Modeling Future Plasma Performance in the HSX Stellarator with a new 70 GHz Gyrotron and Neutral Beam Injection*

The Helically Symmetric eXperiment (HSX) is a neoclassically optimized stellarator operated by the University of Wisconsin, Madison. The experiment uses 48 non-planar modular coils to generate a quasi axisymmetric magnetic field structure that ensures good confinement of trapped particles and allows for weakly damped flows, well known for improved heat and particle confinement. So far, excellent thermal confinement has been demonstrated for the electron channel in the core of the device [1]. To extend studies towards the ion-channel, two major upgrades are planned. First, a new 70 GHz Electron Cyclotron Resonance Heating (ECRH) system is being installed which provides a significantly higher cut-off density (3 x 10¹⁹/m³) compared to the existing 28 GHz system. Hence, higher density operation will become possible with improved coupling between the heated electrons and the ions. Secondly, the installation of a neutral beam injection (NBI) system is planned which features 25 kV and 170 kW of heating power. A tangential injection geometry has been identified for which shinethough is only on the order of 20% and which injects fast ions on to well confined orbits. Predictions of the expected performance of HSX will be presented which suggest that ion temperatures in the range of 200 eV will be achieved during combined ECRH and NBI heating with beta values above 0.1%.

[1] J. M. Canik et al, PRL 98, 085002 (2007)