Study of non-inductive plasma current ramp-up in spherical tokamaks

In spherical tokamaks practically there is no space for central solenoid. That is because the substantial part in the central area of tokamak is necessary to place the toroidal magnetic field coils. Hence in general case the possibility inductively ramp up the current in plasma of spherical tokamak is very limited, the plasma current ramp up problem in such devices becomes a very important. One of the methods for plasma current ramp up in spherical tokamaks could be the combined one, which includes the generation of bootstrap current together with high frequency current drive generation. A result of such method of plasma current ramp up is a production of a very non-monotone plasma current profile, which makes the predictive numerical analysis of plasma equilibrium evolution too difficult. For such analysis in frame of DINA code [1] the special method of plasma fix boundary equilibrium calculation on rectangular grid is developed, which allows solving the plasma fix boundary equilibrium problem with strongly non-monotone current profile in plasma. Presented paper demonstrates the results of study with use of updated DINA code of the non inductive plasma current ramp up scenarios in the spherical tokamak, which is considered in the project of neutron source VNS [2]. The effect of neoclassical plasma conductivity on plasma current density profiles is studied. It is shown, that if only Spitzer conductivity is used, then the shape of current density follows shape of non-inductive current drive density shape. But in the case of neoclassical conductivity the plasma current density on the axis becomes hollow despite of picked external current density, the value of safety factor q on the axis increases, and MHD instabilities on the plasma central region are developed. Special technique, which takes into account such instabilities, is applied and the plasma current evolution is simulated. Importance of careful kinetic control of plasma current and temperature profiles during non-inductive ramp-up process is emphasized.