Bootstrap currents in radiofrequency driven tokamak equilibria

Abstract

As a result of certain experimental observations, interest in the bootstrap current arising from neoclassical transport in tokamaks has increased recently. In this study, the bootstrap current is calculated for a number of radiofrequency (RF) current driven tokamaks. Two-dimensional, self-consistent, steady state tokamak MHD equiJibria are obtained by including both the transport driven bootstrap current and the externally driven RF current. Self-consistency is accomplished by iterating the two-dimensional MHD equilibrium calculation and the current calculation (including bootstrap current and RF ray tracing). The parameter Γ = 1 −(γ/γB) is defined as a measure of the RF current drive power reduction achieved when bootstrap currents are included in the calculation (γ = n̄e R0I/PCD is the current drive efficiency without the benefit of bootstrap current and γB = n̄eR0I/PCD is the higher efficiency obtained when bootstrap effects are included). For INTOR-type reactor parameters with n̄e = (0.7−1.4) × 1020 m−3, Γ ≈ 0.4−0.9 (40-90% reduction of RF power) is obtained. Calculations for other reactor parameters of interest are also carried out. It is found that for reactor-grade plasmas, the bootstrap current contribution to the toroidal current is, in general, important. In some cases, it is even found that the bootstrap current density locally exceeds the required current density of the initial target equilibrium. An approximate scaling law for P, based on the parametric survey performed, is also obtained.