Session Q15: Sherwood III

Turbulent transport of trapped electron modes in collisionless magnetized plasma

A prominent candidate for the electron heat transport in high temperature toroidal plasmas is collisionless trapped electron mode (CTEM) turbulence. Our large scale simulations of CTEM turbulence using gyrokinetic toroidal code (GTC) finds the electron heat transport exhibiting a gradual transition from Bohm to gyroBohm scaling when the device size is increased. The deviation from the gyroBohm could be induced by large turbulence eddies, turbulence spreading and non-diffusive transport process. In the CTEM simulation, radial correlation function shows that the turbulence eddies are predominantly microscopic but with a significant tail in the mesoscale. The macroscopic, linear streamers are mostly destroyed by the zonal flow shearing, which is confirmed by our comprehensive analysis of kinetic and fluid time scales. The mesoscale streamers result from a dynamic process of radial streamers breaking by zonal flows and merging of microscopic eddies. It is further found that the radial profile of the electron heat conductivity roughly follows the global profile of fluctuation intensity, whereas the ion transport tracks more sensitively local fluctuation intensity. This suggests the existence of a non-diffusive component in the electron heat transport, which arises from the ballistic radial drift of trapped electrons due to a combination of the presence of the mesoscale eddies and the weak detuning of the toroidal precessional resonance.   

Rotational Stabilization of Magnetically Collimated Jets

We investigate the launching and stability of extragalactic jets through nonlinear magnetohydrodynamic (MHD) simulation and linear eigenmode analysis. In the simulations of jet evolution, a small-scale equilibrium magnetic arcade is twisted by a differentially rotating accretion disk. These simulations produce a collimated outflow which is unstable to the current driven $m=1$ kink mode for low rotational velocities of the accretion disk relative to the Alfv\'en speed of the coronal plasma. The growth rate of the kink mode in the jet is shown to be inversely related to the rotation rate of the disk, and the jet is stable for high rotation rates. The effect of rigid rotation on the kink mode in a cylindrical plasma is investigated via linear MHD initial value and eigenvalue calculations. The results from both treatments of the problem are shown to be in agreement. These calculations show that rigid-rotation distorts the $m=1$ kink eigenmode reducing its growth rate and reducing the range of unstable non-resonant wave numbers. We surmise that the change in the linear spectrum explains the undistorted propagation in the nonlinear jet simulations in the large disk rotation regime. By removing individual terms in the momentum equation we show that kink stabilization is due to distortion of the eigenmode via the Coriolis force.   

Driving toroidally asymmetric current through the tokamak scrape-off layer to suppress edge-localised modes

The dangerously high divertor heat fluxes impulsively delivered by edge localized modes (ELMs) can be controlled by non-axisymmetric magnetic perturbations that induce enhanced stellarator-like transport and reduce the edge pressure gradient below the peeling-ballooning MHD stability threshold. Unfortunately, the design of the needed perturbation coils is complicated by engineering constraints in a high field and high neutron-flux environment. We suggest driving the needed perturbation current through the scrape-off layer (SOL) plasma itself. Current densities as large as the ion saturation current density, can be driven if the sheath potential differs from the floating potential by O(T$_{e}$/e). If the sheath potential is made to vary toroidally, a non-axisymmetric surface current is generated in the SOL, and the resulting magnetic perturbation can exceed the ELM suppression criterion, as shown by numerical calculations. The combination of non-axisymmetric SOL current and driven convection cells, which radially spread heat flux in the SOL, may be a powerful technique for solving the problem of high target heat fluxes.   

The destabilising effect of dynamical friction on fast particle-driven waves

The non-linear evolution of waves excited by the resonant interaction with energetic particles is known to depend on relaxation processes that restore the unstable distribution function. With Krook type collisions and velocity space diffusion the wave may exhibit steady-state, amplitude modulation, chaotic and explosive (`hard') regimes near marginal stability. However, our recent analysis surprisingly shows that only the explosive behaviour is possible in the near-threshold nonlinear regime when dynamical friction (drag) is the dominant collisional process in the phase space region surrounding the wave-particle resonance. These results indicate that the nonlinear evolution of, e.g., Alfv\'{e}nic instabilities driven by super-Alfv\'{e}nic neutral beam injection (NBI), or by fusion-born alpha-particles with drag-determined distribution functions should be more prone to the `hard' regime than those driven by ion-cyclotron resonance heating (ICRH) with dominant RF quasi-linear diffusion. The experimentally observed differences between the steady-state, amplitude modulation and chaotic regimes of ICRH-driven TAE instabilities on the Join European Torus (JET) and the bursting frequency-chirping TAEs on MAST are then considered. Possible nonlinear scenarios of Alfv\'{e}nic instabilities driven by fusion-born alpha-particles in ITER are also discussed.