tail of energetic electrons. The fractional diffusion method has also been exploited to determine electron acceleration and plasma heating. We anticipate LHW may be responsible for the acceleration of the energetic electrons and anomalous heating in magnetopause. Key Points: Lower hybrid wave (LHW) may be responsible for the acceleration of the energetic electrons and anomalous heating in magnetopauseThe scale size of current sheets and localized structures depends upon the lower hybrid powerDynamical model and computer simulations are used to study LHWs, turbulence and the role of LHWs in electron acceleration and heating [ABSTRACT FROM AUTHOR]

In this study, the impact of a non-inductive current drive, such as electron cyclotron current drive, on three-dimensional (3D) magnetic islands in the high- β equilibrium of the Chinese First Quasi-axisymmetric Stellarator (CFQS) was investigated using the HINT code. In the case of a high- β equilibrium (volume-averaged plasma beta < β > ∼ 0.74% and bootstrap current I _bs ∼ 24.5 kA), two m / n = 4/2 rational surfaces with large magnetic islands develop (Wang et al 2021 Nucl. Fusion 61 036021). The islands can be effectively controlled using a constant or a Gaussian current density profile, depending on the direction and amplitude of the current. With a constant current density amounting to a total current of −6 kA, the rotational transform profile can be modified such that the m / n = 4/2 rational surface is eliminated and the island is suppressed. For the Gaussian current density profile, the magnetic island can also be suppressed using a smaller total current of ∼−2 kA to adjust the iota profile. These results suggest that in the CFQS stellarator, the external current drive might be an efficient approach for controlling 3D magnetic islands and consequently improving plasma confinement.

A one-dimensional chain of elongated anisotropic magnetic islands on a nonmagnetic substrate with dipolar interactions and an applied magnetic field transverse to the chain is considered. With the long axes of the islands perpendicular to the chain, the system allows for three uniform metastable states: (1) tilted dipoles with magnetization at an oblique angle to the chain, (2) transverse dipoles with magnetization perpendicular to the chain, and (3) alternating transverse dipoles with no net magnetization. The uniform magnetic field controls their stabilities and is analyzed for its ability to cause transitions among the states. The energy and frequency eigenvalues are determined for small-amplitude traveling wave deviations of the dipoles. The results are summarized in a phase diagram in the field/anisotropy plane, that highlights the multistable properties of this type of system. • Dipole-coupled magnetic island chains have three metastable uniform states. • Energy and frequency eigenvalues control their stability and linearized dynamics. • Stable regions for each state are determined in an anisotropy/field diagram. • Transitions among the states can be identified in magnetization versus field curves. [ABSTRACT FROM AUTHOR]

At the present time, Magnetic Confinement Fusion (MCF) is considered as a way to produce energy. In this work, one of the Magnetohydrodynamic (MHD) limitations has been discussed. As closed magnetic surfaces, the Magnetic Islands (MIs) which are generated due to pressure effects, need to be surrounded by a separatrix which separate them from the other parts. External Magnetic Fields (EMFs), the safety factor (q) and the pressure profiles would be used to take the MIs under control. This could be achieved through an exterior medium, exclusively the Electron Cyclotron Heating (ECH) as well as the Current Drive (ECCD). Study of the magnetic flux surfaces and the effect of magnetic perturbation on tokamak plasmas, inform us about the formation of the MIs and their locations. In this work, together with the comprehensive review of the MIs and their importance, the conventional methods for improving the magnetic confinement has been introduced and discussed. In this regards, the Hot Limiter Biasing (HLB) method and the Resonant Helical Field (RHF) which is produced by external Helical Coils (HCs) were introduced and used. Then, the plasma current (), the Loop voltage, and the MO were obtained for different states. Finally, the Magnetic Islands Width (W) and their Growth Rate (GR) were calculated and compared with the experimental results.

Various space missions and observations over the past decades have provided unexampled details about the nature of solar wind, the acceleration mechanism, and different nonlinear phenomena responsible for energy transfer and turbulence in the interplanetary space. This review focuses on the role of Alfvénic fluctuations—both kinetic Alfvén wave (KAW) and dispersive Alfvén wave (DAW)—in driving solar wind turbulence and magnetic reconnection at 1 AU. The process of filamentation has been studied through a nonlinear coupling system of KAW/IAW (ion acoustic wave) and relatively high-frequency pump KAW (HKAW, i.e., frequency less than ion cyclotron frequency) in the presence of LKAW (low-frequency KAW, i.e., frequency very much less then ion cyclotron frequency) perturbation by formulating their dynamical equations in the presence of ponderomotive force and using the numerical results for the same. A simplified model is presented to have a deeper insight into the evolution pattern using the results of simulation. The formation of coherent structures and current sheets using a numerical and semi-analytical approach is elaborated near the magnetic reconnection sites. In addition to this, the relevance of the generated turbulence is also depicted through the energy spectrum by examining the spectral index which is noticeable in determining the energy cascade down to smaller scales.

In the present paper, we used two substantially different approaches to analytical description of the merger of magnetic islands. One approach is based on the system of nonlinear hydrodynamic (MHD) equations, while the other is based on the transport equation for the velocity distribution function of the solar-wind particles. In a standard case, these approaches are based on using a single-fluid variant of plasma in which plasma itself represents the fluid. In this work, we apply both approaches to the two-fluid model in which plasma is treated as a combination of an ion and an electron fluids. Physical processes in the solar-wind plasma lead to additional formation of magnetic islands. These processes play an important role in the vicinity of an interplanetary shock front. Merger of magnetic islands is investigated upon realization of the first approach, while the fraction of the high-energy particles in the solar-wind plasma is calculated upon realization of the second. The number of particles acquiring energy exceeding 1 MeV in the solar wind (including processes of magnetic-island merger) is found. Explicit expressions are derived and quantitative estimates for the main parameters of the solar wind are found in all analyzed particular cases. [ABSTRACT FROM AUTHOR]

Recent accumulation of a critical mass of observational material from different spacecraft complete with the enhanced abilities of numerical methods have led to a boom of studies revealing the high complexity of processes occurring in the heliosphere. Views on the solar wind filling the interplanetary medium have dramatically developed from the beginning of the space era. A 2-D picture of the freely expanding solar corona and non-interacting solar wind structures described as planar or spherically-symmetric objects has dominated for decades. Meanwhile, the scientific community gradually moved to a modern understanding of the importance of the 3-D nature of heliospheric processes and their studies via MHD/kinetic simulations, as well as observations of large-scale flows and streams both in situ and remotely, in white light and/or via interplanetary scintillations. The new 3-D approach has provided an opportunity to understand the dynamics of heliospheric structures and processes that could not even be imagined before within the 2-D paradigm. In this review, we highlight a piece of the puzzle, showing the evolution of views on processes related to current sheets, plasmoids, blobs and flux ropes of various scales and origins in the heliosphere. The first part of the review focuses on introducing these plasma structures, discussing their key properties, and paying special attention to their observations in different space plasmas. [ABSTRACT FROM AUTHOR]

Featured Application: This article describes a physical based criterion to track the slowing down of n = 1 instabilities on the ASDEX upgrade (AUG) Tokamak. It can provide automatically the time instance of the locking of the instability in the frame of reference of the laboratory. Immediate applications are possible both in real time and off-line, for mitigation and avoidance purposes respectively. Disruptions in tokamaks are very often associated with the slowing down of magneto-hydrodynamic (MHD) instabilities and their subsequent locking to the wall. To improve the understanding of the chain of events ending with a disruption, a statistically robust and physically based criterion has been devised to track the slowing down of modes with toroidal mode numbers n = 1 and mostly poloidal mode number m = 2, providing an alternative and earlier detection tool compared to simple threshold based indicators. A database of 370 discharges of axially symmetric divertor experiment—upgrade (AUG) has been studied and results compared with other indicators used in real time. The estimator is based on a weighted average value of the fast Fourier transform of the perturbed radial n = 1 magnetic field, caused by the rotation of the modes. The use of a carrier sinusoidal wave helps alleviating the spurious influence of non-sinusoidal magnetic perturbations induced by other instabilities like Edge localized modes (ELMs). The indicator constitutes a good candidate for further studies including machine learning approaches for mitigation and avoidance since, by deploying it systematically to evaluate the time instance for the expected locking, multi-machine databases can be populated. Furthermore, it can be thought as a contribution to a wider approach to dynamically tracking the chain of events leading to disruptions. [ABSTRACT FROM AUTHOR]

This study presents a comprehensive analytical analysis of line start permanent magnet (LSPM) synchronous motors in both steady‐state and transient domains. The PM flux, the back‐EMF and the winding inductances are first calculated in the steady‐state based on the hybrid solution of magnetic circuit and the magnetic islands. Next, the motor voltage relations are mapped into an arbitrary d‐q reference frame to dynamically assess the transient speed response as well as the individual motor torque components. Based on the presented analytical modelling, the parameters of the motor are optimised via genetic algorithm to maximise the back‐EMF voltage and the overall steady‐state performance. Given the parabolic relation between the back‐EMF and the braking torque, the starting capability of the motor is defined as the optimisation constraint. Finally, the analytical results are verified by using a finite element analysis software package. [ABSTRACT FROM AUTHOR]

Disruptions in tokamaks are very often associated with the slowing down of magneto-hydrodynamic (MHD) instabilities and their subsequent locking to the wall. To improve the understanding of the chain of events ending with a disruption, a statistically robust and physically based criterion has been devised to track the slowing down of modes with toroidal mode numbers n = 1 and mostly poloidal mode number m = 2, providing an alternative and earlier detection tool compared to simple threshold based indicators. A database of 370 discharges of axially symmetric divertor experiment—upgrade (AUG) has been studied and results compared with other indicators used in real time. The estimator is based on a weighted average value of the fast Fourier transform of the perturbed radial n = 1 magnetic field, caused by the rotation of the modes. The use of a carrier sinusoidal wave helps alleviating the spurious influence of non-sinusoidal magnetic perturbations induced by other instabilities like Edge localized modes (ELMs). The indicator constitutes a good candidate for further studies including machine learning approaches for mitigation and avoidance since, by deploying it systematically to evaluate the time instance for the expected locking, multi-machine databases can be populated. Furthermore, it can be thought as a contribution to a wider approach to dynamically tracking the chain of events leading to disruptions.