Review of lower hybrid results in the Wega and Petula experiments

Along these lines, a large part of the experimental program of the Grenoble laboratory has been devoted for several years to the study of heating at the lower hybrid frequency, first on the WEGA experiment, (a cooperative experiment with IPP GARCHING) and, more recently, also on the PETULA device. In particular, at HF heating power of the order of the ohmic heating power, significant ion heating (increase of ion temperature by 100%) and electron heating (electron temperature increase by 60%) have been observed on the WEGA device and published in 1976 and 1978. Nevertheless, despite the increasing number of experimental results, detailed investigations of the power balance during the HF heating pulse have remained scarce. Consequently, several important questions such as the identification of the heating mechanism, the origin of the observed fast ion tail and the often observed density increase during the HF remain to be investigated. Based on the power balance carried out in WEGA, a first attempt at an explanation of these various effects was carried out. A summary of this work constitutes the first part of this paper. Another very important aspect of lower hybrid heating consists in the possibility of coupling the lower hybrid wave to the plasma by means of a phased waveguide array, the 'grill'. In order to confirm the advantages of the grill structure up to the power density levels necessary in the future, specific experiments have been carried out in the PETULA device. Several results obtained there are presented in the second part of this paper. In the final section the directions of present research necessary in order to ensure applicability to large machines are briefly discussed