Equilibrium reconstruction in the Sino-United spherical tokamak

An equilibrium reconstruction code named ERST is developed in the MATLAB environment with the EFIT algorithm1,2,3,4. The present code adopts the same polynomial current profile model and Picard iterations as the original EFIT algorithm. At the same time, the following new features are added since the plasma has a sharp current changing rate (30 MA/s) which will induce large vessel currents in the Sino-United Spherical Tokamak (SUNIST), and the plasma has a significant elongation which will cause severe vertical shift instability in Picard iterations. Firstly, the response function method5 is adopted to deduct the vessel currents component in the magnetic diagnostics. Supplementary filaments on the vessel representing the toroidal eddy currents are employed in the following equilibrium reconstruction. Secondly, a free parameter concerning the vertical shift of bulk plasma is admitted in the formulation of the plasma current profile and solved in a least-square sense. This method will help in stabilizing the axial instability. In addition, the code has included the plasma boundary measurement by the Langmuir probe array on the midplane as a constraint which will further improve the reliability of the code. Finally, high fidelity plasma equilibrium reconstruction, which strongly agrees with the fast camera images, can be obtained by the present ERST in the whole discharge, including flat-top and fast ramping up and down phases.