Pulsed RF Schemes for Tearing Mode Stabilization

Abstract

The RF stabilization of tearing modes with current condensation has the potential to increase stabilization efficiency and loosen power localization requirements. Such benefits stem from the cooperative feedback between the RF deposition and resulting island temperature perturbation governed by diffusion. A self consistent treatment of the damping of an rf ray as it traverses the island shows that low damping scenarios can require unfavorably high powers to overcome initial power leakage and effectively capitalize on the nonlinear effect. In this work it is demonstrated that for such regimes, modulated stabilization schemes can achieve significant improvements in heating and current drive contributions to stabilization for the same average power as a continuous wave scheme. The impact of modulation frequency and duty cycle on the performance is explored, the results of which suggest modulation strategies in which the pulsing periods are kept on the order of a diffusive time.

Calculating RF current condensation with self-consistent ray-tracing

“…Furthermore, we will show that the dynamic variation of the poloidal launching angle can help to circumvent the shadowing effect, a nonlinear inhibition effect 40,42 . At high island temperatures, a significant fraction of the wave power may be damped at the island edge before the wave reaches the island centre.…”

“…This not only leads to reduced island temperatures but also to possibly destabilising currents driven close to the island separatrix. One way to bring the power deposition toward the island O-point despite the shadowing effect involves pulsing the input wave power 42 . We will show in this section that the shadowing effect can also be avoided by varying the poloidal launching angle, such that damping at the island edge is reduced.…”

“…V that a bifurcation and hysteresis can be obtained by varying the RF wave launching angles, which could be a pathway for future experimental verification of current condensation. Additionally, the launching angles can be adjusted to circumvent the nonlinear shadowing effect 40,42 .…”