Wavenumber Sensitivity of the NSTX-U Poloidal High-k Scattering System

A 693 GHz, 8-channel, poloidal high-k collective scattering system is under development for the National Spherical Torus eXperiment Upgrade (NSTX-U) device. It will replace the previous 280 GHz, 5-channel, tangential scattering system [1] to study high-k electron density fluctuations, thereby providing a measurement of the k_θ-spectrum of both electron temperature gradient (ETG) and ion temperature gradient (ITG) modes. A tool is under development to calculate the wavenumbers which can be measured and the range of r/a which can be covered in NSTX-U. In the previous work, the tool is limited to placing the interaction regions on the mid-plane and it helps on evaluating the initial design and motivating the new receiver optical design to achieve better performance. The new version of the tool is able to place the interaction regions above or below the mid plane, so larger scattering angle and thus larger fluctuation wavenumber can be achieved. Simultaneously, according to the magnetic field experimental data, it can take the effect of magnetic pitch angle into consideration. We use this tool to evaluate the wavenumber sensitivity of the NSTX-U poloidal high-k scattering system.