Equilibrium and Stability Calculations of MAST Spherical Torus Plasmas in Preparation for MAST-U

Equilibrium and Stability Calculations of MAST Spherical Torus Plasmas in Preparation for MAST-U

Equilibrium and Stability Calculations of MAST Spherical Torus Plasmas in Preparation for MAST-U 150 150 amit lakhani
UKAEA-CCFE-CP(20)100

Equilibrium and Stability Calculations of MAST Spherical Torus Plasmas in Preparation for MAST-U

Disruption prediction and avoidance is necessary in future MAST-U spherical tokamak discharges to enable long-pulse plasma operation. Research examining the stability of plasmas in the MAST database utilizing proposed high quality kinetic equilibrium reconstructions and calculations with various codes, including the Disruption Event Characterization and Forecasting code (DECAF) [[i]], will illuminate relevant physics and enable subsequent analysis and control for MAST-U. For equilibrium analysis, it is important to include currents in the conducting structure, as they can comprise a significant component of the plasma current. An axisymmetric wall model of MAST was created in VALEN, and time-domain calculations were performed using experimental currents in coils with and without plasma current. The DECAF code, which consists of many separate physical event modules that provide warnings and declare occurrences of certain events leading to disruption, has been applied to the MAST database. Disruptivity diagrams indicate where disruptions occur in various parameter spaces, and examination of vertical displacement events show that they occur less often than in NSTX, but when they do occur it is in close time proximity to the disruption current quench. MAST operation was bound by the Greenwald density limit, and this limit was often reached in the current rampdown which lowered the limit below the level of the experimental density. A parametrized forecasting model for βNno-wall implemented in the DECAF code and tested for NSTX used to analyze MAST plasmas with magnetics-only reconstructions does a fairly good job reproducing the measured limit without any modification. Accurate global kinetic MHD mode analysis starts with accurate ideal MHD stability analysis, and a preliminary DCON calculation with the magnetics-only reconstruction shows that DCON can find unstable modes above about βN = 5. The DECAF estimate for βNno-wall was about 4.3 for the same case, however, so a better match between the modeled and calculated limits can probably be achieved with better kinetic equilibrium reconstructions. [[i]] J. W. Berkery et al., Physics of Plasmas 24 (2017) 056103

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46th European Physical Society Conference on Plasma Physics (EPS), Milan, 8-12 July 2019