Lower-hybrid current drive experiments in TORE SUPRA

Lower-hybrid current drive (LHCD) experiments performed in TORE SUPRA [Plasma Physics and Controlled Nuclear Fusion Research, 1988 (IAEA, Vienna, 1989), Vol. 1, p. 9] are reported. Two large ‘‘multijunction’’ launchers have allowed to couple up to 6 MW to the plasma with a maximum power density of 45 MW/m2 and reflection coefficients lower than 3%. The current drive efficiency was about 2×1019 Am−2/W with LH power alone at a volume-averaged electron temperature 〈Te〉=1.4 keV, and a 22 sec long quasistationary discharge could be sustained by applying 2.8 MW during an 18 sec/1.6 MA current flattop at a line-averaged density n̄e=3×1019 m−3. Stable LH current ramp-up assist was achieved, thus reducing the resistive flux consumption with an efficiency of 0.7×1019 Wb m−1/MJ. Experiments with combined LHCD and ion cyclotron resonant heating allowed to inject up to 7.5 MW into the plasma. The electron energy content followed fairly well the Rebut–Lallia scaling law [Plasma Physics and Controlled Nuclear Fusion Research, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 191]. At n̄e=1.5×1019 m−3, sawteeth were suppressed and m=1 MHD (magnetohydrodynamics) oscillations appeared. The central electron temperature then reached 8 keV for 3.6 MW injected. Lower-hybrid power modulation experiments performed at n̄e=4×1019 m−3 showed a delayed central electron heating despite the off-axis creation of suprathermal electrons, thus ruling out the possibility of direct heating through central wave absorption. Successful pellet fueling of a partially LH-driven plasma was obtained, in which 28 successive pellets could penetrate almost to half radius as in Ohmic discharges, with 50% to 80% of the pellet content deposited in the plasma. First attempts to combine LHCD with ergodic divertor discharges showed that, when the plasma edge was subject to a radial magnetic perturbation smaller than the ergodicity threshold, a strong stationary radiation (MARFE) was triggered, locked near the inner wall. The radiated power then amounted to 90% of the total input power with no indication of a radiative collapse.