Perturbative studies of toroidal momentum transport using neutral beam injection modulation in the Joint European Torus: Experimental results, analysis methodology, and first principles modeling

Perturbative experiments have been carried out in the Joint European Torus [Fusion Sci. Technol. 53(4) (2008)] in order to identify the diffusive and convective components of toroidal momentum transport. The torque source was modulated either by modulating tangential neutral beam power or by modulating in antiphase tangential and normal beams to produce a torque perturbation in the absence of a power perturbation. The resulting periodic perturbation in the toroidal rotation velocity was modeled using time-dependent transport simulations in order to extract empirical profiles of momentum diffusivity and pinch. Details of the experimental technique, data analysis, and modeling are provided. The momentum diffusivity in the core region (0.2 < ρ < 0.8) was found to be close to the ion heat diffusivity (χ_ϕ/χ_i ∼ 0.7–1.7) and a significant inward momentum convection term, up to 20 m/s, was found, leading to an effective momentum diffusivity significantly lower than the ion heat diffusivity (χ_ϕ^eff/χ_i^eff ∼ 0.4). These results have significant implications on the prediction of toroidal rotation velocities in future tokamaks and are qualitatively consistent with recent developments in momentum transport theory. Detailed quantitative comparisons with the theoretical predictions of the linear gyrokinetic code GKW [A. G. Peeters et al., Comput. Phys. Commun. 180, 2650 (2009)] and of the quasilinear fluid Weiland model [J. Weiland, Collective Modes in Inhomogeneous Plasmas (IOP, Bristol, 2000)] are presented for two analyzed discharges.