Review of recent advances in heating and current drive on TEXTOR

Abstract

Co-injection (DO --> D+) applied to TEXTOR leads to a hot ion node regime with enhanced confinement. A synergistic increase of the beam effects is observed with the addition of ICRH at omega = 2omega(cD) = omega(cH) (H minority heating scenario) resulting, beside other reviewed effects, in a significant increase of the ion temperature and of the beam driven current (respectively larger than 30 % and 50 % for the addition of an RF power comparable to the NBI one). The large ion heating efficiency of ICRH also remains when ICRH is added to balanced injection and the hot ion mode regime remains up to the maximum achieved beta (=2/3 of the Troyan limit with more than 6MW of auxiliary heating). ICRH also leads to the formation of a more 'isotropic' tail. These results are interpreted with the help of a Fokker-Planck code which computes the beam distribution function in presence of RF and of TRANSP simulations. The amount of RF absorption by the H minority by the ion beam and die bulk plasma is theoretically evaluated. It is shown that a large part of the synergistic effects can be explained by the rise of the electron temperature due to the minority heating which increases the beam slowing down time and its critical energy. A smaller contribution to the effects is due to direct coupling of the RF power to the beam (less than 10 % of the total RF absorbed power) and to the decrease of the plasma toroidal rotation induced by the RF. ICRH has also been added to co-injection at omega = 3omega(CD). In this case no minority heating is present and the RF energy coupling to the beam is one of the dominant effects. It leads to the formation of a very energetic tail of the ion beam with a strong increase of the beam-target neutron reactivity.