Abstract
Efficient off-axis current drive scalable to reactors is a key enabling technology for developing economical, steady state tokamak. Previous studies have focussed on high field side (HFS) launch of lower hybrid current drive (LHCD) in double null configurations in reactor grade plasmas and found improved wave penetration and high current drive efficiency with driven current profile peaked near a normalized radius, ρ, of 0.6-0.8, consistent with advanced tokamak scenarios. Further, HFS launch potentially mitigates plasma material interaction and coupling issues. For this work, we sought credible HFS LHCD scenario for DIII-D advanced tokamak discharges through utilizing advanced ray tracing and Fokker Planck simulation tools (GENRAY+CQL3D) constrained by experimental considerations. For a model and existing discharge, HFS LHCD scenarios with excellent wave penetration and current drive were identified. The LHCD is peaked off axis, ρ∼0.6-0.8, with FWHM Δρ=0.2 and driven current up to 0.37 MA/MW coupled. For HFS near mid plane launch, wave penetration is excellent and have access to single pass absorption scenarios for variety of plasmas for n|| =2.6-3.4. These DIII-D discharge simulations indicate that HFS LHCD has potential to demonstrate efficient off axis current drive and current profile control in DIII-D existing and model discharge.
Highlights
Efficient off-axis current drive (OACD) scalable to reactors is a key enabling technology for economical, steady state tokamak
high field side (HFS) launch potentially mitigates plasma material interaction and coupling issues due to quiescent HFS scrape off layer (SOL)[2], lower plasma particle[3] and neutron flux, and improved impurity screening[4]
In addition to primarily RF constraints, the analysis presented here optimizes for scenarios compatible with DIII-D advanced tokamak and scenario development research.[5]
Summary
Efficient off-axis current drive (OACD) scalable to reactors is a key enabling technology for economical, steady state tokamak. Double null plasmas utilizing high field side (HFS) launch lower hybrid current drive (LHCD) have found improved wave penetration enabling efficient OACD.[1] Further, HFS launch potentially mitigates plasma material interaction and coupling issues due to quiescent HFS scrape off layer (SOL)[2], lower plasma particle[3] and neutron flux, and improved impurity screening[4]. On the HFS, the toroidal field is higher and allows launch of lower n|| waves that penetrate farther into the plasma core before damping. A further benefit to lower n|| launch is that the waves are absorbed at higher Te yielding a higher current drive efficiency, 1/n||2, because wave momentum is transferred to less collisional electrons.[15] Looking towards reactor concepts where current drive efficiency is the primary parameter to be optimized, the poloidal position is selected to balance the effects of toroidicity. Proper poloidal positioning allows for waves to penetrate and damp in region ~0.6-0.8
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