Expected performance of the upgraded HSX stellarator experiment

The Helically Symmetric eXperiment (HSX) at UW Madison, Wisconsin is the world's first neoclassically optimized stellarator. It started operation in 2001 and has since then significantly contributed to the understanding of neoclassical and turbulent transport in 3D magnetic field geometries. To further extend the operational space of HSX, the device is currently undergoing a major upgrade, consisting of a new 70 GHz electron cyclotron resonance heating system and an increase of the magnetic field strength to 1.25T. This upgrade will allow plasma operation at three times higher densities, which will significantly increase the ion temperature thanks to better coupling between electrons and ions and reduced charge exchange losses. Hence, studies of ion-temperature gradient modes, as well as the investigation of neoclassical transport of ions in the long mean free path regime, might become possible. In addition, stronger flows in the helical direction thanks to reduced neutral damping and smaller density gradient lengths are expected. This will likely reduce the growth rates and non-linear heat fluxes of collisionless trapped electron modes, which are believed to be the dominant type of instability in HSX plasmas limiting plasma performance.