Experimental validation of momentum transport theory in the core of a tokamak plasma

Accurate prediction of plasma rotation profiles in future fusion devices requires a comprehensive understanding of momentum transport. This is crucial as plasma rotation affects neoclassical and impurity transport, MHD stability, turbulence, and confinement.

The advanced momentum transport analysis framework presented herein can uniquely, separately, and concomitantly determine the contributions of diffusion, convection, and intrinsic torque to the momentum transport within the core plasma from torque modulation experiments. The analysis, self-consistently, incorporates the time dependencies of all transport mechanisms, which is essential to compensate for changes in the transport synchronous with the torque perturbation, to separate the momentum fluxes and closely match experiment. The transport coefficients inferred from ASDEX Upgrade experiments show quantitative agreement with gyrokinetic predictions for the Prandtl and pinch number, providing an unprecedented degree of validation. Scaling laws derived from a database of gyrokinetic calculations are also compared to the experimental results.

Of particular interest is the intrinsic torque, which remains the largest uncertainty in predicting the rotation of future fusion devices. This work demonstrates that the intrinsic torque is co-current directed and mainly originates at the edge of the plasma in discharges dominated by ITG according to gyrokinetic calculations. Furthermore, the magnitude of this edge intrinsic torque correlates with the plasma pressure gradient in the pedestal. The intrinsic torque in the plasma core, conversely, correlates most strongly with local turbulence properties. This work generates novel experimental opportunities for validating momentum transport theory and holds the potential to provide the first consistent, physics-based, and validated predictions of momentum transport for future reactor scenarios.