Exploring ion heat transport during neutral beam heated plasmas at W7-X
The stellarator concept is an attractive approach for fusion energy production, as the confining 3D magnetic field structure can be generated without strong internal plasma currents. However, recent results from the neoclassical-transport-optimized W7-X stellarator show that the required excellent ion heat confinement is only observed during transient phases in presence of peaked density profiles. Theory suggests that the improved confinement is related to a strong reduction in ion-temperature gradient (ITG) turbulence in presence of local density gradients, but more studies addressing this important problem from different perspectives are required. Here, we propose to perform a detailed comparison of experimental data vs. theory for W7-X plasmas with strong neutral beam injection (NBI) heating.
NBI heat deposition profiles and the level of charge exchange losses will be determined and validated using Balmer alpha spectroscopy, as these two quantities are essential for accurate calculations of the ion heat diffusivity during NBI heated experiments. Moreover, information on the ion heat-pulse diffusivity will be obtained from NBI modulation experiments. Such perturbation experiments are a widely applied method in fusion research and have become feasible at W7-X thanks to a new charge exchange spectroscopy system operated by UW Madison that allows for measurement of local impurity ion temperatures at different radial positions.
The obtained heat diffusivities will be compared to neoclassical transport predictions to estimate the remaining anomalous transport. The resulting anomalous heat diffusivities will then be analyzed using linear and non-linear gyrokinetic simulations. Synthetic diagnostics will be developed to additionally compare the gyrokinetic simulation results to fluctuation measurements that will provide a better understanding of the specific impact of the 3D geometry and profiles on turbulence. In addition, the validated nonlinear simulations will themselves be used to develop reduced turbulent transport models. The vision is to work towards the identification of mechanisms to alter the magnetic field configuration resulting in reduced ITG turbulent transport.