TJ-II:Effect of pellet injection on the radial electric field profile of stellarators: Difference between revisions

Pellet injection modifies in a very abrupt manner the density and electron temperature profiles, see e.g. <ref>McCarthy 2015 ISWH, McCarthy 2016 EPS</ref>, and this is known to affect radial transport (at least transiently) in such a way that can be described by neoclassical theory, see e.g. <ref>Velasco 2016 PPCF</ref>. In particular, through modification of the density and electron temperature gradients, it can affect the radial electric field profile, which is set by ambipolarity of the neoclassical fluxes. Positive radial electric fields, connected to hollow density profiles, have been found in the Large Helical Device after injection of a pellet close to the edge <ref>Dinklage 2016 IAEA</ref>. This would be a manifestation of the well-known positive ion-root feature <ref>[Baldzuhn 1999 PPCF]</ref>.

In principle, a positive electric field should lead outward convection for impurities, but this was not the case of W7-AS plasmas with core electron root and positive ion root at intermediate radial positions <ref>Baldzuhn 1999 PPCF, Hirsch 2008 PPCF</ref>. We will nevertheless keep an eye on the radiation signals from the bolometers as they can provide (when normalized by the electron density) an indication of the impurity content of the plasma.