The evolution of current density profile in MAST discharges

A characteristic feature of small aspect ratio tokamaks is the high fraction of banana trapped particles that do not conduct the plasma current. In comparison with the large aspect ratio tokamak plasma, the current conductivity is much reduced and is more peaked. This gives the effect of faster current penetration into the plasma column if we take the plasma parameters, density and electron temperature, approximately the same. It can be said that plasma current is pushed into the plasma core by the banana particles. Special experiments were performed to test the current penetration on MAST at conditions when neoclassical conductivity plays an important role (R=0.7m, a=0.5m, Ipl~0.5MA, <n>~ 1*1019m-3, Te ~ 0.5keV). The experimental information on the current profile characteristics is taken from the electron temperature and density profiles obtained by the MAST Thomson Scattering diagnostic, the time of the appearance of the q=2 and q=1 magnetic surfaces, and the location of the sawtooth inversion radius observed for the SXR cameras. Results have been obtained showing different penetration at different conditions. The calculations of current profile evolution are carried out using the ASTRA code [1] with neoclassical conductivity. Modelling with ASTRA shows that the increase of the plasm