Recent progress in L–H transition studies at JET: tritium, helium, hydrogen and deuterium

Abstract

We present an overview of results from a series of L–H transition experiments undertaken at JET since the installation of the ITER-like-wall (JET-ILW), with beryllium wall tiles and a tungsten divertor. Tritium, helium and deuterium plasmas have been investigated. Initial results in tritium show ohmic L–H transitions at low density and the power threshold for the L–H transition (PLH) is lower in tritium plasmas than in deuterium ones at low densities, while we still lack contrasted data to provide a scaling at high densities. In helium plasmas there is a notable shift of the density at which the power threshold is minimum (ne,min) to higher values relative to deuterium and hydrogen references. Above ne,min(He) the L–H power threshold at high densities is similar for D and He plasmas. Transport modelling in slab geometry shows that in helium neoclassical transport competes with interchange-driven transport, unlike in hydrogen isotopes. Measurements of the radial electric field in deuterium plasmas show that Er shear is not a good indicator of proximity to the L–H transition. Transport analysis of ion heat flux in deuterium plasmas show a non-linearity as density is decreased below ne,min. Lastly, a regression of the JET-ILW deuterium data is compared to the 2008 ITPA scaling law.