Theory Weekly Highlights for March 2007

March 30, 2007

Development and validation of a predictive sawtooth model is an important research topic for present day tokamaks and ITER. Analysis using ONETWO to model the changes in sawtooth behavior in DIII-D discharges due to the interactions between FW and NBI fast ions after FW heating is applied, predicts the observed reduction in the axis safety factor q0 due to the current profile evolution. A preliminary analysis indicates that the predicted drops in q0 within a sawtooth cycle follow closely the experimental values reconstructed from the EFIT code using MSE data. In the ONETWO simulations, the evolution of q0 within a sawtooth period is modeled with neoclassical resistivity and the experimental density and temperature profiles in two neighboring giant sawtooth cycles. So far, most of the analyses are limited to a single sawtooth cycle. Initial analysis using the Kadomtsev sawtooth mixing model indicates that the sawtooth crash can be qualitatively reproduced with an appropriately chosen triggering parameter. The Porcelli sawtooth model is being implemented into ONETWO to more comprehensively simulate the sawtooth crash.

March 23, 2007

In an extensive set of code benchmarking exercises, excellent agreement was found between ELITE, GATO and DCON, to better than 5% in the onset threshold for edge-localized modes when the equilibria are sufficiently well resolved. MHD stability codes are widely used to study tokamak stability, and recently have been used to study onset conditions for edge-localized modes (ELMs) and the resulting constraints on the height of the edge barrier (or pedestal). The benchmarking calculations considered both the growth rates of instabilities, as well as the onset thresholds for edge-localized modes. For under-resolved equilibria, the results naturally vary more due to differences between codes in the mapping onto high resolution grids. The resolution commonly used in experimental studies (nr x nz = 129×129 grid points from EFIT) is generally sufficient, allowing a difference between stability results that is substantially smaller than the experimental uncertainty. However, significant improvement in agreement between the codes is obtained by using higher resolution (257×257 or 513×513) equilibria. Results will be shown at the Edge Coordinating Committee code verification session at next month's TTF meeting.

March 16, 2007

Analytical expressions of the critical wave field amplitude for stochastic motion of particles in phase space in the presence of Ion Cyclotron Range of Frequency (ICRF) waves have been generalized using standard mapping theory. The expressions include finite k and ion Finite Larmor Radius (FLR) effects. In the limit of k = 0 and no FLR effect, previous results, which are only valid for the fundamental harmonic interaction, are reproduced. Results using the generalized formulas show that the FLR effect significantly affects the critical electric field at high harmonic resonances. The required field for the onset of stochastic motion turns out to be significantly higher when compared to that with no FLR. This new result qualitatively agrees with numerical studies by ORBIT-RF. Further study is underway for more quantitative evaluation.

Valerie Izzo attended the 7th Symposium—Current Trends in International Fusion Research: A Review, in Washington, DC, where she gave a talk on MHD simulations of gas jet disruption mitigation. The meeting covered a range of tokamak and innovative confinement topics and included participation from both US and international scientists (from China, Japan, India and Europe). Observers were present from the US Government Accountability Office (GAO), which is presently conducting an audit of the DOE OFES. On Monday, March 12, Dr. Izzo presented a seminar at the University of Wisconsin, Madison, on the same topic.

March 09, 2007

In collaboration with the UCSD Dept. of Mechanical and Aerospace Engineering and the University of Wisconsin, Madison, a synthetic beam emission spectroscopy (BES) diagnostic has been developed for use with GYRO. The synthetic BES diagnostic convolves spatial transfer functions calculated for the upgraded BES system deployed on DIII-D, which model both light collection volumes and beam “smearing” effects due to finite excitation lifetimes with GYRO output to generate a series of time traces corresponding to the expected BES channel signals. Initial results indicate that the synthetic BES channels are well correlated with the corresponding GYRO time traces, with RMS fluctuation levels are reduced by approximately 25% - 33% due to the spatial averaging. The results also indicate that there may be significant spatial overlap in channels, leading to an increase in the observed correlation lengths if not corrected for via deconvolution of the transfer function response. Comparisons between the synthetic diagnostic and experimental BES measurements of core L-mode fluctuations are planned for presentation at the upcoming TTF and Sherwood meetings.

March 02, 2007

In DIII-D experiments near the end of the last run period, the ONETWO transport code was successfully launched in analysis mode automatically between shots. In analysis mode, the external sources (RF and neutral beam) are determined using the geometry from the fitted equilibria at each time slice. This provides the instantaneous snapshot information needed for control room analysis. For the shots tested, each containing on order of 200 time slices, the transport analyses completed in two minutes or less on the 6 new nodes of the STAR cluster. This marks the first time at DIII-D that a 1 1/2D transport code has been run between shots for an entire discharge in this mode.

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