Stable Small Plasmas at the Density Limit in the W7-X Stellarator

Experiments in the Wendelstein 7-X (W7-X) stellarator with plasma density beyond the established density limit in stellarators [Scalings of energy confinement and density limit in stellarator/heliotron devices, Nucl. Fusion 30, 11 (1990).NUFUAU0029-551510.1088/0029-5515/30/1/002] resulted in unusual, reduced size, fully radiative plasmas lasting stably for many confinement times [Small, stable plasmas, fully decoupled from the plasma-facing components in W7-X, Stellarator News 175, 1 (2021).]. In these experiments, the plasma had shrunk to a significantly smaller minor radius due to intense edge radiation from a radiating mantle and had no contact with material components, yet it remained relatively well confined for many confinement times with electron temperatures on the order of 1 keV. To our knowledge, this is the first definitive experimental long-time stability demonstration of such high-temperature contactless plasmas in the laboratory. It is shown that the onset and extent of this fully radiative small plasma state is predictable. The stability of the plasma size is elucidated with a simple power-balance model. In other helical devices such as the Large Helical Device (LHD) and the Wendelstein 7-AS (W7-AS), plasmas that start shrinking suffered from core impurity accumulation leading to a collapse whereas this was generally not observed for these W7-X plasmas.