Lithium coating trick backfires in fusion reactors, speeds up metal pollution
Researchers at Japan's Large Helical Device found that spraying lithium granules—a standard technique to improve plasma performance—unexpectedly accelerates the escape of heavy metals like molybdenum from fusion cores. The discovery matters because stellarators and other fusion designs rely on keeping contaminants contained; if wall coatings degrade confinement, it could undermine efficiency gains and complicate reactor operations.
Originaltitel: Enhancement of mid-/high-Z impurity transport by continuous Li-granule dropping in a stellarator plasma
An enhancement of core impurity transport is observed in high-density plasmas of the stellarator large helical device, heated by neutral beam injection, when continuous lithium (Li) granule dropping is performed. In the reported experiments, in which the tracer-encapsulated solid pellet is employed to inject trace amounts of titanium (Ti) and molybdenum (Mo) into the plasma core, confinement times for these impurities are seen to reduce significantly when Li dropping is applied, this reduction being more notable for Mo. To gain some initial insight into these observations, simulations are performed using the drift-kinetic transport code SFINCS for the Mo case. These simulations indicate that, while neoclassical transport prevails for the main plasma components (electrons, majority ions, and low-Z impurities), the classical contribution appears dominant for transporting Mo impurities. In summary, this work reports the first experimental observation of the degradation of mid-Z and high-Z impurity confinement induced by the continuous dropping of Li granules into a high-density stellarator plasma. In the case of the Mo impurity, simulations suggest that classical transport is the key mechanism underlying the enhanced impurity transport.