Theory Weekly Highlights for May 2009

May 29, 2009

In the limit in which the plasma is assumed to behave as a vacuum, the response to external resonant perturbing non-axisymmetric magnetic fields from MARS-F has been shown to have excellent agreement with that computed previously by SURFMN, a Biot-Savart code with realistic coil arrangements. In this comparison, both SURFMN and MARS-F are also shown to agree with an analytic model for the fields produced by the external coils. With the agreement in the vacuum response, MARS-F was then used to compute the plasma response in the ideal plasma limit. The 2D MHD response computed by MARS-F showed that the net ideal plasma plus vacuum response depends strongly on the arrangement of the external perturbing fields.

May 22, 2009

A detailed and complete description of the implementation and application the synthetic beam emission spectroscopy (BES) and correlation electron cyclotron emission (CECE) diagnostics has been published in Physics of Plasmas. This will serve as the primary reference for current and future validation studies that make use of these tools. The paper describes direct, quantitative comparisons of local density and electron temperature fluctuation amplitudes and frequency spectra, as well as density correlation lengths, against experimental measurements for DIII-D discharge #128913 in an unprecedented level of detail. Comparisons of all fluctuation statistics, as well as nonlinear predictions for turbulent energy fluxes, showed good agreement with experiment at r/a = 0.5. Interestingly, good agreement was found in the shapes of the fluctuation spectra and density correlation lengths at r/a = 0.75, despite an under-prediction of amplitudes there. The paper also quantifies the sensitivity of the results to uncertainties in ion temperature gradients and ExB shear, as well as issues of frequency resolution and numerical convergence.

May 15, 2009

A new tool has been developed to facilitate using the Trapped Gyro-Landau Fluid (TGLF) fast linear stability code to perform drift wave stability analysis in the DIII-D control room between discharges. This analysis tool was developed to support ECH modulation experiments and enabled the session leader to monitor how effective the changes to discharge conditions had been in achieving the targeted changes in drift wave turbulence in real time. Instructions for use can be found on the Transport Model Validation task force page at (

May 08, 2009

MHD relaxation events observed during the ramp in the bean and oval sawtooth experiments (see October 03 highlight at were analyzed and shown to be associated with a nonmonotonic q profile near unity and an underlying ideal quasi-interchange mode. Application of the Porcelli model at these times suggests that the quasi-interchange mode is initially unstable even with stabilization from the kinetic thermal and fast ion trapped particles right before the observed onset time but transitions rapidly to a resistive mode during the observed relaxation time. After the event, the axis q0 is below one. A likely hypothesis then is that the relaxation events involve reconnection at the innermost q = 1 surface which restabilizes the modes. Rough calculations indicate that the corresponding reconnection time is about the same as the observed duration of the relaxation event. In that sense, the events are like mini sawteeth but affecting only the small region inside the inner q = 1 surface.

May 01, 2009

In collaboration with PPPL, significant progress has been made in extending the nonlinear but axisymmetric M3D-C1 code to treat linear nonaxisymmetric stability as a tool for studying peeling-ballooning stability and resistive wall modes with finite Larmor radius and two-fluid effects. Recent benchmarks show good agreement with ELITE and GATO for ballooning-mode growth rates in the ideal limit. The capability for initializing M3D-C1 using equilibria from EFIT and TOQ has also been implemented.

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