Some observations, conclusions, and plans

I have several observations:

  1. The default settings for k_yrho are suppose to correspond the most unstable modes. However, these setting were found in scans for conventional tomakak parameters (primary for H-mode parameters at DIII-D). These default settings do not necessarily correspond to the most unstable modes at NSTX.
  2. The modes with different k_yrho might be more unstable in different plasma regions. The DRIBM model with default parameter k_yrho=0.2 does not show any anomalous transport in the plasma core for rho< 0.3. However, we found that there might be anomalous transport driven by smaller scale instabilities that have k_yrho=0.6.

In order to address these observations, several improvements in the MMM8.1 model can be considered. These improvements include:

  1. Loops over spectrum of k_yrho needs to be implemented for DRIBM and Weiland models. Individual values of k_yrho that correspond to maximum growth rates need to be selected for every flux surface for every numerical iteration. Depending on the k_yrho ranges and granularity, these change will make the Multi-Mode model order of magnitude slower, but it will increase the robustness of the results.
  2. The ExB flow shear effect should depend on k_yrho value. There is no theory-based model at this time that can be implemented in MMM8.1. We can develop such a model by running gyro-kinetic simulations (for example, using the GYRO code). We can perform several scans with respect to k_yrho values. Using these scan, we can develop a parametrized dependencies of the ExB flow shear factor with respect to k_yrho.

These developments introduce significant changes to the model and are not currently funded. They might be included as new tasks to the NSTX proposal.

Next steps:

  1. Compile the scan with respect to k_yrho and ExB flow shear factor
  2. Run the PTRANSP simulations with adjusted k_yrho and ExB flows shear factors from t=0.364 sec to t=0.482 sec and to compare the predicted temperature profiles with the experimental observations.
  3. Compare the diffusivities from the Multi-Mode model with the diffusivities from other models (TGLF).
  4. Verify the ETG model results with other models (GLF23, TGLF and possibly GYRO).