The paper summarizes the experiments performed with ion cyclotron resonance heating (ICRH) on ASDEX, from November 1984 until March 1986; the most interesting results are reported and discussed in detail. Heating and confinement studies using the hydrogen second harmonic scheme and the hydrogen minority scheme (PIC < 2.6 MW, tIC < 1.5 s) show a typical L-mode behaviour, i.e. a power dependent confinement degradation, which is rather similar to that found with neutral beam injection (NBI) heating. ICRH is accompanied by a slightly improved particle and energy confinement compared with that of NBI; this is also true for a combined ICRH + NBI scheme, up to Ptot ≈ 4.5 MW, absorbed in the plasma. Particular efforts have been devoted to investigations of the second harmonic regime in H/D plasmas with nH/ne ≈ 0.1 - 1, with a view to heating mixtures in reactor relevant plasmas. The achievement of H-mode transitions with ICRH alone in the hydrogen minority scheme at an absorbed RF power of about 1.1 MW supports the assumption of common confinement properties in auxiliary heated tokamaks, since they appear to be widely independent of the additional heating method. ICRH specific impurity problems, such as the strong release of iron from the vessel walls, have been overcome by applying extensive in situ wall carbonization. The mechanisms responsible for impurity generation have partly been identified and analysed; however, the problem still remains to be solved. Impurities preferentially released from the ICRH antenna do not pose problems.

The H mode in ion cyclotron-resonance-heated plasmas has been investigated with and without additional neutral beam injection. Ion cyclotron-resonance heating can cause the transition into a high-confinement regime (H mode) in combination with beam heating. The H mode, however, has also been realized—for the first time—with ion cyclotron-resonance heating alone in the D (H)-hydrogen minority scheme at an absorbed rf power of 1.1 MW.Received 4 September 1986DOI:https://doi.org/10.1103/PhysRevLett.58.124©1987 American Physical Society

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.

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.

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