Reduction of Neoclassical Transport and Observation of a Fast Electron Driven Instability with Quasisymmetry in HSX

The Helically Symmetric Experiment (HSX) is the first operational quasisymmetric stellarator, with neoclassical transport at low collisionality greatly reduced compared to a conventional stellarator. Experimental differences in the density and temperature profiles are observed between the quasihelically symmetric (QHS) configuration and a configuration with the symmetry intentionally broken. The central electron temperature in the QHS configuration is significantly higher than in the non-symmetric configuration with the same launched power. The density profile in the QHS configuration is always centrally peaked, regardless of heating location, whereas in the non-symmetric configuration the core density profile is flattened with central heating. Transport analysis shows that this flattening is due to neoclassical thermodiffusion, which is reduced in the QHS configuration. Measurements show that the thermal conductivity is reduced in the QHS configuration by an amount comparable to the neoclassical reduction. An unexpected consequence of the improved confinement in the QHS configuration is that a fast particle population drives an Alfvénic instability. We report on experimental observations of a fast electron driven Alfvénic mode in QHS plasmas. These modes are coherent and global, peaking in the plasma core. The measured frequency range, propagation direction, and scaling with ion mass density are consistent with theoretical predictions for the Global Alfvén Eigenmode. When symmetry is broken, the mode is no longer observed.