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

Measurements of the time evolution of the current-density distribution in ASDEX show that lowerhybrid current drive leads to broader profiles, whereby q increases from q < 1 to q > 1 in the plasma central region. Simultaneously, the electron temperature is observed to peak, thus demonstrating that the lower-hybrid-driven current distribution is decoupled from the classical conductivity profile.

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We have continued our investigations on particle losses from cesium plasmas in the Wendelstein stellarator. At the Culham Conference we reported on measurements 1 obtained in a stellarator magnetic field with helical windings of type I = 2. 2 We found particle-loss rates to be much less than the anomalously high "pumpout" losses usually encountered in stellar ator s. 3 Moreover, the relationship observed between ion input flux and the resultant particle density distribution was in agreement with calculations which assumed resistive diffusion across the magnetic confining field and recombination on the insulating surfaces of the supports of the plasma source, the latter constituting the predominant loss process. 4 In recent experiments, which will be described in this Letter, surface recombination losses within the plasma volume were made negligibly small by minimizing the surfaces of the supports of the plasma source. In this way we have been able to show the radial transport of the plasma to be governed by resistive diffusion.Our machine has a race-track shape with an axial length of 319 cm and a tube diameter of 5 cm. The main magnetic field can be pulsed up to 15 kG for about one second; the 1 = 2 helical windings yield a maximum angle of rotational transform of 0.477 at a main field strength of 11 kG. A small correction field transverse to the magnetic axis can be applied from two pairs of auxiliary windings which encircle the machine. The plasma is produced by contact ionization on a hot tantalum sphere, 5 mm in diameter, which is hung from a thin (25 /im in diameter) tungsten wire. The emitting sphere is heated by bombardment with a beam of energetic electrons from a gun outside the plasma volume. This electron beam is switched off during the time of the experiment and the emitting sphere is allowed to assume its floating potential. The total ion inpu: flux is determined from the ion saturation current drawn when a voltage is applied between the emitting sphere and the vacuum vessel with no magnetic field present. Two small cylindrical electrostatic probes (50 /im diam, 5 mm length)

Xeutral beam (NI), ion cyclotron resonance (ICRH) and lower hybrid resonance (LIIKV) heating on ASDEX are discussed with regard to their effect on plasma confinement. Comparison of SI and ICRH shows that the L and H-regimes are universal confinenent nodes of auxiliary-heated tokamak plasmas (i.e. independent of the heating nethod), and that the edge electron temperature (or a related parameter) dictates which mode prevails. In this connection it is noted that carbonization of the vessel walls impedes transition to the H-mode in the case of SI heating. Studies of energy confinement in the intermediate regime from ohmic to SI heating reveal a gradual transition from ohnic (-se) to neutral injection L-mode (-I) scaling. At the same tine a renarkable invariance of electron tenperature profile shape with increasing heating power is observed. Changing the XI power deposition profiles fron central to off-axis leaves gross energy confinement tines unchanged while central confinement is substantially inproved. Detailed investigations of the O-linit confirn the relation &$ [ %] = 2.8 I/(a B) [HA*n-l*T-l]. They show that the &limit is independent of heating method (NI, ICRH) and confinenent regine (L or H), but strongly dependent on the current density profile. indirect evidence suggests ballooning (and possibly surface kink) nodes to be responsible for the limitation in plasma pressure. LH heating and current drive show improved energy confinement (conpared to OH) at low densities (necl -2 x 1013 ~n -~, suprathermal electrons), but a degradation in confinement at higher densities.