No abstract

No abstract

Thermal energy confinement times in National Spherical Torus Experiment (NSTX) dimensionless parameter scans increase with decreasing collisionality. While ion thermal transport is neoclassical, the source of anomalous electron thermal transport in these discharges remains unclear, leading to considerable uncertainty when extrapolating to future spherical tokamak (ST) devices at much lower collisionality. Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX show they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the transport ($98%), perturbed field line trajectories are globally stochastic, and a test particle stochastic transport model agrees to within 25% of the simulated transport. Most significantly, microtearing transport is predicted to increase with electron collisionality, consistent with the observed NSTX confinement scaling. While this suggests microtearing modes may be the source of electron thermal transport, the predictions are also very sensitive to electron temperature gradient, indicating the scaling of the instability threshold is important. In addition, microtearing turbulence is susceptible to suppression via sheared E Â B flows as experimental values of E Â B shear (comparable to the linear growth rates) dramatically reduce the transport below experimental values. Refinements in numerical resolution and physics model assumptions are expected to minimize the apparent discrepancy. In cases where the predicted transport is strong, calculations suggest that a proposed polarimetry diagnostic may be sensitive to the magnetic perturbations associated with the unique structure of microtearing turbulence. V C 2012 American Institute of Physics. [http://dx.

Recent advances in GYRO allow simulations to map out the linear stability of many eigenvalues and eigenvectors of the gyrokinetic equation (as opposed to only the most unstable) at low computational cost. In this work, GYRO's new linear capabilities are applied to a pressure scan about the pedestal region of DIII-D shot 131997. MHD calculations in the infinite-n limit of the ideal ballooning mode, used in the very successful EPED model to predict pedestal height and width, demonstrate clear onset of the instability at 70% of the experimental pressure. Presented GYRO results first demonstrate that the ion temperature gradient driven mode and microtearing mode are dominant at the top of the pedestal, while an unnamed group of drift waves are found to be most unstable in the peak gradient region of the pedestal. The peak gradient modes have very extended ballooning structure, peak near the inboard midplane and have drift frequencies at or near the electron diamagnetic drift direction, even for very low wavenumbers (k θ ρ s ∼ 0.2). Connection is made to the MHD calculations by demonstrating the kinetic ballooning mode (KBM) is present but subdominant in the DIII-D pedestal, and the pressure required for onset of the KBM in the gyrokinetic limit is in near agreement with MHD predictions. Finally, comparisons and analysis of GYRO with two independent gyrokinetic codes, GEM (initial value) and HD7 (1D eigenvalue), are presented.

No abstract