Simple relations between scrape-off layer parameters of high recycling divertors Part I: The relation between `upstream' density and temperature

Abstract

It has been found that values of neu and Teu in the SOL of singlenull, high recycling divertor discharges in JET and Alcator C-Mod arerelated approximately as Teu ∝ neu1/2, where (Teu,neu) are thevalues of plasma temperature and density on any given flux tube at the`upstream' end, for example, halfway between the two targets. Using thestandard two point model for the high recycling (conduction limited)divertor, this result is shown to be directly related to criteriabased on plasma collisionality - which have to be satisfied if theflux tube is to be in the conduction limited (high recycling) regimerather than the sheath limited (low recycling) regime or the detachedregime: the (Teu, neu) points for conduction limited flux tubesare predicted to fill a (somewhat narrow) band in the (Teu, neu)plane. Secondly, a tendency for the collisionality, as calculated forupstream conditions, to remain constant across the SOL is noted andan explanation is given. The latter then implies a tendency forTeu ∝ neu1/2, to hold, not just at the separatrix but across theSOL. Thirdly, for the JET data a further re-enforcing factor tendingto cause a rather strong Teu ∝ neu1/2 correlationis identified in the dependence of χSOL⊥ on the plasma parameters. Thevalues of χSOL⊥ were extracted from the JET datausing an onion skin method analysis. The first effect is expectedto be general to tokamaks operated in the highrecycling/conduction limited regime, although it only states that(Teu, neu) values will fall within a certain band, and adecreasing value of Teu with increasing neu is not ruled out. It isnot known at this time how general the second two effects are.