Experiments on the DIII-D device in the negative triangularity (NT) regime of tokamak operation demonstrate core conditions that offer advantageous stability properties. Long duration, stationary discharges in this scenario maintain performance metrics that scale to viable reactor gain. Deleterious global modes of toroidal mode number n=1 are infrequent in these plasmas, which operate free of core instability cycles that can kick off global instabilities.
These plasmas operate free of edge instability cycles that would damage reactor components, as do all strongly shaped NT plasmas. Reproducible access to high-power stationary states was developed at two values of q95, the edge magnetic winding number or “safety factor”. Core MHD instabilities manifest in one form of internal ideal mode, the quasi-interchange mode (QI), found to be consistent with modeling of the profiles and parameter space in which NT operates. The GATO and DCON ideal MHD codes are used to characterize the limits to normalized pressure in NT, finding global kink modes with strong poloidal harmonic m=1,2 components at normalized plasma pressure βN=3-3.5. Limits to β_N are predicted to be mostly insensitive to plasma boundary shape in NT and similar at both q95 values obtained in experiments. Average triangularity is shown to affect ideal limits, when modified at the outer midplane.
A similar result is obtained with the RDCON resistive MHD code, which is used to characterize the stability to resistive “tearing modes”. Experimental NT equilibria and equilibria across shape scans were investigated. Only outer midplane modifications affected tearing calculations. Ideal kink modeling and experimental observations of sporadic QI mode provide an explanation for current diffusion not predicted by neoclassical theory. This effect is found in experiments at q95=3, analyzed with the ONETWO transport code’s facility to evolve magnetic flux over a discharge consistently with measured profiles and reconstructed magnetic flux surfaces. This result is compared with GATO calculations and ONETWO flux diffusion analysis of a conventional shape, ITER baseline demonstration discharge that is shown to have an intrinsically 3D core. Radiation from accumulated plasma impurities seems to alter the core q profile.
This makes unstable a QI mode that spurs formation of a helical core, sustained by anomalous magnetic flux diffusion. NT experiments at q95=4 are limited in energy confinement by poor fast ion confinement, as a result of nondisruptive core 3/2 tearing modes. Analysis with ONETWO shows agreement with neoclassical flux diffusion predictions in these cases, corresponding to a removal of core instabilities and elevation of minimum safety factor values qmin to unity. This understanding of the core MHD, performance, and operational limits of NT scenarios in DIII-D advances the development of negative triangularity scenarios and informs the core phenomena observed in experiments spanning the regime.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/6647-qr29 |
| Date | January 2024 |
| Creators | Boyes, William Samuel |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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