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A tokamak scenario with q(0) close to 1 has been achieved on HL-2A tokamak, which is an internal transport barrier (ITB) at low central shear or a steady-state ITB combined with an H-mode edge barrier. In this scenario, the formation of an ITB with a steep ion temperature profile is observed to be closely linked to the q = 1 magnetic surface and magnetohydrodynamic (MHD) activities around it, such as long-lived mode (LLM) or fishbone activities. Experimental evidence and simulation analysis suggest that the fishbone activities can induce a poloidal flow, which is beneficial for the suppression of turbulence in the plasma core region. Furthermore, an application of central electron cyclotron resonance heating (ECRH) to such beam-heated weak shear ITB discharges leads to a substantial effect on central MHD stability, converting the LLM into fishbone activity and hence enhancing the strength of the ITB with a much stronger gradient. Moreover, ITBs in combination with an H-mode barrier were achieved for 10 confinement times with βN = 2.0. [ABSTRACT FROM AUTHOR]

Impurity effects on trapped electron modes (TEMs) in tokamak plasmas with inverted electron density profile (IEDP) are numerically investigated with a gyrokinetic integral eigenmode equation. It is found that different from the negative gradient of normal electron density profile, the positive gradient of the IEDP has a stabilizing effect on TEM in the presence of impurity ions. The electron temperature gradient threshold for TEM excitation increases not only with the increasing absolute value of IEDP but also with increasing impurity content. Furthermore, the effects of different impurity species and different impurity peaking profiles on TEMs with the IEDP are analyzed in detail. It is shown that there is a transition point of impurity density profile, on both sides of which the impurity has opposite effects on TEM. The dependence of such a transition point on electron temperature and density gradients is obtained numerically. Besides, the synergistic effects of ion temperature gradient and impurity density gradient are studied, in which a similar transition point of the ion temperature gradient is also identified in the case of outwardly peaked impurity density profile. In addition, impurity effects on the characteristics of mode structure and on the radial transport coefficients in positive and negative magnetic shear regions are discussed as well based on quasi-linear mixing length estimation. [ABSTRACT FROM AUTHOR]

The explosive behavior of neo-classical double tearing mode (NDTM) is numerically investigated by a reduced four-field-drift-magnetohydrodynamic code [Ye et al., Nuclear Fusion 59, 096044 (2019)] with the bootstrap current effect. It is numerically observed that the explosive burst of NDTM can be absolutely suppressed by diamagnetic drift flow. The dependence of the critical value of diamagnetic drift flow V 0 * c for avoiding the explosion on typical plasma parameters is numerically studied in detail. It is found that V 0 * c decreases with increasing Δ r s , while it increases with an increasing bootstrap current fraction. The plasma viscosity and resistivity can raise the threshold through their effects on the rotation of magnetic islands and the instability of system, respectively. Furthermore, the perpendicular transport and parallel transport, which are significant to the driven effects of the bootstrap current, have the opposite effects on V 0 * c . The underlying mechanisms are discussed. [ABSTRACT FROM AUTHOR]

By the gyrokinetic integral eigenmode equation, we numerically investigate the short-wavelength ion-temperature-gradient (SWITG) mode in tokamak plasmas with hollow density profiles. This paper finds that the critical ion temperature gradient (ITG) R / L TiC exists for the hollow density profile. Over such a gradient, the SWITG mode is unstable. The R / L TiC for a hollow density profile is slightly lower than that for a peaked density profile, though far away from the threshold, the mode in the latter case is more unstable. Furthermore, the scaling of the ITG threshold increasing with ion-to-electron temperature ratio T i / T e has been calculated, suggesting that the SWITG mode is more difficult to excite in hot ion plasmas as compared with that in hot electron plasmas. For the slightly hollow density profiles, it is shown that in the case of flat (peaked) electron temperature profiles, the SWITG modes are destabilized (stabilized) by the trapped electrons. For the steep hollow density profiles, however, the trapped electrons reduce the growth rate substantially, and thus, the SWITG stability threshold is raised significantly. In addition, the dependence of the threshold on local plasma parameters is analyzed in detail. It is found that the SWITG modes become more unstable for the high safety factor, small Larmor radius, weak magnetic shear, and high electron-to-ion temperature ratio. [ABSTRACT FROM AUTHOR]

Effects of sub-Alfvénic shear flows on the complicated properties of three types of unstable eigenmodes, namely, coupled triple tearing mode (M1), coupled double tearing mode (M2), and three single tearing modes (M3), in a triple current sheet configuration are studied by means of an eigenvalue analysis. It is identified that the difference of velocity among three resonant surfaces rather than the local flow shear rate plays the more important role in the three types of eigenmodes. Thus, it is found that for the symmetric shear flow (SSF), the growth rate of the M1/M3 mode decreases/increases with the increasing strength of the shear flow, while the growth rate of the M2 mode is almost uninfluenced by SSF. For the antisymmetric shear flow (ASF), it is interestingly observed that in the small wavenumber regime, the M1 and M2 modes degenerate into similar eigenmode states with the same growth rate and opposite traveling frequencies. In the large wavenumber regime, the M2 and M3 modes degenerate into similar eigenmode states. Moreover, for both SSF and ASF, the growth rates of the three types of modes are almost the same in the small and large wavenumber limits. [ABSTRACT FROM AUTHOR]

The linear properties of both even and odd double tearing modes (DTMs) in the presence of plasma shear flow are studied based on a reduced resistive MHD model in slab geometry. It is found that for the anti-symmetric shear flow, the linear growth rates γ (k y) of the even (odd) eigenmode of DTMs decrease (increase) with increasing the strength of shear flow. Indeed, in the small wavenumber k y regime, γ (k y) of the even eigenmode is larger than that of the odd eigenmode, while γ (k y) of two kinds of eigen states coalesce with each other (the same growth rate and opposite frequencies) when the wavenumber k y exceeds a critical value k y C . It is demonstrated that k y C decreases with decreasing η for a fixed separation between two resonant surfaces x s , while decreasing x s increases the critical value of k y C for a fixed shear velocity. In the nonlinear regime for a low value of resistivity, it is observed that by increasing the strength of anti-symmetric shear flow, in the early phase of the abrupt regime a monster plasmoid and subsidary small plasmoids are generated, and later they are ejected from the DTM current sheet and merged with the main DTM islands. [ABSTRACT FROM AUTHOR]

We perform a numerical study of linear stability of the ion temperature gradient (ITG) mode and the trapped electron mode (TEM) in tokamak plasmas with inverted density profiles. A local gyrokinetic integral equation is applied for this study. From comprehensive parametric scans, we obtain stability diagrams for ITG modes and TEMs in terms of density and temperature gradient scale lengths. The results show that, for the inverted density profile, there exists a normalized threshold temperature gradient above which the ITG mode and the TEM are either separately or simultaneously unstable. The instability threshold of the TEM for the inverted density profile is substantially different from that for normal and flat density profiles. In addition, deviations are found on the ITG threshold from an early analytic theory in sheared slab geometry with the adiabatic electron response [T. S. Hahm and W. M. Tang, Phys. Fluids B 1, 1185 (1989)]. A possible implication of this work on particle transport in pellet fueled tokamak plasmas is discussed. [ABSTRACT FROM AUTHOR]

Nonlinear multi-scale interactions between the tearing mode and the ion temperature gradient (ITG) mode are investigated by means of numerical simulations in a self-consistent 5-field Landau-fluid model. It is observed that there exists a threshold of magnetic island width in the nonlinear evolution of interaction, above which the ITG turbulence can enhance the island growth significantly. Dependence of the threshold on basic plasma parameters is deeply analyzed. It is found that the higher ion viscosity may raise the threshold through its effect on the E×B drift and the diamagnetic drift of electron density gradient in different ways, both of which play a synergetic role in determining the threshold. Moreover, the effects of plasma resistivity, gradient length of equilibrium current sheet as well as magnetic shear of field line on the threshold are discussed based on the analyses of the initial growth rate of islands. [ABSTRACT FROM AUTHOR]

An electromagnetic particle simulation model has been formulated and verified for nonlinear processes of lower hybrid (LH) waves in fusion plasmas. Electron dynamics are described by the drift kinetic equation using either kinetic momentum or canonical momentum. Ion dynamics are treated as the fluid system or by the Vlasov equation. Compressible magnetic perturbation is retained to simulate both the fast and slow LH waves. Numerical properties are greatly improved by using the electron continuity equation to enforce the consistency between electrostatic potential and vector potential, and by using the importance sampling scheme. The simulation model has been implemented in the gyrokinetic toroidal code (GTC), and verified for the dispersion relation and nonlinear particle trapping of the electromagnetic LH waves. [ABSTRACT FROM AUTHOR]

Turbulence induced toroidal momentum transport in boundary plasma is investigated in H-mode discharge using Langmuir-Mach probes on EAST. The Reynolds stress is found to drive an inward toroidal momentum transport, while the outflow of particles convects the toroidal momentum outwards in the edge plasma. The Reynolds stress driven momentum transport dominates over the passive momentum transport carried by particle flux, which potentially provides a momentum source for the edge plasma. The outflow of particles delivers a momentum flux into the scrape-off layer (SOL) region, contributing as a momentum source for the SOL flows. At the L-H transitions, the outward momentum transport suddenly decreases due to the suppression of edge turbulence and associated particle transport. The SOL flows start to decelerate as plasma entering into H-mode. The contributions from turbulent Reynolds stress and particle transport for the toroidal momentum transport are identified. These results shed lights on the understanding of edge plasma accelerating at L-H transitions. [ABSTRACT FROM AUTHOR]

Kinetic effects of both trapped thermal and energetic particles on the resistive wall mode (RWM) and on the fishbone-like mode (FLM) are investigated in theory. Here, the trapped thermal particles include both ions and electrons. The FLM is driven by trapped energetic particles. The results demonstrate that thermal particle collisions can either stabilize or destabilize the RWM, depending on the energetic particle pressure βh. Furthermore, the critical value of βh for triggering the FLM is increased when the thermal particle contribution is taken into account. The critical value sensitively depends on the plasma collision frequency. In addition, the plasma inertia is found to have a negligible influence on the FLM. [ABSTRACT FROM AUTHOR]

Linear and nonlinear evolution of ion-temperature-gradient (ITG) modes is numerically investigated by using a Landau-fluid model in a sheared slab geometry. Dependence of the linear growth rates of ITG modes with different radial mode numbers on various equilibrium parameters is discussed in detail. It is found that the ITG mode with a higher radial mode number is the dominantly unstable one in a narrow current sheet system with a low magnetic shear. In nonlinear simulations with different magnetic shears, two types of nonlinear states, specifically, oscillatory relaxation to a steady state and an intermittent bursting state, are observed in the weak turbulence regime. Similar phenomena of both nonlinear states are also observed in the simulations with different temperature gradients. The basic physical processes of both nonlinear states are thoroughly analyzed by means of quasilinear simulations. It is found that the system relaxes to a steady state when the ITG mode is slightly unstable, whereas it enters an intermittent burst state when the ITG mode is more unstable. During each burst, the ITG mode with a higher radial mode number becomes the dominant one due to the energy cascade in kx space, which is induced by self-generated zonal flows. [ABSTRACT FROM AUTHOR]

The linear behavior of plasmoid instability in double current sheet configurations, namely, double plasmoid mode (DPM), is analytically and numerically investigated within the framework of a reduced magnetohydrodynamic model. Analytical analysis shows that if the separation of double current sheets is sufficiently small [ κxs«κ2/9S1/3L], the growth rate of DPMs scales as κ2/3S0L in the non-constant-ψregime, where κ=kLCS/2 is the wave vector measured by the half length of the system LCS/2, 2s is the separation between two resonant surfaces, and SL=LCSVA/2κ is Lundquist number with VA and η being Alfven velocity and resistivity, respectively. If the separation is very large [κxs»κ2/9S1/3L], the growth rate scales as κ-2/5S2/5L in the constant-ψ regime. Furthermore, it is also analytically found that the maximum wave number scales as xs-9/7sS3/7L at the transition position between these two regimes, and the corresponding maximum growth rate scales as xs-6/7SL-2/7there. The analytically predicted scalings are verified in some limits through direct numerical calculations. [ABSTRACT FROM AUTHOR]

Effects of poloidal rotation profile on tearing and Kelvin-Helmholtz (KH) instabilities in a cylindrical plasma are investigated by using a reduced magnetohydrodynamic model. Since the poloidal rotation has different effects on the tearing and KH modes in different rotation regimes, four unstable domains are numerically identified, i.e., the destabilized tearing mode domain, stabilized tearing mode domain, stable-window domain, and unstable KH mode domain. It is also found that when the rotation layer is in the outer region of the rational surface, the stabilizing role of the rotation can be enhanced so significantly that the stable window domain is enlarged. Moreover, Alfven resonances can be induced by the tearing and KH modes in such rotating plasmas. Radially wide profiles of current and vorticity perturbations can be formed when multiple current sheets on different resonance positions are coupled together. [ABSTRACT FROM AUTHOR]

Error-field penetration in reversed magnetic shear (RMS) configurations is numerically investigated by using a two-dimensional resistive magnetohydrodynamic model in slab geometry. To explore different dynamic processes in locked modes, three equilibrium states are adopted. Stable, marginal, and unstable current profiles for double tearing modes are designed by varying the current intensity between two resonant surfaces separated by a certain distance. Further, the dynamic characteristics of locked modes in the three RMS states are identified, and the relevant physics mechanisms are elucidated. The scaling behavior of critical perturbation value with initial plasma velocity is numerically obtained, which obeys previously established relevant analytical theory in the viscoresistive regime. [ABSTRACT FROM AUTHOR]

Stabilization of ion temperature gradient (ITG) driven mode by a large-scale vortex flow is investigated using a gyrofluid model in slab geometry. Extending a recent work [Z. X. Wang et al., Phys. Rev. Lett. 103, 015004 (2009)], the focus in this work is put on the effect of magnetic shear. It is shown that the vortex flow can effectively stabilize the ITG mode by inducing both radial and poloidal mode couplings. Furthermore, decreasing magnetic shear is identified to weaken the stabilizing role of the vortex flow. The effects of the magnetic shear on the ITG mode structure and on the growth rate in the presence of various shear flows are obtained and the relevant mechanisms are discussed in detail. [ABSTRACT FROM AUTHOR]

Characteristics of ion temperature gradient (ITG) instability in the presence of a magnetic island are investigated numerically using a gyrofluid model. It is shown that when the magnetic island is wide enough to produce a broad distribution of rational surfaces near the O-point region, the ITG perturbations at these rational surfaces form a radially global-type eigenmode with a fast growth rate, which is referred to as the magnetic-island-induced ITG mode. Moreover, the magnetic island also causes both radial and poloidal mode couplings, which play a stabilizing role. [ABSTRACT FROM AUTHOR]

The resistive wall mode (RWM) instability is numerically investigated for a toroidal resistive plasma, with results compared to those of an analytic cylindrical model. The full toroidal code MARS-F [Liu et al., Phys. Plasmas 7, 3681 (2000)] is applied for a computational study. The results indicate that there are two branches of unstable RWMs, when the toroidal favorable average curvature effect (the GGJ effect) is taken into account in the resistive layer. In addition, the GGJ physics not only directly affects the mode growth rate, but also indirectly modifies the mode stability by changing the continuum damping through modifying the mode frequency in the plasma frame. Furthermore, the plasma resistivity can either stabilize or destabilize the RWM, depending on the regime of key plasma parameters (e.g., the plasma rotation). Similarly, the plasma rotation can stabilize or destabilize the RWM, depending on the plasma resistivity. These numerical results from MARS-F are qualitatively confirmed by an analytic theory model which includes the GGJ effect. [ABSTRACT FROM AUTHOR]

Radio frequency (RF) waves can provide heating, current and flow drive, as well as instability control for steady state operations of fusion experiments. A particle simulation model has been developed in this work to provide a first-principles tool for studying the RF nonlinear interactions with plasmas. In this model, ions are considered as fully kinetic particles using the Vlasov equation and electrons are treated as guiding centers using the drift kinetic equation. This model has been implemented in a global gyrokinetic toroidal code using real electron-to-ion mass ratio. To verify the model, linear simulations of ion plasma oscillation, ion Bernstein wave, and lower hybrid wave are carried out in cylindrical geometry and found to agree well with analytic predictions. [ABSTRACT FROM AUTHOR]

The beta-induced Alfvén eigenmode (BAE) excited by energetic particles in toroidal plasmas is studied in the global gyrokinetic simulations. It is found that the nonlinear BAE dynamics depends on the deviation from the marginality. In the strongly driven case, the mode exhibits a bursting state with fast and repetitive chirping. The nonlinear saturation is determined by the thermal ion nonlinearity and has no clear dependence on the linear growth rate. In the weakly driven case, the mode reaches a nearly steady state with small frequency chirping. The nonlinear dynamics is dominated by the energetic particle nonlinearity. In both cases, the nonlinear intensity oscillation and frequency chirping are correlated with the evolution of the coherent structures in the energetic particle phase space. Due to the radial variation of the mode amplitude and the radially asymmetric guiding center dynamics, the wave-particle interaction in the toroidal geometry is much more complex than the conventional one-dimensional wave-particle interaction paradigm. [ABSTRACT FROM AUTHOR]