Ion cyclotron resonance heating in the divertor tokamak ASDEX

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The main topics of ICRF investigations in ASDEX are the influence of the divertor on impurity production and transport in ICRH heated discharges, and the heating efficiency and plasma confinement in various scenarios (minority and harmonics regimes). The first experiments were conducted in November 1984 at 67 MHz, corresponding to second harmonic heating of a hydrogen plasma at B0=2.2 T. A transmitted power of 2.5 MW has been reached so far, the total capability being 3 MW. A linear increase of the central electron and ion temperature with the rf power is observed in Ohmically preheated plasmas (ΔTe∼280 eV, ΔTi∼500 eV, ΔWp∼17 kJ at a power of 1.2 MW coupled to the plasma and n̄e=3.5×1013 cm−3). The total radiation increases linearly with the power, too, and the ratio Prad/Ptot∼0.35 stays approximately constant. However, first investigations indicate that with a divertor, ICRF operation is also accompanied by a significant increase in impurity production. The presence of neutral beam injection in addition to ICRH clearly enhances the absorption of the wave energy from about 50% to up to 90% with respect to the coupled power. With neutral beam injection (PNI≤3.5 MW) the increment of the plasma energy content due to ICRH (Prf≤2 MW) is found to be almost twice as large as in case OH+ICRH. Global heating efficiencies of up to 3×1013 eV/kW cm3 compare quite well with other ICRH experiments. First observations indicate a degradation of plasma confinement with ICRH to values in between L-type and OH confinement.