The Role of Divertor and SOL Physics for Access to H-mode on JET

To achieve the goals in ITER operation in the high confinement H-mode regime is crucial. H-mode is accessible above a threshold heating power, Pthr, marked by the formation of an edge transport barrier. Regression of data from todays tokamaks provides an empirical scaling of the form: PthrMartin'08 = 0.049n0.72 BT0.80 S0.94. Here ne20 is the line averaged density in 1020 m­3, BT the magnetic field at the geometric axis in T and S the plasma surface area in m2. This scaling law, however, is only applicable above a certain minimum density, ne,min, since a U shaped dependence of Pthr as function of density is commonly observed. According to a detailed analysis of the heat flux through the plasma edge on AS DEX Upgrade the density at the minimum Pthr scales like nemin 0.71 IP0.34 BT0.62 a­0.95(R/a)0.4 and is determined by the collisional coupling between electrons and ions. Other explanations involve the parallel heat transport regime of the scrape-off layer (SOL), although in this work the closure of the equations and the assumption of a constant SOL collisionality leading to the observed U shape is technically wrong. However, this can be rectified by using the equally valid radial density decay length in the S OL as substitution for the unknown heat-flux decay length rather than the temperature decay length. The impact of divertor and SOL geometry on H-mode access has long been known and is an active area of research on many devices. Whilst the good agreement of the scaling of nemin with the measurements in several devices presented in [6] is a strong support for the role of the energy exchange term, the differences of Pthr as function of density measured on JET with C as plasma facing components (JET-C) compared to the ITER like W/Be wall (JET-ILW) in the same divertor geometry (MKII-HD) points towards the role of the SOL. The JET-ILW data shows U shaped density dependence of Pthr and a reduction of about 30% in the high density branch compared to JET-C (MKII-HD), where in the same density range Pthr µ n0.7 . However, a minimum of Pthr with ne20 had been observed in JET-C previously with the more closed MKII-GB divertor. In this paper we compare recent results from JET-ILW with both strike points on the vertical targets (VT) to the JET-ILW data
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