Estimation of Effective Momentum Diffusivity and Its Correlation with Neutral Particle Density Based on Toroidal Rotation Profiles in the TCABR Tokamak

Abstract

An equation for estimating the effective diffusivity \(\chi _\varphi ^\textrm{eff}\) based 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 \(\chi _\varphi ^\textrm{eff}\)—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 \(\chi _\varphi ^\textrm{eff} \approx 20\) m\(^2/\)s near the plasma edge at \(r/a \simeq 0.87\), 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.