2024 – Test Off-axis Helicon Current Drive using High Beta_e Plasma coupling

Test Off-axis Helicon Current Drive using High Beta_e Plasma coupling

2024 Research Campaign, Heating and Current Drive

Purpose of Experiment

The purpose of this experiment is to demonstrate absorption and current drive at large minor radius in the core of a high electron-beta plasma using high-power helicon waves at 476 MHz. Helicon current drive is predicted to be two to three times more efficient than outside launch ECCD at mid-radius in plasmas with high densities and high electron temperatures. The installation and commissioning of the MW-level system at DIII-D enables the first tests of this technology in reactor-relevant plasmas, where full single-pass absorption is expected. Extensive modeling of helicon CD in equilibria similar to those targeted in this proposal has been carried out, leading to the presently installed helicon system at DIII-D. The predictions converge to a 60-70 kA of volume-integrated current driven by 1 MW of fast wave power at radius rho=0.4-0.6.

Experimental Approach

To make the experiment successful, measurements of the power deposition profiles, and of the current drive profiles are essential, along with measurements of the antenna-plasma coupling. The current drive profiles are determined from measurements of the internal magnetic field structure by the motional Stark effect (MSE) diagnostic, which yields both the total parallel current density and the parallel electric field from the time derivatives of the poloidal flux.A factor of two improvement of signal-to-noise ratio can be obtained by comparing a pair of otherwise identical discharges with co- and counter-injection of the helicon waves. Two separate half days are planned for this one-day experiment: one for co- and one for counter-IP helicon current drive. The target plasma scenario is a high normalized fusion performance scenario that requires current drive at large minor radius for sustainment. The scenario uses Ip and Bt ramps to transiently generate the broad current profile that supports stable operation at betaN~4, H98Y2~1.5, and betaT>4%. The purpose of helicon CD is to demonstrate a method to help sustain the current profile obtained via the ramps, in a way that is compatible with steady-state.