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A laser-based technique known as Doppler-free saturation spectroscopy (DFSS) has been designed, fabricated, and installed on the DIII-D National Fusion Facility to measure the helicon wave electric field vector in the edge plasma. These experimental measurements quantify phenomena resulting in decreased current drive efficiency due to wave/edge plasma interactions. This implementation of DFSS on DIII-D is the first of its kind on a tokamak and thus presents unique engineering challenges, including integration of the system onto an existing multidiagnostic port flange without impacts to system serviceability, as well as maintaining precise laser alignment over a 2-m distance during disruptions and thermal drift of the vessel. These challenges were resolved using innovative design approaches such as a novel decoupled shutter system to facilitate serviceability of the in-vessel mirror assemblies without the need for personnel vessel entry, as well as an ex-vessel piezo-mirror-based optical system for laser beam shaping and real-time steering of the measurement location. The solutions to these engineering challenges were demonstrated during the successful installation and operation of these diagnostic components during the 2022 DIII-D vent and subsequent experimental campaign.

An equation for estimating the effective diffusivity χφeffbased on the thermal and frictional forces arising from the interactions with neutral particles is provided. These interactions are primarily attributed to charge-exchange processes between ions and neutral particles. Our analysis indicates that the proposed forces are strong candidates to act as sources or sinks of momentum at the plasma edge. Furthermore, although we only have an estimated profile of neutral particles, the results demonstrate a strong dependence of the effective diffusivity on the ion-neutral collision frequency and a comparatively weaker dependence on ion temperature. We showed that the toroidal rotation profile in the TCABR tokamak is well described by the Helander model for toroidal rotation velocity in the Pfirsch-Schlüter regime, which depends on the ion temperature gradient in the presence of the neutral particles. The toroidal rotation profile has been reconstructed by a sum of Bessel functions, derived from the solution of a second-order differential equation for the toroidal velocity. In this initial study on momentum transport in TCABR, our findings indicate that χφeff—which accounts for both the diffusivity and pinch terms in the momentum equation—increases significantly in the outer plasma region. Additionally, it achieves a reasonable maximum value of approximately χφeff≈20m2/s near the plasma edge at r/a≃0.87, in the same radial position of the maximum ion-neutral collision frequency, demonstrating that the applied method successfully captures the general behaviour of this component as reported in other works, and guiding future experimental validation.