Analysis of Equilibrium Pressure and Current Dynamics in SPARC*

The plasma shape of the SPARC tokamak changes as a function of pedestal height and current density distribution. The equilibrium sensitivity to these dynamics is assessed using both the “TokaMaker” time-dependent Grad-Shafranov solver [1] and the time-independent EFIT solver. This investigation informs shape control tolerances, an essential step in avoiding damage to the divertor and first-wall during plasma operation. Changes to the strike point locations as a function of pedestal height are found to be sensitive to the bootstrap current fraction and fidelity of real-time equilibrium reconstructions. This suggests potential challenges in constraining strike point sweep locations if an unexpected L-to-H transition occurs. Additionally, the sensitivity of magnetics-only equilibrium reconstructions to tearing-unstable current profiles is investigated, assessed without the benefit of Motional Stark Effect. These analyses will be further generalized using a more precise treatment of current diffusion and equilibrium time evolution, with the goal of anticipating uncertainties that may interfere with shape control and profile stability during SPARC operation. 

[1] C. Hansen et al 2017 Physics of Plasmas 24 (4): 042513