Langmuir Probe Characterization of the C-2W Scrape-Off-Layer Plasmas

The C-2W experiment is a field-reversed configuration (FRC) which uses neutral beam injection and an edge biasing scheme to suppress turbulence and stabilize the plasma. The FRC core is surrounded by open field lines in the edge region of the confinement vessel (CV), called the scrape-off-layer (SOL), which terminate on electrode plates located in the inner and outer divertors. Therefore, settings in the divertors can significantly affect the plasma conditions and stability in the SOL. A motorized triple Langmuir probe with 24 inches of travel has been installed on the CV, 66 cm from the mid-plane to make critical measurements of the SOL plasma parameters. Characterization of the electron density (n_e), electron temperature (T_e), floating potential, plasma potential, and edge fluctuations in the C-2W SOL are presented. Initial measurements show SOL T_e ~20 eV as the FRC is formed, after which T_e gradually decreases, and SOL n_e ~10¹⁸ m^-3, consistent with Thomson Scattering and FIR diagnostic data measured at the CV mid-plane. Finally, details of a design for a new insertable probe platform system and initial measurements in the C-2W inner divertor will be discussed.