Nonlinear Generation of Zonal Fields by the Beta-Induced Alfven Eigenmode in Tokamak

The zonal fields effect on the beta-induced Alfven eigenmode(BAE)destabilized by the energetic particles in toroidal plasmas is studied through the gyrokinetic particle simulations.It is found that the localized zonal fields with a negative value around the mode rational surface are generated by the nonlinear BAE.In the weakly driven case,the zonal fields with a strong geodesic acoustic mode(GAM)component have weak effects on the nonlinear BAE evolution.In the strongly driven case,the zonal fields are dominated by a more significant zero frequency component and have stronger effects on the nonlinear BAE evolution.     

Turbulence-Driven Tearing Modes in Inhomogeneous Magnetized Plasma

The effects of density inhomogeneity in the nonlinear growth of tearing modes in the presence of background density, electrostatic and magnetic fluctuations are investigated. The nonlinear interaction is assumed to occur in the inner layer of the tearing mode, where the fields and their variations are greatest. Our results demonstrate that turbulence-generated dissipative effects replace the collisional resistivity as the driving force of the tearing mode. The inhomogeneity introduces a real frequency to the nonlinear tearing mode, as well as new unstable tearing-mode–related drift waves.     

Magnetic-island-induced ion temperature gradient mode: Landau damping,equilibrium magnetic shear and pressure flattening effects

Characteristics of the magnetic-island-induced ion temperature gradient (MITG) mode are studied through gyrofluid simulations in the slab geometry,focusing on the effects of Landau damping,equilibrium magnetic shear (EMS),and pressure flattening.It is shown that the magnetic island may enhance the Landau damping of the system by inducing the radial magnetic field.Moreover,the radial eigenmode numbers of most MITG poloidal harmonics are increased by the magnetic island so that the MITG mode is destabilized in the low EMS regime.In addition,the pressure profile flattening effect inside a magnetic island hardly affects the growth of the whole MITG mode,while it has different local effects near the O-point and the X-point regions.In comparison with the non-zero-order perturbations,only the quasi-linear flattening effect due to the zonal pressure is the effective component to impact the growth rate of the mode.     

New method for rekindling the explosive waves in Maxwellian space plasmas

Our interest is to study the nonlinear wave phenomena in complex plasma constituents with Maxwellian electrons and ions. The main aim is to use a new method known as the (G'/G) method to exhibit the effects of dust charge fluctuations on the evolution of nonlinear waves. The coherent features of the shock and solitary waves along with the generation of high-energy waves have been amplified through the solution of the Korteweg–de Vries–Burgers equation, and the different natures of the waves were found successfully. Results are discussed graphically with the thoughtful choice of typical plasma parameters from different space plasma environments, exactly those found in cosmic dusty plasmas laden in ionospheric auroral region, radial spokes of Saturn's rings, planetary nebulae and solar F-corona region. All conclusions are in good accordance with the actual occurrences and could be of interest to further investigations of space. Moreover, the applicability of the present method is hoped to be a great enhancement by its use as ingenious mechanism used to evaluate the soliton dynamics and Burgers shock waves.     

Effects of sawtooth heat pulses on edge flows and turbulence in a tokamak plasma

Enhancements of edge zonal flows,radial electric fields,and turbulence are observed in electron cyclotron resonance heating-heated plasmas(Zhao et al 2013 Nucl.Fusion 53 083011).In this paper,the effects of sawtooth heat pulses on flows and turbulence are presented.These experiments are performed using multiple Langmuir probe arrays in the edge plasmas of the HL-2A tokamak.The edge zonal flows,radial electric fields,and turbulence are all enhanced by sawteeth.Propagation of the zonal flow and tu...     

Transition from fishbone mode to β-induced Alfvén eigenmode on HL-2A tokamak

In the presence of energetic particles(EPs) from auxiliary heating and burning plasmas, fishbone instability and Alfvén modes can be excited and their transition can take place in certain overlapping regimes. Using the hybrid kinetic-magnetohydrodynamic model in the NIMROD code, we have identified such a transition between the fishbone instability and the β-induced Alfvén eigenmode(BAE) for the NBI heated plasmas on HL-2 A. When the safety factor at magnetic axis is well below one, typical kink-fishbone transition occurs as the EP fraction increases. When q0 is raised to approaching one, the fishbone mode is replaced with BAE for sufficient amount of EPs. When q0 is slightly above one, the toroidicity-induced Alfvén eigenmode dominates at lower EP pressure, whereas BAE dominates at higher EP pressure.     

Free-boundary plasma equilibria with toroidal plasma flows

Magnetohydrodynamic equilibrium schemes with toroidal plasma flows and scrape-off layer are developed for the 'divertor-type' and 'limiter-type' free boundaries in the tokamak cylindrical coordinate. With a toroidal plasma flow, the flux functions are considerably different under the isentropic and isothermal assumptions. The effects of the toroidal flow on the magnetic axis shift are investigated. In a high beta plasma, the magnetic shifts due to the toroidal flow are almost the same for both the isentropic and isothermal cases and are about 0.04a0 (a0 is the minor radius) for M₀ = 0.2 (the toroidal Alfvén Mach number on the magnetic axis). In addition, the X-point is slightly shifted upward by 0.0125a0. But the magnetic axis and the X-point shift due to the toroidal flow may be neglected because M0 is usually less than 0.05 in a real tokamak. The effects of the toroidal flow on the plasma parameters are also investigated. The high toroidal flow shifts the plasma outward due to the centrifugal effect. Temperature profiles are noticeable different because the plasma temperature is a flux function in the isothermal case.     

Flow Shear Stabilization of Ion Temperature Gradient Modes in an Internal Transport Barrier

Ion temperature gradient(ITG) driven instability is studied with gyro-kinetic theory in an internal transport barrier(ITB) of tokamak plasmas.The stabilization effects of a parallel velocity shear on the modes are investigated.It is found that the mode structures and stability properties,as well as the effects of a velocity shear,in an ITB are significantly different from that in off-ITB regions.     

Viscous effects on plasmoid formation from nonlinear resistive tearing growth in a Harris sheet

In this work, the evolution of a highly unstable m = 1 resistive tearing mode, leading to plasmoid formation in a Harris sheet, is studied in the framework of full MHD model using the Non-Ideal Magnetohydrodynamics with Rotation, Open Discussion simulation. Following the initial nonlinear growth of the primary m = 1 island, the X-point develops into a secondary elongated current sheet that eventually breaks into plasmoids. Two distinctive viscous regimes are found for the plasmoid formation and saturation. In the low viscosity regime (i.e. P_r ≲ 1), the plasmoid width increases sharply with viscosity, whereas in the viscosity dominant regime (i.e. P_r ≳ 1), the plasmoid size gradually decreases with viscosity. Such a finding quantifies the role of viscosity in modulating the plasmoid formation process through its effects on the plasma flow and the reconnection itself.     

Some nonlinear problems in transport theory

The integro-differential Boltzmann equation governing the diffusion of the particles of a mixture is formulated by incorporating the nonlinear effects due to the collisions of a same kind of particles between themselves. For the case of - cross sections the integral version of the original Boltzmann equation is derived, and the general features of the associated nonlinear evolution problem are discussed. A simple application of the theory is developed with regard to the problem of the heating of a plasma.     

Anomalous transport driven by ion temperature gradient instability in an anisotropic deuterium-tritium plasma

In this work, the anomalous transport driven by the ion temperature gradient instability is investigated in an anisotropic deuterium-tritium (D-T) plasma. The anisotropic factor α, defined as the ratio of perpendicular temperature to parallel temperature, is introduced to describe the temperature anisotropy in the equilibrium distribution function. The linear dispersion relation in local kinetic limit is derived, and then numerically evaluated to study the dependence of mode frequency on the anisotropic factor α of D and the fraction of T particle ε_T by choosing three sets of typical parameters, denoted as the cyclone base case, ITER and CFETR cases. Based on the linear results, the mixing length model approximation is adopted to analyze the quasi-linear particle and energy fluxes for D and T. It is found that choosing small α and large εT is beneficial for the confinement of particle and energy for D and T. This work may be helpful for the estimation of turbulent transport level in the ITER and CFETR devices.     

Investigation of stimulated growth effect using pulsed cold atmospheric plasma treatment on Ganoderma lucidum

Cold atmospheric plasma (CAP) driven by pulsed high-voltage power has specific advantages in various fields, such as the growth promotion of edible fungi in plasma agriculture. Compared with other strains, Ganoderma lucidum has the significant advantage of high medicinal value, but the shortcomings of low yield, long growth cycle, and an uneven market quality. In this study, Ganoderma lucidum was treated with a CAP powered by a homemade pulsed high-voltage power supply. Three groups of Ganoderma lucidum were treated under different conditions. The stalk lengths and pileus areas of Ganoderma lucidum were recorded to evaluate growth status. Results showed that treatment frequency affected growth status considerably. Ganoderma lucidum was treated with 40 s plasma once a week and showed an improved growth status with a 33.63% increase in average stalk length and a 28% increase in the number of individuals whose pileus areas was greater than the average. Meanwhile, the growth speed was accelerated. However, treatment at an excessive frequency would damage and slow down the growth of Ganoderma lucidum.     

A brief review: experimental investigation of zonal flows and geodesic acoustic modes in fusion plasmas

Zonal flows self-generated by turbulence play an important role in regulating turbulence, reducing transport level, and thus improve plasma confinement in fusion plasmas. The zonal flows and geodesic acoustic modes have been identified in various devices. The related issues, such as the poloidal and toroidal symmetries, coupling to turbulence, effects on turbulence and transport, nonlinear energy transfer between turbulence and zonal flows, dependence of the plasma parameters, roles in the confinement regime transitions etc are overviewed briefly in this paper. The interaction between zonal flows and magnetic islands is emphasized.     

Effects of resonant magnetic perturbations on radial electric fields in DIII-D tokamak

Gyrokinetic simulations of DIII-D tokamak equilibrium find that resonant magnetic perturbation (RMP) drives a neoclassical non-ambipolar electron particle flux, which causes a rapid change of equilibrium radial electric fields consistent with experimental observations during the suppression of the edge localized mode (ELM). The simulation results provide a support for the conjecture that RMP-induced changes of radial electric fields lead to the enhanced turbulent transport at the pedestal top during the ELM suppression (Taimourzadeh et al 2019 Nucl. Fusion 59 046005). Furthermore, gyrokinetic simulations of collisionless damping of zonal flows show that resonant responses to the RMP decrease the residual level of the zonal flows and damp the geodesic acoustic mode.     

Effects of resonant magnetic perturbations on radial electric fields in DⅢ-D tokamak

Gyrokinetic simulations of DⅢ-D tokamak equilibrium find that resonant magnetic perturbation(RMP) drives a neoclassical non-ambipolar electron particle flux,which causes a rapid change of equilibrium radial electric fields consistent with experimental observations during the suppression of the edge localized mode (ELM).The simulation results provide a support for the conjecture that RMP-induced changes of radial electric fields lead to the enhanced turbulent transport at the pedestal top during the ELM suppression (Taimourzadeh et a12019 Nucl.Fusion 59046005).Furthermore,gyrokinetic simulations of collisionless damping of zonal flows show that resonant responses to the RMP decrease the residual level of the zonal flows and damp the geodesic acoustic mode.     

Sawtooth-like oscillations and steady states caused by the m/n = 2/1 double tearing mode

The sawtooth-like oscillations resulting from thedouble tearing mode (DTM) are numerically investigated through the three-dimensional, toroidal, nonlinear resistive-MHD code (CLT). We find that the nonlinear evolution of theDTM can lead to sawtooth-like oscillations, which are similar to those driven by the kink mode. The perpendicular thermal conductivity and the external heating rate can significantly alter the behaviors of the DTM driven sawtooth-like oscillations. With a high perpendicular thermal conductivity, the system quickly evolves into a steady state withmagnetic islands and helical flow. However, with a low perpendicular thermal conductivity, the system tends to exhibit sawtooth-like oscillations. With a sufficiently high or low heating rate, the system exhibits sawtooth-like oscillations, while with an intermediate heating rate, the system quickly evolves into a steady state. At the steady state, there exist the non-axisymmetric magnetic field and strong radial flow, and both are with helicity ofLike the steady state withradial flow, which is beneficial for preventing the helium ash accumulation in the core, the steady state withradial flow might also be a good candidate for the advanced steady state operations in future fusion reactors. We also find that the behaviors of the sawtooth-like oscillations are almost independent of tokamak geometry, which implies that the steady state with saturatedislands might exist in different tokamaks.     

Effects of plasma radiation on the nonlinear evolution of neo-classical tearing modes in tokamak plasmas

The effects of plasma radiation on the nonlinear evolution of neo-classical tearing modes are investigated based on a set of reduced magnetohydrodynamic equations. It is found that the radiation can reduce the pressure near the rational surface. During the nonlinear evolution, the magnitude of perturbed bootstrap current is drastically enhanced in the presence of the radiation. Besides, the radiation can increase the growth rate of the magnetic islands by diminishing the pressure, such that the magnetic islands do not saturate compared with that without radiation. On the other hand, with the increase of the ratio of parallel to perpendicular transport coefficient ${\chi }_{\parallel }/{\chi }_{\perp },$ the reduction of pressure can further increase the growth rate of magnetic islands in the presence of plasma radiation. Finally, the mechanisms of the destabilizing effects driven by the radiation are discussed in detail as well.     

Kinetic Theories of Geodesic Acoustic Modes: Radial Structure,Linear Excitation by Energetic Particles and Nonlinear Saturation

Geodesic Acoustic Modes (GAMs) are oscillating zonal mode structures unique to toroidal plasmas and are capable of regulating microscopic turbulence and associated transports. In this paper, we investigate three important aspects of GAM dynamics: (1) GAM continuous spectrum and its mode conversion to kinetic GAM (KGAM); (2) linear excitation of energetic particle induced GAM (EGAM) and its coupling to the GAM continuum, and (3) nonlinear saturation of EGAM via wave particle trapping. The analogy between the GAM-EGAM dynamics and the well known beam-plasma instability is also discussed.     

Impact of the mass isotope on plasma confinement and transport properties in the HL-2A tokamak

The impact of the mass isotope on plasma confinement and transport properties has been investigated in Ohmically-heated hydrogen and deuterium plasmas in the HL-2A tokamak. Experimental results show that under similar discharge parameters the deuterium plasma has better confinement and lower turbulent transport than the hydrogen one, and concomitantly, it is found that the magnitude of geodesic acoustic mode zonal flows, the tilting angle of the Reynolds stress tensor and the turbulence correlation lengths are all larger in the edge region of the deuterium plasma. The results provide direct experimental evidence on the importance of the nonlinear energy coupling between ambient turbulence and zonal flows for governing the isotope effects in fusion plasmas.     

Tokamak Plasma Flows Induced by Local RF Forces

The tokamak plasma flows induced by the local radio frequency(RF) forces in the core region are analyzed. The effective components of local RF forces are composed of the momentum absorption term and the resonant parallel momentum transport term(i.e. the parallel component of the resonant ponderomotive forces). Different momentum balance relations are employed to calculate the plasma flows depending on different assumptions of momentum transport.With the RF fields solved from RF simulation codes, the toroidal and poloidal flows by these forces under the lower hybrid current drive and the mode conversion ion cyclotron resonance heating on EAST-like plasmas are evaluated.