Considering a Novel Detached Divertor Solution Based on Thermionic Emission*

Thermionic emission from tungsten has an important influence on the PMI in tokamaks. Emission is often viewed as unfavorable since it occurs when the surface is hot and is thought to weaken the sheath and further raise the heat flux. Here, we will show that if thermionic emission across the divertor plates exceeds a critical threshold, it will cause a dramatic cooling of the target plasma temperature, which can be used favorably to induce detachment. This idea stems from recent work showing that the classical sheath breaks down and transitions to an inverse sheath when the emission coefficient exceeds unity [1]. In the inverse regime, a high density of cold thermionic electrons (tenths of eV) dominates the quasineutral plasma near the surface.  If induced in a divertor, it will ensure the formation of a cold target plasma with high recombination rates. This opens up the possibility of maintaining detachment without injecting impurities that contaminate the core.

[1] M.D. Campanell and M.V. Umansky, PoP 24, 057101 (2017) and PRL 116, 085003 (2016).