Systematic investigations of the scrape-off layer (SOL) in the midplane of ASDEX have been carried out in He, D2 and H2 for diverted ohmic discharges over a wide range of plasma conditions: ne ∼ 0.5−4.7 × 1013 cm−3, Ip = 200–450 kA, BT = 16–23 kG, qa∼ 2.4–4.4 and POH = 200–480 kW. For the first two cm outside the separatix, ne is found to decay exponentially with an e-folding length λn given by λn = kqα (He, k = 1.32 cm, α = 0.52; D2, k =1.29 cm, α = 0.35; H2, k = 1.18 cm, α = 0.4) when from which follows for qa = 3: λn(D2) ∼ λn(H2) ∼ 0.8 λn(He). The qα scaling is roughly predicted by the simple formula λn = D⊥ Lυ∥ under the assumption D⊥ ∝ mi−0.5 (as has been observed on ASDEX for H2 and D2). There appears to be no explicit λn dependence on heating power. λn varies strongly with ne in the range ne ≤ 1 × 1013 cm−3, decreasing for example (D2,H2; qa = 3.0), from λn ≥ 3 cm at ne ∼ 0.5 × 1013 cm−3 to λn ∼ 1.9 cm for ne ≥ 1.5 × 1013 cm−3, ne at the separatrix is primarily a function of ne.

Measurements of the time evolution of the current-density distribution in ASDEX show that lower-hybrid current drive leads to broader profiles, whereby q increases from q\ensuremath{\lesssim}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.

Neutral beam (NI), ion cyclotron resonance (ICRH) and lower hybrid resonance (LHRH) heating on ASDEX are discussed with regard to their effect on plasma confinement. Comparison of NI and ICRH shows that the L and H-regimes are universal confinement modes of auxiliary-heated tokamak plasmas (i.e. independent of the heating method), 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 NI heating. Studies of energy confinement in the intermediate regime from ohmic to NI heating reveal a gradual transition from ohmic ( approximately ne) to neutral injection L-mode ( approximately I) scaling. At the same time a remarkable invariance of electron temperature profile shape with increasing heating power is observed. Changing the NI power deposition profiles from central to off-axis leaves gross energy confinement times unchanged while central confinement is substantially improved.

Confinement properties of ASDEX discharges are studied when NI and ICRH or NI and LH were applied simultaneously. ICRH leads to a larger change in plasma energy content when applied together with NI. At high power the beta p-increases of NI and ICRH are additive corresponding to the total heating power. In the combination of NI and LH the beta p-increase is the same when NI is applied to inductively or non-inductively driven plasma current despite the different current profiles. The effect of sawteeth on central confinement is described in detail in ohmically and beam heated plasmas as is the suppression of sawteeth by LH. The suppression of sawteeth by LH in low qa beam heated plasmas leads to a substantial increase of beta p. The H-mode can be realized with NI under current drive conditions. ICRH can cause the H-transition in combination with NI. The H-mode, however, can also be realized with ICRH alone.

High resolution X-ray spectroscopy was used to evaluate the fractional population of non-thermal electrons generated when electromagnetic waves in the lower hybrid frequency range are launched in the ASDEX tokamak. In complete current drive discharges a typical value of 1% is found for this parameter, in agreement with a theoretical estimate. Furthermore, it is shown that the toroidal electric field is very effective in enhancing tail electron generation, as confimed by β measurements, and that the wave penetration to the plasma centre is inhibited when interaction of the waves with plasma ions is the dominant damping mechanism.

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 (IHFIp≈0.5). The stabilization is due to a flattening of the current profile j(r) and an increase of q(0) 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.Received 2 June 1986DOI:https://doi.org/10.1103/PhysRevLett.57.1137©1986 American Physical Society

Interaction of lower hybrid waves at 1.3 GHz with ions and electrons was studied in the density range 0.2-5*1013 cm-3 in the ASDEX tokamak. At high densities, ne>or approximately=4*1013 cm-3, fast ions with mainly perpendicular velocities are produced by the RF power at the plasma periphery. They are not well confined and do not lead to any bulk plasma heating. At lower densities, 2*1013<or approximately=ne<or approximately=4*1013 cm-3, electron and ion heating is observed. The heating is better in deuterium than in hydrogen plasmas. At very low densities, ne<or approximately=2*1013 cm-3, the discharge becomes suprathermal as soon as the RF power is switched on. Launching an asymmetric spectrum of waves in a low density plasma leads to the generation of an RF-driven DC-plasma current.