In previous analyses of a DIII-D discharge where successive decaying, and ultimately destabilized, 3/2 modes are triggered by a series of saweeth, the decay/growth rates of each 3/2 island qualitatively correlated with changes in Δ' calculated from PEST-III. New detailed analyses have made this correlation quantitative in both the decay rates of the 3/2 islands and the onset time of the final unstable island. Taking the computed linear Δ' values from reconstructed equilibria at eight time slices, plus the calculated neoclassical terms and the measured island widths w at the same time slices, the dw/dt values were calculated from the Modified Rutherford Equation (MRE). These were compared directly to the dw/dt values inferred from the measured island decay rates. The resulting growth and decay rates agree very well, including the change in sign from a decaying to a growing mode. A single free parameter was retained in the polarization term. This was assumed to be constant throughout the discharge and adjusted to obtain the best fit. The best fit value is stabilizing, and also agrees well with the most commonly used theoretical polarization model. This is strong confirmation that the MRE, including the often-disputed polarization model, and linear Δ', is a valid quantitative model for island evolution.
The renormalized GLF23 confinement model was installed in the ONETWO code and benchmarked against the XPTOR code for several experimental DIII-D discharges as well as several proposed burning plasma experiments (BPX) currently under consideration. Several of the BPX simulations included sawtooth modeling in combination with GLF23. The results indicate that agreement between the codes is satisfactory and the (small) differences that are observed can be explained in terms of understood differences in the codes.
Oral talks were presented at the recent European Physical Society (EPS) meeting in Montreux, Switzerland by J. Candy and A. Turnbull describing the Theory Group's work on the development of the GYRO code and the MHD stability analysis for DIII-D. Both were well received. L. Lao and A. Turnbull also made presentations at the IEA Workshop on ELMs at JET following the EPS meeting. These covered our work on developing the model for Type I ELMs based on ideal stability of intermediate n modes and complemented experimental talks by T. Osborne and A. Leonard.
These highlights are reports of research work in progress and are accordingly subject to change or modification