Ultra-High Neutral Pressures in the Helical Divertor of the Large Helical Device

In the Large Helical Device (LHD), plasma operation is possible with a helical divertor and inward-shifted configurations. For plasmas with a small distance between the divertor and the plasma, ultra-high neutral pressures of up to 2.4 Pa were measured in the helical divertor–ten times higher than theoretically predicted. This enables high particle exhaust rates, comparable to tokamaks such as ASDEX Upgrade and DIII-D, and is caused by a condensation instability near the divertor targets. Like a classical Marfe, it exhibits an asymmetric radiation pattern, but the new phenomenon is dominated by hydrogen radiation. We denote it as HYDRA, which stands for HYdrogen Dominated Radiation Asymmetry. The appearance of the HYDRA induces detachment of the divertor plasma, which, together with the ultra-high divertor neutral pressure, leads to a favorable divertor state for a fusion reactor based on the stellarator concept.