Measurements of the edge current evolution and comparison with neoclassical calculations during MAST H-modes using motional Stark effect

Edge localized modes (ELMs), that are present in most tokamak H-(high confinement) modes, can cause significant damage to plasma facing components in fusion reactors. Controlling ELMs is considered necessary and hence it is vital to understand the underlying physics. The stability of ELMs is typically expressed in terms of the pressure gradient del p in the edge and the edge current density j(phi) Both del p and j(phi) are usually derived from profiles fitted to the measured edge density and temperature profiles, where for the calculation of j(phi) neoclassical theory is used. This paper presents direct measurements of the magnetic pitch angle gamma(m) evolution in the edge and the derived j(phi). These provide a method to validate the j(phi) as derived with neoclassical theory and they open up the possibility to find a complete, self-consistent set of edge profiles, that fit density, temperature and gamma(m) measurements, hence allowing for a more accurate stability analysis.