Peculiar properties of internal transport barrier formation near the q = 1 and q = 2 surfaces in tokamaks

Abstract

A new type of internal transport barrier (ITB) was found in the T-10 tokamak plasma for the first time. It is created by sawtooth oscillations which are almost damped by off-axis electron cyclotron resonance heating and electron cyclotron current drive (ECRH and ECCD). During the ITB formation, the electron temperature, Te, at a relative radius r/a ≤ 0.45 rapidly rises to a new quasi-steady state, while the turbulence level, which is measured at the ITB, falls below its pre-crash value, and the turbulence spectrum shrinks in a wide range of frequencies. The ITB appears near the q = 1 magnetic surface and exists for approximately 50–70% of the plasma energy confinement time. During this process, accumulation of impurities was not observed. An abrupt and non-local reduction of the electron heat flux (ITB-event) around the q = 1 surface (within 1 ms in the spatial zone with a relative radius 0.2 < r/a < 0.5) was observed for the first time for the case with a gas puffing cut-off in OH T-10 discharges with an increasing high density. In contrast to the ITB that forms during the ECR heating, in this case, impurities begin to accumulate simultaneously with the ITB event. Analysis of the D-III-D experiments with a weak reverse magnetic shear shows that the appearance of a qmin = 2 surface leads to this ITB event in a very wide spatial region (the relative radius of the plasma column is 0 < r/a < 0.7).