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The sawtooth oscillations in tokamak discharges with Ohmic and neutral-beam heating could be suppressed when a large part of the plasma current was driven by lower hybrid waves (IHF/IP ^ 0.5). The stabilization is due to a flattening of the current profile j(r) and an increase of q(Q) above 1. Higher central electron temperatures are obtained with neutral-beam heating if the sawteeth are stabilized. The increase in total energy content in this case was 30% higher than in the presence of sawteeth.PACS numbers: 52.50.Gj, 52.35.Hr, 52.55.FaThe sawtoothlike oscillations usually observed in tokamaks have been considered for a long time as beneficial for stable and clean discharges. The timeaveraged power loss due to the sawtooth instability is small during Ohmic heating (OH) and is still not serious with additional neutral-beam heating. The situation has changed drastically with high-power ioncyclotron heating where giant sawteeth, with modulation of the central electron temperature by up to 50%, have been observed. 1 The resulting clamping of the central energy content could strongly impair core ignition in large fusion experiments. Methods for stabilization of the sawteeth are therefore of greatest interest.Application of lower hybrid (LH) waves may lead to the suppression of sawteeth in OH discharges. 2 * 3 On the ASDEX tokamak we have studied the influence of LH waves on the sawtooth period. Appropriate wave spectra and power requirements for suppression of sawteeth are discussed. Local current-profile measurements allowed clarification of the mechanism of sawtooth stabilization. Application to neutral-beam heating finally demonstrates the possible gain in total energy content in sawtooth-free discharges.The experiments reported here were performed in the divertor tokamak ASDEX (i? = 165 cm, a = 40 M). LH waves were injected with powers of up to F LH = 1 MW and pulse lengths of up to 1.5 s. Details of the LH system and of the plasma behavior with LH heating and current drive are presented elsewhere. 4 " 6

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First ICRF experiments on ASDEX have been performed at 67 M z , corresponding to ZECH-heating of a hydrogen plasma at Bo = ?.2T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up t o 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The rf heating is found to depend strongly on plasma preheating. In conjination with neutral beam injection the ICRF heating efficiency is even higher than the one of SI. Confinement degrades with ICRH to values in between XI-L-type and OH confinement.

Efficient radio frequency (rf) heating of a high-density plasma is obtained for any value of the containing magnetostatic field larger than that corresponding to electron cyclotron resonance.