[en] In addition to their effect on the linear stability of tearing modes, energetic particles can influence the nonlinear evolution of a magnetic island through an uncompensated cross field current due to the effect of charge separation when the orbit width of an energetic particle is much larger than the island width. The corresponding return parallel current may compensate the loss of bootstrap current in the magnetic island. This nonlinear effect depends on the island's propagation frequency (the rotation frequency of the island relative to the plasma), the density gradient of energetic ions and magnetic shear. If the island's propagation frequency is positive, the effect of the uncompensated current plays a stable role on neoclassical tearing modes. When the magnetic shear is sufficiently small, this effect becomes significant and can partially cancel or even overcome the destabilizing effect of the perturbed bootstrap current. In ITER this provides a possibility of using energetic ions to suppress the neoclassical tearing mode for the steady state and hybrid scenarios with weak weak magnetic shear. (author)

[en] Full text: The effects of trapped energetic ions (TEI) on double tearing modes (DTMs) are studied by hybrid simulation. It is shown that TEI have a stabilizing effect on DTMs for small energetic ion beta. A new energetic particle driven mode is found when energetic ion beta larger than a threshold. This mode is an ideal mode, which is a fishbone-like mode. The threshold increases with resistivity, and the resistivity tends to reduce the growth rate. The dependence of TEI effects on energetic ion beta, gyro-radius and speed is studied systematically. It suggests that a fishbone-like mode will be triggered with a reversed shear q profile. (author)

[en] The dissipation mechanisms of reconnection and the pressure gradient effects on tearing mode with guide magnetic field are analyzed systematically by including the electron pressure tensor in electron magnetohydrodynamics. It is found that which dissipation mechanism dominates, either pressure-based dissipation or inertia-based dissipation, has a great relation with the relative scaling orders between the electron thermal Larmor radius and electron inertia skin depth. The effects of pressure gradient also depend on the relative magnitude between parallel and perpendicular equilibrium pressure gradients. When the pressure-based dissipation is dominant, the condition that pressure drives or suppresses tearing mode instability also depends on the relative magnitude between parallel and perpendicular equilibrium pressure gradients.

[en] The general time evolution of tearing mode due to electron viscosity μ_e from linear to nonlinear phase in the framework of electron magnetohydrodynamics (EMHD) is derived by quasilinear theory. Tearing mode grows exponentially in the linear phase and slows to an algebraic growth behavior Ψ₁∝(μ_et)^2/3 (Ψ₁ is the magnetic flux) in the nonlinear phase. These two phases are separated by a transition during which the growth behavior is much different from that in the linear and nonlinear phases. It behaves as Ψ₁∝μ_e^5/8t^1/2 approximately.

[en] The general dispersion relation of collisionless reconnection instability due to electron viscosity μ_e in the whistler frequency is derived. In the framework of electron magnetohydrodynamics (EMHD), the evolution of magnetic reconnection instability is studied, and the linear growth rates are obtained. The scaling laws of the reconnection instability growth rate with respect to the electron viscosity in constant-ψ (used in the tearing mode) and low-k regimes are obtained, respectively, and compare with those obtained in standard magnetohydrodynamic theory. In the constant-ψ regime for 'tearing-mode-like' instability, the growth rate is proportional to μ_e^1/4, while in the low-k regime, it is proportional to μ_e^1/8

[en] The general dispersion relation of tearing mode with pressure gradient effect in pair plasmas is derived analytically. If the pressure gradients of positron and electron are not identical in pair plasmas, the pressure gradient has significant influence at tearing mode in both collisionless and collisional regimes. In collisionless regime, the effects of pressure gradient depend on its magnitude. For small pressure gradient, the growth rate of tearing mode is enhanced by pressure gradient. For large pressure gradient, the growth rate is reduced by pressure gradient. The tearing mode can even be stabilized if pressure gradient is large enough. In collisional regime, the growth rate of tearing mode is reduced by the pressure gradient. While the positron and electron have equal pressure gradient, tearing mode is not affected by pressure gradient in pair plasmas.

[en] The evolution of neoclassical tearing modes (NTMs) in the presence of electrostatic drift wave turbulence is investigated. In contrast with anomalous transport effect induced by turbulence on NTMs, a new mechanism that turbulence-driven current can affect the onset threshold of NTMs significantly is suggested. Turbulence acts as a source or sink to exchange energy with NTMs. The turbulence-driven current can change the parallel current in magnetic islands and affect the evolution of NTMs, depending on the direction of turbulence intensity gradient. When the turbulence intensity gradient is negative, the turbulence-driven current enhances the onset threshold of NTMs. When the turbulence intensity gradient is positive, it can reduce or even overcome the stabilizing effect of neoclassical polarization current, leading to a small onset threshold of NTMs. This implies that NTMs can appear without noticeable magnetohydrodynamics (MHD) events. (paper)

[en] The general dispersion relation of the tearing mode with charge separation and pressure gradient effects in the whistler frequency is analytically derived in the framework of electron magnetohydrodynamics (EMHD). It is shown that pressure gradient effect enhances the growth rate, and makes the EMHD tearing mode drift. The growth rate of the EMHD tearing mode is significantly affected by the pressure gradient effect in the large pressure gradient limit. Furthermore, in this limit, the growth rate in the compressible EMHD fluid is much different from that in the incompressible EMHD fluid

[en] The general dispersion of tearing modes due to the effects of electron inertia and resistivity in pair plasmas is derived analytically, and is discussed in two cases: ${△<!--\; ???\; -->}^{\mathrm{\prime <!--\; ???\; -->}}\gg <!--\; ???\; -->1$ and ${△<!--\; ???\; -->}^{\mathrm{\prime <!--\; ???\; -->}}\ll <!--\; ???\; -->1$, where ${\mathrm{\Delta <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ is the instability criterion of the tearing mode. It is found that the conditions under which either resistivity or electron inertia dominates depend strongly on the limit of ${\mathrm{\Delta <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ considered. (paper)

[en] In this paper, a new method to derive the Fokker-Planck coefficients defined by a non-Maxwellian velocity distribution function for the field particles is presented. The threefold integral and the new Debye cutoff parameter, which were introduced by CHANG and LI, are applied. Therefore, divergence difficulties and the customary replacement of relative velocity g by thermal velocity v_th are naturally avoided. The probability function P(v,Δv) for non-Maxwellian scattering is derived by the method of choosing velocity transfer Δv, which is a true measure of collision intensity, as an independent variable. The method enables the difference between small-angle scattering and small-momentum-transfer collisions of the inverse-square force to be well clarified. With the help of the probability function, the Fokker-Planck coefficients are obtained by a normal original Fokker-Planck approach. The friction and diffusion coefficients of the Fokker-Planck equation are modified for non-Maxwellian scattering and are used to investigate the relaxation processes for the weakly coupled plasma. The profiles of the relaxation rates show that the slowing down and deflection processes are weakened in the conditions of non-Maxwellian scattering. (basic plasma phenomena)