Joint European Torus results with both fast and lower-hybrid wave consequences for future devices

Abstract

Heating and current drive studies were performed during the JET [Phys Fluids B 3, 2209 (1991)] 1990/91 operation using two large systems capable of generating either fast waves in the ion cyclotron range of frequencies (ICRF) or slow waves at a frequency above the lower-hybrid resonance (LH). The maximum wave power coupled to the torus reached 22 MW for ICRH and 2.4 MW for LH. The results obtained in plasma heating experiments qualify ICRH as a prime candidate for heating reactor grade plasmas. A centrally localized deposition profile in the cyclotron damping regime was demonstrated in a wide range of plasma density resulting in (i) record value nd τE Ti0 ≂ 7.8 × 1020 m−3 sec keV in ‘‘thermal’’ conditions Ti = Te ≂ 11 keV at high central densities generated by pellet injection; (ii) large normalized confinement 2.5 ≤ τE/τGoldston≤4. The large values of τE/τGoldston are reached in H-mode discharges (I≤1.5 MA) with large bootstrap current fraction IBS/I ≤ 0.7 ± 0.2; (iii) the highest to date D–3He fusion power (140 kW) generated with 10–14 MW of ICRH in the L-mode regime at the 3He cyclotron frequency. All specific impurity generations have been reduced to negligible levels by proper antenna design and the generic difficulty of wave–plasma coupling has been greatly reduced using feedback loops controlling in real time the antenna circuits and the plasma position. Current drive efficiencies γ=ICD〈ne〉R/P ≂ 0.4 × 1020 m−2 A/W have been reached in 1.5 MA L-mode plasma with zero loop voltage by combining LHCD and ICRH. Fast electrons are driven by LHCD alone to tail temperatures of up to 70 keV. The fast electron density is peaked in the plasma center at lower densities (ne0 ≤ 2.6 × 1019 m−3) and high field (Bφ ∼ 3.1 T). In these conditions, the fast electrons are further accelerated (even at zero loop voltage) to tail temperatures above 150 keV by heating the plasma with ICRF in monopole phasing. Direct electron damping of the fast wave on the fast electrons created by LH appears to be the driving mechanism of this synergism which produces fast wave current drive even without phasing the ICRH antennas. Finally, the first results obtained in the minority current drive regime are reported. The sawtooth instability is considerably modified when the resonance is located near the q=1 resonance. The effect is reversed from stabilization to destabilization when the phase of the ICRH antennas is reversed from +90° to −90°.