Speaker
Description
See the full abstract here http://ocs.ciemat.es/EPS2019ABS/pdf/P5.1097.pdf
The Wendelstein 7-X superconducting stellarator at IPP Greifswald has a neoclassically optimized magnetic field, such that turbulent transport can be a major loss channel. Its variable three-dimensional field geometry is expected from analytical and numerical models to provide venues to modify both spatial localization and strength of turbulent mode activity. Turbulence in W7-X has therefore been experimentally investigated in the recently completed experimental campaign that demonstrated long pulse operation at high density and triple product.
This contribution focuses on results from the phase contrast imaging (PCI) diagnostic, which provides poloidally resolved (k⊥ρs ≈ 1) density fluctuation measurements along a line of sight through the magnetic axis. The system is a collaboration between the MIT PSFC and IPP and has been in operation for the last operation phase OP1.2 (2017-2018).
While ion-scale density turbulence is regularly observed in the core plasma which scales well in magnitude with the diamagnetic energy over a wide range of discharge scenarios, several remarkable exceptions are found. In particular, a transient increase of the diamagnetic energy with a simultaneous reduction of density fluctuations are observed in discharge scenario in which the equilibrium temperature and density profiles undergo changes in magnitude and location of maximum gradient lengths. This is the case after pellet injections (hydrogen ice or tracer impurity), and after rapid changes in heating power from both the electron cyclotron heating and neutral beam injection systems. The PCI frequency-wavenumber-spectra are then significantly modified, displaying multiple phase velocities which suggests the coexistence of turbulent modes. This is supported by reflectometer data and radially resolved impurity transport experiments, which indicate radial localization of this feature.
Gyrokinetic simulations (both linear and non linear) further support these findings by showing that growth rates are minimized when the ion temperature and density gradients are of similar amplitude and spatially overlap.
