Te profile invariance under transient conditions on ASDEX

The resilience of the electron temperature profile in a tokamak plasma is a well known phenomenon, but not at all understood. On ASDEX it was found that the normalized shape of the electron profile can be represented as a fairly straight line with constant slope outside the q=1 surface; the different Te profile shapes at various qa values are only due to changes within the q=1 surface; a similar behaviour has also been found on other machines. As the safety factor seems to be the only clear parameter to influence the temperature profile, it is often argued that the current density profile is actually invariant and the stiff coupling j∼T3/2 causes the Te profile consistency. This early analysis was carried out under steady state conditions, and is now extended to transient ASDEX plasmas with the plasma current being stepped up and down within the same discharge. The conductivity of the plasma column was calculated radially and time resolved including neoclassical trapped particle effects using the measurements of Te(r,t) and ne(r,t) with the 60 Hz Nd:YAG laser scattering system. Current profiles were then calculated with a diffusion code starting with a radially constant electric field from a steady state discharge and the electric field at the plasma edge derived from the measured loop voltage around the torus as boundary condition. (author) 2 refs., 4 figs