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FY2021 Research Milestones

R(21-1):  ST pedestal transport modeling using gyrokinetics

Description: Milestone work in FY20 (R20-1) used multiple gyrokinetic codes (CGYRO, GENE, XGC-1) to predict and compare linear microinstability growth rates and thresholds with various NSTX H-mode pedestal profiles. Multiple mechanisms are predicted to be important including KBM, ETG, MTM, and TEM. Nonlinear ETG simulations were initiated in a few cases to test whether predicted transport can account for experimental electron thermal losses in the bottom half of the pedestals. Research in FY21 will continue to test the ability of nonlinear simulations to predict saturated turbulent transport, for ETG and the other ion-scale mechanisms. Effort will be given to expand tests of numerical resolution to ensure the fidelity of the simulations. Where appropriate, scans will be pursued to characterize the dominant sensitivities of the various transport mechanisms. These results will be compared to experiment and will form the basis for development of reduced pedestal transport models.

R(21-2):  Prepare tools for commissioning

Description: Critical aspects of the plasma control system and routine analysis tools must be updated to reflect changes to the NSTX-U device. For example, models used to generate 2D and 3D magnetic field calculations must be updated to reflect changes in the poloidal field coils and conductive wall elements. The updated models will be used to redesign breakdown scenarios for inductive startup and quantify the ability to control the vertical position of the plasma at high elongation. This milestone also contains activities that advance the capabilities of the plasma control system based on the experience of Commissioning activities in 2016. This includes improving the ability to control the plasma boundary shape for specific scenarios derived from discharges in 2016 and offline modeling. Activities will improve between-shot data availability and visualization, such as restoring and refining between-shot TRANSP (BEAST). New automated routines for refining the calibration of gas flow rate and magnetic sensors will also be developed and possibly put into action near the end of FY21.

R(21-3): Optimization of NBI mix for AE-mitigated scenarios

Description: A broad deposition profile from neutral beam injection, e.g. by NBI aiming tangentially on the outboard midplane, is usually assumed to reduce the drive for Alfvénic instabilities (AEs) by reducing the radial gradient of the fast ion density profile. However, NSTX-U results from the FY16 campaign show evidence that tangential NB injection, including off-axis injection near the plasma mid-radius, can also lead to undesired effects such as the destabilization of AEs, thus potentially leading to enhanced fast ion redistribution. Based on previous work, dedicated experiments will be performed to assess the optimal mix of on- and off-axis NBI that simultaneously maximizes NB current drive with mitigated or suppressed AE activity. Predictive time-dependent analysis with the TRANSP code will inform on the expected fast ion distribution function resulting from different NBI configurations. TRANSP results will then be used as input for AE stability analysis, which in turn provides input for updated TRANSP simulations including enhanced fast ion transport by AEs through the “kick” and RBQ fast ion transport models interfaced with TRANSP. Predictions from the TRANSP/AE stability/transport loop will be tested against available data from NSTX-U, and against data from the MAST-U and DIII-D devices.

R(21-4): ST pedestal stability modeling and determination of stability thresholds associated with ELMs

Description: In the frame of this milestone we model instabilities related to the pedestal in various H-mode NSTX discharges, employing the state of the art extended-MHD code M3D-C1. We perform linear stability simulations to enhance our understanding of the role of non-ideal MHD effects on the stability of peeling-ballooning modes in spherical tokamaks. Considering ELMing and non-ELMing narrow, wide and enhanced pedestal discharges we determine the peeling-ballooning stability boundary and locate the discharges relative to this boundary. In addition to the macroscopic modeling, we want to perform gyrokinetic simulations of microscopic instabilities (in particular KBMs) and transport phenomena to advance our understanding of the local constraints on the pedestal structure.