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The effects of nonresonating hot ions on the spectrum of magnetohydrodynamic (MHD) waves and instabilities in tokamaks are studied in the limit when the width of the hot ion drift orbits is much larger than the radial scale length of the MHD perturbations. Due to the large magnetic drift velocities the hot ions cannot contribute to the MHD perturbations directly, but two main effects of the hot ions, the hot-ion density-dependent effect and the hot-ion pressure-dependent effect, influence the MHD perturbations indirectly. The physics of both effects is elucidated and it is shown that both these effects can be described in MHD approach. A new code, MISHKA-H (MISHKA including the hot-ion indirect effects), is developed as an extension of the ideal MHD code MISHKA-D [Huysmans et al., Phys. Plasmas 8, 4292 (2002)]. Analytical benchmarks for this code are given. Results of the MISHKA-H code on Alfven spectrum in a shear-reversed discharges with ion-cyclotron resonance frequency (ICRF) heating are presented. Mo...

Since the determination of the magnetic field topology and of the safety factor profile during the crash stage of the sawtooth instability is a difficult problem both theoretically and experimentally, a complementary approach based on a mapping technique usually referred to as the Tokamap [Balescu et al., Phys. Rev. E 58, 951 (1998)] is proposed to reconstruct the stochastic magnetic field evolution. It is shown that this method, when combined with the constraints on the magnetic fluxes provided by the theories of the sawtooth instability, is able to generate poloidal cross sections of the magnetic field topology during the crash phase of the instability that have behavior similar to the experimental ones. The method is applied to both the complete and the incomplete reconnection of the magnetic field.

The cycling dynamics of the internal kink mode, which drives sawtooth oscillations in tokamak plasmas, is studied using the three dimensional, non-linear magnetohydrodynamic (MHD) code XTOR-2F [H. Lütjens and J.-F. Luciani, J. Comput. Phys. 229, 8130 (2010)]. It is found that sawtooth cycling, which is characterized by quiescent ramps and fast crashes in the experiment, can be recovered in two-fluid MHD provided that a criterion of diamagnetic stabilization is fulfilled. The simulation results indicate that diamagnetic effects alone may be sufficient to drive sawteeth with complete magnetic reconnection in high temperature Ohmic plasmas.