Session H16: Sherwood II

Gyrokinetic Simulations with External Resonant Magnetic Perturbations: Island Torque and Nonambipolar Transport with Rotation

Static external resonant magnetic perturbations (RMPs) have been added to the $\delta f$ gyrokinetic code GYRO. This allows nonlinear gyrokinetic simulations of the nonambipolar radial current flow $j_r$ and the corresponding plasma torque (density) $R[j_rB_\theta/c]$, induced by islands that break the toroidal symmetry of a tokamak. This extends previous GYRO simulations for the transport of toroidal angular momentum (TAM) [1,2]. The focus is on full torus radial slice electrostatic simulations of induced q=m/n=6/3 islands with widths 5\% of the minor radius. The island torque scales with the radial electric field $E_r$ the island width $w$, and the intensity $I$ of the high-n micro-turbulence, as $wE_rI^{1/2}$. The net island torque is null at zero $E_r$ rather than at zero toroidal rotation. This means that there is a small co-directed magnetic acceleration to the small diamagnetic co-rotation corresponding to the zero $E_r$ which can be called the residual stress [2] from an externally induced island. Finite-beta GYRO simulations of a core radial slice demonstrate island unlocking and the RMP screening.\par \vskip6pt\noindent [1]~R.E. Waltz, et al., Phys. Plasmas {\bf 14}, 122507 (2007).\par\noindent [2]~R.E. Waltz, et al., Phys. Plasmas {\bf 18}, 042504 (2011).   

A Self-Consistent Mechanism for Incomplete Reconnection in Sawteeth

A prevailing impediment to core confinement in fusion devices is the occurrence of large sawtooth events. Experiments show that the crash phase often ends before all available magnetic flux is reconnected, i.e., reconnection is incomplete, but this is inconsistent with the Kadomtsev model. We present a model for incomplete, or partial, reconnection in sawtooth crashes [1]. The reconnection inflow self-consistently convects the high pressure core and low pressure edge of a tokamak toward the m=n=1 rational surface, thereby increasing the pressure gradient at the reconnection site. If the pressure gradient at the rational surface exceeds a threshold, incomplete reconnection will occur. We show that predictions of this model are borne out in large-scale simulations of reconnection. The predictions are also consistent with data from the Mega Ampere Spherical Tokamak. Physically, we attribute the suppression to the interaction of the exterior pressure gradient with the pressure quadrupole that inherently occurs during collisionless (Hall) reconnection with a strong guide-field. The results should apply across tokamaks, including ITER.\\[4pt] [1] M. T. Beidler and P. A. Cassak, Phys. Rev. Lett., 107, 255002 (2011)   

Rotation of tokamak halo currents

Halo currents, which can be tenths of the total plasma current, flow at the plasma edge along the magnetic field lines that intercept the chamber walls. Non-axisymmetric halo currents are required to maintain force balance as the plasma kinks when the edge safety factor drops to about two in a vertical displacement event. The plasma quickly assumes a definite toroidal velocity $v_a(r)$ with respect to the magnetic kink, where $v_a(r)$ is determined by the radial electric field required for ambipolarity. The plasma velocity near the edge is set by interaction with neutrals or by the radial derivative of the electric potential in the halo required for quasi-neutrality on open magnetic field lines, so the magnetic kink tends to rotate. If the magnetic field lines of the halo plasma intercept the wall at locations of very different electrical conductivity, the toroidal rotation of the halo currents can intermittently lock, as seen in experiments, at wall locations of high conductivity though the toroidal velocity of the magnetic kink itself is essentially smooth. A major concern cited by ITER engineers is that the time varying force of the rotating halo could substantially increase the disruption loads on in-vessel components.