Physical origin of ultra-high neutral pressures in the Large Helical Device

Abstract Efficient particle exhaust remains a key challenge in magnetic confinement fusion, particularly in stellarators, where divertor neutral pressures are typically low. Recently, exceptionally high neutral pressures-up to 2.4 Pa-were measured in the divertor of the Large Helical Device, exceeding theoretical predictions and previously observed stellarator values by an order of magnitude. We demonstrate that such high pressures can only be sustained through an additional particle exhaust channel formed by plasma condensation. At the same time, the divertor plasma is detached and exhibits strong hydrogen radiation. This divertor regime therefore appears to be an excellent candidate for fusion reactors based on the stellarator concept.