Theory Weekly Highlights for January 2012

January 27, 2012

A simulation of massive gas injection (MGI) in DIII-D with the NIMROD MHD code shows how 2D and 3D flows, particularly those associated with the m=1/n=1 instability, mix impurities from the edge to the core. Even when the neon impurity injection is poloidally uniform, the neon distribution becomes concentrated on the outboard midplane prior to the saturation of the 1/1 mode due to 2D poloidal flow patterns. The 3D flows associated with the 1/1 mode then pull the poloidally localized neon blob into the core, causing a rapid increase in core density. The impurity injection source in NIMROD has been modified to inject impurities into only the high-field- or low-field-side. The pre-thermal-quench phases of these simulations indicate that the timing of the thermal quench trigger does not vary with the poloidal distribution of the impurities. The simulations are being extended through the thermal quench and early current quench phases in order to evaluate the relative efficiency of impurity mixing for the various poloidal injection profiles.

January 20, 2012

A formulation, suitable for computation, was derived to compute the kinetic diffusivity for energetic particles. Kinetic refers to the radial transport as well as velocity space diffusion of energetic particles (EP) as a function of energy, pitch angle, and direction (co- vs counter-current). These radial and velocity space quasilinear and passive diffusivities will be incorporated in a fast particle kinetic transport code like NUBEAM. “Passive” diffusivity assumes no EP driven modes (like TAE/EPM) are present to drive “active” EP diffusion. This can then be used to predict any decrease in NBI current drive efficiency from background ITG/TEM turbulence provided by the TGLF model transport code simulations. TGLF already computes the passive velocity space averaged radial transport diffusivity from a Maxwellian or thermalized EP distribution with a large effective “temperature”. The TGLF thermal diffusivity is used for the quasilinear normalization of the kinetic diffusivities. TGLF may also be able to treat TAE/EPM modes providing active EP diffusion. The first step is to do carry out a transport run from PTRANSP or ONETWO, using TGLF and the velocity space independent TGLF thermal EP radial diffusivity in the NUBEAM simulation of DIIID NBI current drive. This work is part of the GSEP SciDAC project.

January 13, 2012

ReviewPlus has been updated to use the IDL Virtual Machine (VM) by default and is available on both the Venus and LSF clusters. This change should help alleviate recent IDL license shortage problems as ReviewPlus will no longer take up IDL licenses. A large majority of the original ReviewPlus functionality is retained so that most users will see no difference, but there are some limitations, since there is no access to the IDL compiler within the virtual machine.

January 06, 2012

In response to recent fast wave (FW) heating and current drive experiments in DIII-D, the ORBIT-RF code was upgraded to model the simultaneous interaction of fast ions with two fast waves. In the experiments, a synergistic effect was observed when the 6th harmonic 90 MHz FW is applied to the plasma heated by neutral beams and the 4th harmonic 60 MHz FW. The measured fast ion energy ranged from 30 keV to 60 keV for both tangential and vertical components and the fast ion diagnostic array (FIDA) signals showed a significant increase in the vertical component with the addition of the 90 MHz FW, while no increase was found in the tangential component. Using the new version of ORBIT-RF in conjunction with the AORSA code, the simulated FIDA signals obtained from the synthetic diagnostic code FIDASIM for the tangential fast ion component in the observed energy range show no change, in agreement with the experiment. However, those for vertical component show a small decrease, in contrast to the FIDA measurements. Investigation of this discrepancy is underway.

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