Theory Weekly Highlights for August 2012

August 31, 2012

The NEO code has been used to assess the limitations of commonly used model collision operators for realistic edge-relevant parameters by comparing the neoclassical transport levels predicted by the exact linearized full Fokker-Plank collision operator to those from various model operators in DIII-D L-mode plasmas. Previously for these cases NEO simulations of the deuterium and carbon flows was shown to agree with measurements upon approach to the last closed flux surface. We considered four model collision operators: test particles with an ad hoc field particle operator, the full Hirshman-Sigmar, the zeroth order Hirshman-Sigmar, and the Connor models. For both the high and low density cases, the ad hoc field particle operator and the zeroth order Hirshman-Sigmar are the most accurate for the flows as well as for the bootstrap current, with less than ~10% error up to ρN =0.9 for j and 20 to 30% for the flows. The Connor and zeroth order Hirshman-Sigmar models, which are the two most closely related in that they contain only the Lorentz operator with a simple momentum-restoring term, underestimate j, while the ad hoc field particle and full Hirshman-Sigmar operators, which both contain energy diffusion terms, generally overestimate j. All of the models underestimate the carbon flow further in the edge. A notably large inaccuracy of the full Hirshman-Sigmar model, especially at higher collision frequency closer to the edge, is surprising, although it has been shown to significantly underestimate the ion flow coefficient for pure plasmas at large collisionality. Overall, the results show that using the linearized full Fokker-Planck operator becomes more important further into the edge.

== August 24, 2012 ==
A comparison of a set of four NIMROD simulations of massive gas injection in DIII-D found that the location of the toroidal radiation peak is entirely determined by the toroidal phase of the 1/1 mode, and independent of the location of the toroidally peaked Ne injection source. The simulations compared MHD mixing effects and toroidal radiation peaking for high field side (HFS) versus low field side (LFS) neon injection, as well as toroidally peaked versus toroidally uniform injection. In some cases, the location of the peak is displaced 180 degrees from the injection source. The toroidal peaking factor of the radiated power is dependent on the relationship between the Ne source location and the n=1 mode phase; it is larger when the two are in-phase and smaller when they are out of phase. The mixing efficiency for the injected Ne is dependent on this same phase relationship. The single HFS injection simulation was found to have much weaker particle mixing than the LFS simulations but this appears to be a coincidental result of the n=1 phase relationship to the Ne source in each case, and not intrinsically related to the HFS or LFS injection site. This hypothesis will be tested in further simulations in which the Ne sources are moved 180 degrees toroidally. In the NIMROD simulations, the plasma and the mode do not rotate toroidally. Although MGI does generally reduce plasma rotation significantly, the relevance of these results to experiments will have to be considered further.

== August 17, 2012 ==
A practical quaslinear model for passive energetic particle (EP) diffusion in radial and velocity space has been recently developed. The model is embodied in a new code DEP installed in the GACODE suite with TGLF and TGYRO, GYRO, and NEO. DEP computes the passive energetic diffusivity DEP as a function of energy, pitch angle, and parallel direction (co- or counter-). The passive energetic particle diffusion is driven by ITG and TEM thermal plasma turbulence modeled with TGLF. DEP is a positive definite 2×2 matrix with fluxes driven by the radial and energy gradients (-dfEP/dr and -dfEP/dE) of any general energetic particle distribution function fEP. The off diagonal elements drive fluxes and diagonal elements represent diffusive effects. The DEP code computes a theory-based formula for the quasilinear ratio of [DEP/χi]n, fitted to GYRO simulations, for each ITG/TEM unstable mode n from the linear growth rates obtained from TGLF, which is then convolved over the low-k modes with the TGLF spectral weights [χi n /χi] for the thermal ion energy diffusivity χi to provide the summed [DEP χi]. This can then be multiplied by χiTGLF or χi>EXP to obtain the DEP matrix. The DEP matrix will be connected to NUBEAM to treat the effect of ITG/TEM turbulence on the DIII-D off-axis NBI current drive experiments.

== August 10, 2012 ==
Efitviewer and Efitloader have been updated to use the IDL Virtual Machine (VM) by default and are available on both the Venus and LSF clusters. This change should alleviate recent IDL license shortage problems, as Efitviewer and Efitloader will no longer take up IDL licenses. Efitviewer was also updated to improve portability and recognition of EFIT generated equilibria from multiple experiments.

== August 03, 2012 ==
The EPED-based working model for RMP ELM suppression has been tested on a pair of DIII-D discharges and the results are consistent with the experimental observations of ELM suppression. In the first discharge, the value of q95 is inside the resonant window, and ELMs are fully suppressed. In the second discharge, q95 is outside the resonant window and ELMs remain. The EPED model predicts a critical pedestal width of ~ 3%, above which the ELM is expected to be triggered. In the ELM-suppressed discharge, the pedestal width is constrained below this value (~ 1.9%) and hence no ELMs are expected, as observed. In the discharge with ELMs, the pedestal width reaches the predicted critical width just before the ELM is triggered. These results are consistent with the model, in which a “wall” at some radius, provided by strong RMP transport associated with rational surfaces, blocks the inward expansion of the pedestal, limiting the pedestal width and thereby suppressing the ELMs. Further tests of the working model for RMP-ELM suppression are ongoing.

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