Simulation of a High Field Side Lower Hybrid Current Drive (LHCD) Coupler for DIII-D

In DIII-D, an high field side LHCD launch scenario with r/a~0.6-0.8 deposition and high efficiency has been proposed for current profile control. A multi-junction (MJ) coupler is simulated in COMSOL and MFEM to optimize spectrum, directivity, and reflection coefficient for a range of plasma conditions. Coupler directivity and n|| spectrum depend critically on correct phasing and power splitting between MJ arms; characteristics that vary with plasma vacuum gap at the antenna aperture, local density and density gradient. The COMSOL model utilizes a lossy dielectric benchmarked against ALOHA [1] simulations. COMSOL analysis allows for rapid optimization of the coupler geometry and verification of proper n|| launch spectrum for given plasma edge conditions. The MFEM model allows more complex coupler features including curvature and a warm plasma wave solver. The latest coupler modelling results including COMSOL with a cold plasma model will be presented.

[1]J. Hillairet et al, Nucl. Fusion 50, 125010 (2010).