Calculating RF current condensation with consistent ray-tracing and island heating

Abstract

By exploiting the nonlinear amplification of the power deposition of RF waves, current condensation promises new pathways to the stabilization of magnetic islands. We present a numerical analysis of current condensation, coupling a geometrical optics treatment of wave propagation and damping to a thermal diffusion equation solver in the island. Taking into account the island geometry and relativistic damping, previous analytical theory can be made more precise and specific scenarios can be realistically predicted. With this more precise description, bifurcations and associated hysteresis effects could be obtained in an ITER-like scenario at realistic parameter values. Moreover, it is shown that dynamically varying the RF wave launching angles can lead to hysteresis and help to avoid the nonlinear shadowing effect.

On the merit of hot ion mode for tearing mode stabilization

“…1. At rf powers past the bifurcation point, the temperature in the island will continue to grow until encountering additional physics not included here, such as stiffness 18 or ray depletion 17,19,20 . We opt for the simplified rf model used here to isolate the physics of the condensation effect in hot ion mode, without introducing particular depletion mechanisms; in exchange, the present investigation is limited to weak absorption regimes, i.e.…”

“…In general, γ 0 = γ, h 0 = h and c 0 = c. Eqs. (18)(19) can be converted in to a relationship between η, γ 0 , c 0 , and h 0 :…”

“…This amplification and focusing are termed the current condensation effect. [16][17][18][19][20] As the nonlinear enhancement is dictated by the electron temperature perturbation (P r f ∼ exp(w 2 0 ∆T e /T e0 ), it naturally follows that additional heat sources to the electrons would further amplify the condensation effect and associated stabilization properties. Such a supplementary heat source could be provided by operating in a "hot ion mode", in which the ions are maintained at significantly higher temperatures than the electrons.…”

Two-fluid model of rf current condensation in magnetic islands

“…53 A new code, OCCAMI, coupling GENRAY to a thermal diffusion equation solver within the island to provide higher fidelity simulations of the current condensation effect, is in development. 34 This tool has already been used to show the feasibility of accessing the bifurcation point and related hysteresis phenomena in an ITER-like scenario with realistic parameters.…”

“…[21][22][23][24][25][26][27][28][29] Only recently, however, has the presence of the island and its effect on rf deposition been accounted for. [30][31][32][33][34] The thermal insulation provided by the closed magnetic topology and reduced cross field transport within the island [35][36][37][38] produces significant temperature perturbations relative to the background plasma in the presence of rf heating. Relative temperature increases as large as 20% have been reported.…”