Relevant parameter space and stability of spherical tokamaks with a plasma center column

Abstract

A spherical tokamak (ST) with a plasma center column (PCC) can be formed inside a simply connected chamber via driven magnetic relaxation. From a practical perspective, the ST-PCC could overcome many difficulties associated with the material center column of the standard ST reactor design. Besides, the ST-PCC concept can be regarded as an advanced helicity injected device that would enable novel experiments on the key physics of magnetic relaxation and reconnection. This is because the concept includes not only a PCC but also a coaxial helicity injector (CHI). This combination implies an improved level of flexibility in the helicity injection scheme required for the formation and sustainment phases. In this work, the parameter space determining the magnetic structure of the ST-PCC equilibria is studied under the assumption of fully relaxed plasmas. In particular, it is shown that the effect of the external bias field of the PCC and the CHI essentially depends on a single parameter that measures the relative amount of flux of these two entities. The effect of plasma elongation on the safety factor profile and the stability to the tilt mode are also analyzed. In the first part of this work, the stability of the system is explained in terms of the minimum energy principle, and relevant stability maps are constructed. While this picture provides an adequate insight into the underlying physics of the instability, it does not include the stabilizing effect of line-tying at the electrodes. In the second part, a dynamical stability analysis of the ST-PCC configurations, including the effect of line-tying, is performed by numerically solving the magnetohydrodynamic equations. A significant stability enhancement is observed when the PCC contains more than the 70% of the total external bias flux, and the elongation is not higher than two.

Spherical tokamaks with a high current carrying plasma center column

“…For these reasons, they have been used to introduce and study many features of ST-PCC plasmas in the past. 5,9 However, the k profiles observed during sustainment are not uniform because different current drive mechanisms act on the open and closed flux regions. 19 In this sense, the model can be improved by using a tanh k profile such as 20,21 kðwÞ…”

“…A related configuration is the spherical tokamak with a plasma center column (ST-PCC), whose properties, relevant parameter space, and tilt stability have already been studied. 5,8,9 Those studies, however, were restricted to current density profiles corresponding to fully relaxed equilibria.…”

“…14 For this reason, alternative sustainment scenarios using auxiliary non-inductive current drive a) Electronic mail: lealamp@cab.cnea.gov.ar (CD) systems, such as beam injection or RF have also been proposed in this context. 5,15 In this work, the analysis of ST-PCC equilibria 5,9 is extended to consider arbitrary current density profiles (Sec. II).…”