Power balance in a reversed-field-pinch plasma with partial poloidal current drive by neutral beams

Zero-dimensional power balance is analyzed, and an operation boundary is deduced in a ''beam-assisted reversed-field pinch'', the latter utilizes partial poloidal current drive by neutral beams so that transport losses arising from magnetohydrodynamics (MHD)-dynamo, i.e., tearing mode instability are reduced. Changes of power flow and heat conductivity due to a beam driven current are treated by considering an MHD-dynamo-based power balance model that assumes linear dependence of magnetic fluctuation level on the externally driven current. It is shown that a ratio of a beam driven current to a dynamo current must not exceed similar to 40% regarding a beta-limit in the next generation of plasma experiments (minor radius/major radius = 0.6 m/1.8 m, plasma current = 1 MA, poloidal beta = 0.1). At that point, the energy confinement time is predicted to increase by a multiple or so of that estimated from the MHD dynamo model without a current drive.