[en] Three aspects of the physics of magnetic islands are investigated. (a) The threshold for the onset of the island in tokamak plasmas. For a sufficiently high temperature plasma like that of ASDEX-Upgrade, the stability of a small island is found to be mainly determined by the electron diamagnetic drift frequency ω*_e and the heat diffusivity, and it can be driven unstable by the electron temperature gradient for a certain range of ω*_e. In the nonlinear stage the saturated island width decreases for sufficiently large ω*_e. (b) The heat diffusion across stochastic magnetic field. With the increase of the ratio between the parallel and the perpendicular heat diffusivity, the enhanced radial heat diffusivity due to the parallel transport along the field lines is found to be determined first by the additive effect of individual islands and then by the field ergodicity. (c) The stabilization of the island by rf current drive. When the rf wave deposition width is larger than the island width, the modulated rf current drive has a stronger stabilizing effect than a non-modulated one. A more effective way for stabilizing the large island is found by using both a rf current drive and a resonant helical field. (author)

[en] The growth of m/n=2/1 tearing mode is studied numerically in a new kind of equilibrium magnetic configuration: a zeroth-order axisymmetric equilibrium field superposed with a small m/n=7/4 static helical field, where m and n are respectively the poloidal and toroidal mode numbers. The amplitude of the magnetic flux perturbation Ψ_2/1 is found to be reduced as the magnitude of the m/n = 7/4 helical field increases. [Ψ_2/1] can be reduced to zero when the m/n=7/4 magnetic island is large enough that it overlaps the q=2 flux surface. Oscillatory [Ψ_2/1] is also excited with the appropriate magnitude of the m/n=7/4 helical field. These results are of practical interest for tokamak reactor design. 12 refs., 2 figs

[en] A theory for the growth of the double tearing mode in the early nonlinear stage is presented. The equations governing the nonlinear evolution of double tearing modes are formulated and are solved in different parameter regimes. When the phase difference between the two islands is π, the coupling of the two islands plays a destabilizing role. The island growth is of the form w∼t in the early nonlinear stage. The instability condition for the nonlinear double tearing mode is found, and is the same as that for the linear double tearing mode. When current profile is flattened near the two rational surfaces, the coupling plays the dominant role in determining the nonlinear growth.copyright 1996 American Institute of Physics

[en] The formation process of negative ions in a flashover diode plasma is investigated. A coaxial magnetically insulated diode, whose cathode can be cooled by liquid nitrogen or helium, is driven with a 5-Ohm/60nsec Blumlein line. Effects of voltage polarity, prepulse level, source materials, and surface structure of the electrode, on the diode characteristics are examined and the potentiality of the plasma as a source of intense negative ion beam is discussed

[en] The structural mechanisms of the hardening of a Ti-2Al-2.5Zr alloy by 75 keV nitrogen ion implantation with doses of 3x10¹⁷ and 8x10¹⁷ N/cm² is investigated. The surface characterization of samples was studied by XPS, XRD and Vickers hardness tests. The results of XPS and XRD analysis show that TiN is formed in the surface region. Nitrogen implantation increases the surface hardness up to 340 and 260% for doses of 8x10¹⁷ and 3x10¹⁷ N/cm², respectively, due to formation of TiN and irradiation-enhanced hardening

[en] The non-linear evolution of the neoclassical double tearing mode (DTM) is investigated by numerical modelling. As predicted by an analytical theory, the perturbed bootstrap current is found to have a stabilizing effect on the inner island but a destabilizing effect on the outer island. The coupled tearing modes are much more unstable than single modes, especially when the two rational surfaces are close (but not so close as to be reduced to a single tearing mode). Differential plasma rotation decouples the modes and stabilizes, therefore, the DTM. However, for large islands the relative phase between the two islands will be locked. The outer island width is found to be an important parameter in determining the inner island width and the stability of the non-monotonic q profile. (author)

[en] The drift-tearing mode stability is studied numerically using the (reduced) two fluid equations. With a set of experimental data as input, the mode is found to be driven unstable by the electron temperature gradient when the classical perpendicular electron heat conductivity is taken into account. In the nonlinear phase two saturation regimes are found, a small magnetic island regime existing for an intermediate electron diamagnetic drift frequency and a large island regime for a sufficiently high bootstrap current fraction.

[en] Based on the earlier work for a single magnetic island and a weak stochastic magnetic field (Yu 2006 Phys. Plasmas 13 062310), the heat diffusion across a local fully stochastic magnetic field is studied numerically and compared with the analytical theories. The ratio between the parallel and the perpendicular heat diffusivity, χ_paraller/χ_{perpendicular}, is found to be an important parameter in characterizing the transport. With the increase in χ_paraller/χ_{perpendicular}, the transport is dominated first by the additive effect of individual islands and then by the field ergodicity and χ_paraller

[en] Three aspects of the physics associated with magnetic islands are investigated. (a) The threshold for the onset of magnetic island in tokamak is studied using the two fluids equations. For a sufficiently high beta plasma like that of ASDEX-Upgrade, the stability of a small island is found to be mainly determined by the electron diamagnetic drift frequency and the perpendicular heat diffusivity, and it can be driven unstable by the electron temperature gradient for a certain range of the diamagnetic drift frequency. With experimental data as the input, the spontaneous growing tearing mode observed on ASDEX-Upgrade is obtained from the numerical simulation. In the nonlinear stage the island decreases the local electron temperature gradient, which in turn leads to the mode saturation at a small amplitude. When including the bootstrap current perturbation, the mode can further develop into a large amplitude. The saturated island width is found to decrease for large diamagnetic drift frequency. (b) The heat diffusion across a local stochastic magnetic field is studied numerically. With the increase of the ratio between the parallel and the perpendicular heat diffusivity, the enhanced radial heat diffusivity due to the parallel transport along the field lines is found to be determined first by the additive effect of individual islands and then by the field ergodicity. A quasi-linear analytical theory are developed, which agrees with the numerical result. (c) Numerical modelling on the stabilization of NTMs by localized RF current drive are carried out. When the RF deposition width is much larger than the island width, the modulated RF current drive to deposit the RF current around the island's o-point is found to have a stronger stabilizing effect than a non-modulated one. A more effective way for stabilizing NTMs is found by using both the RF wave and a resonant helical field. The helical field decreases the island rotation frequency, leading to a longer island rotation period comparing with the slowing down time of the fast electrons and therefore a larger stabilizing effect by the RF current. (author)

[en] A theory for the nonlinear neoclassical double tearing mode is presented. The equations governing the nonlinear evolution of the mode are obtained for both the locked islands and the islands with different rotation frequencies. The coupling of the two islands plays a destabilizing role in the island growth, especially when the phase between the two islands is locked. The perturbed bootstrap current has a strong stabilizing effect on the inner island but a weak destabilizing effect on the outer island. The inner island width is decreased when there is a relative rotation between the two islands, while the outer island is less affected by plasma rotation. The outer island width is found be an important parameter in determining the inner island width and the stability of the nonmonotonic q-profile. copyright 1997 American Institute of Physics