Relationship between parametric and classical absorption mechanisms of lower hybrid waves in tokamak plasmas

A new parametric instability of the lower hybrid waves has been discovered previ-ously on the FT-2 tokamak at Ioffe Institute [1]. This instability is localised in a nar-row spatial region several millimetres wide with local densityne = (0.3–0.5)ne,LH, where ne,LH is the lower hybrid (LH) density. A very slow daughter wave with a frequency of about 0.7f0, f0 being the pump frequency, was observed here. Its spectrum consists of a single line several MHz wide, which indicates a highly coherent nature of the parametric process. A similar instability has also been ob-served in cold laboratory plasma [2]. An internal instability with a wide frequency spectrum has been observed on the ATC device [3]. It has been shown in the above FT-2 experiments that the wave absorption, fast ion generation and bulk ion heating are associated with this parametric decay instability [1]. Experiments carried out in 1993 and based on this mechanism could then achieve successful bulk ion heating with an increase of ion temperature from 100 to 400 eV and electron heating with an increase of electron temperature from 500 to 1000 eV [4–6]. The main device and discharge parameters are: the major radius R = 55 cm, minor radius at the limiter a = 8 cm, toroidal magnetic field B = 1–2.5 T, plasma current Ip up to 40 kA, plasma density (0.5–5.0)×1013 cm−3, electron temperature Te = 500 eV, ion temperature Ti = 100 eV. LH waves are produced using a 300 kW magnetron generator operating at frequency f = 920 MHz. The power is launched into the plasma by means of a double waveguide grill from the low field side of the torus. The typical N ‖ is about 2–3. It appears, however, that the most effective heating of ions was observed at a density of 5 × 1013 cm−3, where a lower hybrid resonance exists in the plasma centre. Therefore the possibility of classical LH resonance ion heating can not be excluded. An abrupt reduction of ion heating was observed when the density was increased above 0.5 × 1013 cm−3. This seems to be associated with a peripheral instability different from the above internal one.