One-dimensional modeling of lower hybrid current ramping in tokamaks

For the tokamak to be a viable reactor concept, it is highly desirable that it operate steady state. Whether or not this can be achieved by radio-frequency (rf) current drive along remains an open question. Recently, it was proposed that the reactor be operated in the so-called hybrid transformer mode, which exploits the relatively good lower hybrid (LH) current drive efficiency at low densities, as found in recent experiments. In this paper, they use a one-dimensional model to examine the physics and engineering issues related to the hybrid transformer mode, in terms of its feasibility and operational procedure. In particular, they focus on the transition between the burn and recharging phases, which requires a detailed time-dependent analysis of LH current ramping. As such, their analytic tool consists of a three-dimensional, LH wave ray-tracing code, which they couple to WHIST, a 1 1/2-dimensional transport, magnetohydrodynamic equilibrium code developed at Oak Ridge.