Equilibrium simulation for the magnetic confinement of the Spherical Tokamak MEDUSA-CR

The low aspect ratio spherical tokamak (ST) MEDUSA-CR is currently being re-commissioned at Instituto Tecnológico de Costa Rica. One of the proposed first tasks is to simulate the magnetic confinement considering the particular coils arrangement of the device. This work presents the resulting shape of the plasma cross-section. MEDUSA-CR main specifications are: plasma major radius R0 approx. 0.14 m, plasma minor radius a approx. 0.10 m, toroidal field at the vessel geometrical center BT smaller than 0.5 T, plasma current Ip lesser than 40 kA, central electron temperature Te lesser than 140 eV, discharge duration lesser than 3 ms [1]. The free boundary equilibrium solver Fiesta has been used to obtain the plasma shaping and some equilibrium parameters. The code works the magnetic confinement for the static Magneto-Hydro-Dynamic (MHD) equilibrium case solving the Grad-Shafranov equation. A central solenoid was added to the original magnetic configuration aiming to create a limiter with an ergodic behavior in order to enhance the plasma confinement [2], [3]. The resulting cross-section (without divertor, i.e., natural divertor) for the plasma volume is a “bean” shape. The elongation is found to be 1.43 and the triangularity 0.547. A bean shape cross section is found to produce a higher triangularity, which have a significant effect on MHD stability [4]. Further simulations with FIESTA could be used as proof of principle for the effect of the plasma shape on transport and stability.