Metamaterial Lens for Improved Transverse Resolution of mm-wave Diagnostics, applied to ECE at DIII-D

Millimeter wave diagnostics of plasmas typically cover bands of several GHz (reflectometry, scattering), tens of GHz (radiometry) or even hundreds of GHz (Michelson interferometry), but their focus is optimized for a single frequency. For other frequencies, the measuring volume is far from the beam waist. This results in a loss of resolution in the poloidal direction, especially at higher poloidal mode numbers (e.g., Alfv\`{e}n Eigenmodes). Ideally the beam should be focused at different locations for different frequencies. Our recent numerical study suggests that a zoned planar metamaterial lens can achieve this result in the 8-12GHz band [W.J. Capecchi \textit{et al}., \textit{Optics Express} \textbf{20}, 8761 (2012)]. Here we present the design and full-wave simulations of a lens for possible use with Electron Cyclotron Emission (ECE) at DIII-D in the 80-130GHz band, discuss the fabrication challenges due to the required miniaturization, and present a system allowing the adjustment of the focal points to accommodate changes in the magnetic field. Because ECE at DIII-D undergoes one of the largest variations of optimal focal length, similar metamaterial lenses can more easily be designed for other mm-wave diagnostics and/or smaller devices.