2D Hall-MHD investigation of the magnetic pressure balance during merging start-up in spherical tokamaks

Abstract

In this work, the formation and merging of two spherical tokamak (ST) plasmas have been studied by the mean of the 2D Hall-MHD simulation. It was found that during the ST plasma formation, an internal toroidal field is formed due to the changes in the flux surface. The expanding force of this toroidal field pushes the formed STs toward the mid-plane just after the pinch-off, while the magnetic tension and mutual attraction of plasma toroids mainly drive the merging at the mid-plane. During the merging, the internal toroidal field doubles inside the separatrix due to the shrinking of the flux surface while the total toroidal and poloidal fluxes are conserved. After the merging, the single plasma goes through a relaxation process, reaching a force-free state.

MHD-FiT: MHD-based dynamic reconstruction of tokamak plasma configuration

“…Figure 5 illustrates the safety factor profile after the end of merging, which is found to be greater than 1 in the TS-6 spherical tokamak. Notably, a small reconstruction error is observed between the measured and reconstructed q profiles as a function of normalized poloidal flux Ψ N , while the reconstructed profile by EFIT has a much larger error 5.1% in comparison to the MHD-FiT 1.4% which is most likely due to the presence of plasma motion (not equilibrium state) and paramagnetic internal toroidal field [20] at this time step.…”

“…The governing MHD system of equations of the MHD-FiT approach is solved using the two-dimensional implementation of high-order finite difference framework BOUT++ [19][20][21]. In this approach, equations are normalized by presenting each dimensional variable ξ as ξ = ξ 0 ξ, where ξ 0 is a dimensional constant and ξ is the normalized variable.…”

2D MHD Study of the Plasma Formation Using the Induction of In-Vessel Poloidal Field Coils inside the Spherical Tokamak

“…As mentioned before, the experimental measurements of the magnetic field during the merging proposed the presence of a toroidal flux inside the ST plasmas [9]. Figure 4 illustrates the time evolution of the generated toroidal flux inside the separatrix (hence it is referred to as the internal toroidal flux) produced by poloidal plasma current in the case of double PF coils [10].…”

“…The toroidal flux is generated due to the increase in the poloidal current during the plasma formation and the merging. Yet, the poloidal current itself is generated due to the shrinking of flux surfaces which is higher during the merging [10].…”