Theory Weekly Highlights for August 2014

August 29, 2014

A NIMROD simulation of massive gas injection (MGI) into DIII-D predicts that reversing the toroidal current direction on DIII-D should produce measurable changes in the toroidal radiation asymmetry compared with forward-Ip MGI experiments already performed. The primary physics reason for the expected change is the tendency of the injected gas plume in the simulations to spread preferentially toward the high-field-side (HFS) as it expands parallel to the field lines. In the simulations, this phenomenon is explained in terms of a “magnetic nozzle” effect that accelerates the plume in the direction of converging field lines. Evidence for this predicted asymmetry has not been found in experiments to date, but determining the role the nozzle mechanism plays in MGI plume spreading is important for ITER because the effect should weaken as device size increases. According to the simulations, when Ip is reversed in DIII-D, the toroidal direction corresponding to the preferred poloidal direction will change and the gas distribution relative to the two radiated power diagnostic locations will be altered. This reverse-Ip experiment is scheduled for early September and should be a good initial test of the NIMROD predictions regarding asymmetric plume spreading.

August 22, 2014

NEO, which has recently been extended to include toroidally non-axisymmetric flux surfaces, was used to study the effects of a ripple-like perturbation on neoclassical transport in DIII-D plasmas with an H-mode edge. Unlike previous 3D neoclassical calculations, the NEO 3D simulations include the full, linearized Fokker-Planck collision operator and kinetic electron dynamics. It was found that the ion and electron particle fluxes, energy fluxes, and torque densities scale as the square of the non-axisymmetric magnetic perturbation strength. The implications of this are that the enhancement of the transport for reasonable expected values of ripple (delta B/B0 ~ 10-3) can push the ion particle and energy fluxes from standard neoclassical values to values which are competitive with turbulence levels, ~1-10 gyroBohm units. The effect of the non-axisymmetry on the flows is much smaller. An overall enhancement of the bootstrap current by a few percent is seen due to the increased effect of the non-axisymmetry on the ion flow, which dominates over the more slowly growing magnitude of the electron current density in the opposite direction. This is in contrast with previous analytically based scaling studies which predicted a large (~10%) reduction of the bootstrap current due to ripple, most likely due to their overestimation of the ripple strength.

August 15, 2014

James Penna from MIT has concluded his NUF internship at GA. This summer James developed a module within the OMFIT integrated modeling framework to enable time-dependent evolution of the plasma transport and equilibrium solution in a self-consistent manner. The workflow allows incorporating time-dependent changes to the equilibrium and heating sources, reflective of how an experimentalist would introduce changes during an experiment. The transport solution is calculated within the ONETWO code using a Neural Network transport model that has been recently developed (see Highlight from May 9 2014 at Theory Weekly Highlights for May 2014). The EFIT equilibrium code provides the equilibrium solution. The neural network transport model is computationally very efficient, and 200 milliseconds of plasma evolution can be calculated in less than 10 minutes. Preliminary simulation results showed good agreement with experiment. This work sets the basis for enabling predictive simulation planning of DIII-D shots.

August 08, 2014

Modeling of a DIII-D discharge with a dithering H-mode transition or limit cycle oscillation (LCO) using parallel and toroidal mean field momentum transport equations has been extended to 1-D. The frequency of the LCO and the radial profile of the mean electric field have been successfully matched with a simplified model of the Reynolds stress due to turbulence. The transport coefficients of the model were constrained by data in the L-mode phase. The radial profile of the main ion poloidal velocity predicted by the model agrees qualitatively with published L and H mode data from DIII-D Helium plasmas [J. Kim, PPCF 1994]. These new 1-D results build on the previous 0-D modeling that established that the timescale of the L/H transition and the LCO frequency were consistent with mean field momentum transport.

August 01, 2014

Dr. Gaimin Lu of SWIP, in Chengdu, China, has completed a one-year visit to the General Atomics theory group. During her visit, she worked with members of the theory group and DIII-D experimental team to perform critical gradient and transport stiffness analyses of a dedicated DIII-D H-mode stiffness experiment. The motivation was to look for correlations between the measured value of the critical value for ITG onset, and the value predicted by TGLF transport solutions. The analysis found that the critical gradient for ITG onset is approximately constant with injected power and torque, whereas the measured values increased, albeit somewhat weakly. The predicted gradients, however, generally lie between 40% and 60% of the measured gradients; the predicted critical gradient is generally lower than the measured values, with the larger differences at higher heating levels. The results were presented at the 2014 Sherwood theory meeting, and are being prepared for publication. Further investigation of the implications is continuing.

These highlights are reports of research work in progress and are accordingly subject to change or modification