Characterization of the Scrape-off Layer of Lithium Tokamak eXperiment-$\beta$ (LTX-$\beta$) using a Retarding Field Energy Analyzer

The Lithium Tokamak eXperiment (LTX) is a spherical tokamak device designed to study lithium plasma facing components (PFCs). The lithium coated wall of LTX has been demonstrated to produce a low density, high temperature, and hence low collisionality plasma edge. The recent upgrade to LTX-$\beta$ includes installation of a neutral beam, which will provide further heating and fueling of the plasma. Core and edge diagnostics will also be expanded. As part of this expansion, a Retarding Field Analyzer (RFEA) has been developed for the scrape-off layer (SOL) of LTX-$\beta$. Measurements of the ion temperature, ion energy distribution, and the local space potential will be performed in the SOL plasma using this RFEA. Since a high temperature, low collisional edge is expected for LTX-$\beta$, the plasma in the SOL will be mirror-trapped, and could produce an ambipolar potential via differential loss of the electrons and ions, known as the Pastukhov potential in the literature [1]. A simple numerical prediction of the ambipolar potential profile will be presented, along with the design of the RFEA system. $^1$ Pastukhov, V. P., Nucl. Fusion \textbf{14}.1 (1974).