Localized heat flux due to lower hybrid wave coupling in the Ergodic Divertor configuration on Tore Supra

Abstract

Lower hybrid (LH) current drive experiments were carried out in the ergodic divertor (ED) configuration. In this configuration, at low density (〈 n e〉=1.7 × 10 19–2.3 × 10 19 m −3), up to 4.5 MW of LH power was coupled and 65% of the plasma current (here equal to 1.4 MA to match the resonant edge safety factor needed for divertor operation) was driven by the LH waves. With an optimized position of the grills, it was possible to obtain a reasonable steady state temperature of the neutralizers ( T neut < 800°C) and 3.9 MW were coupled for 20 s leading to a record total injected energy of 93 MJ in this configuration. The power deposition can be derived from the surface temperature of the boron carbide coating of the neutralizers by infrared thermography. A well localized heat flux is observed on the divertor neutralizers magnetically connected to the grills. Such heat flux is known, from previous work, to be due to LH power dissipation near the grills. This heat flux is here studied in detail. In the shots analyzed here, parallel heat fluxes up to 15 MW/m 2 were measured, but values exceeding 50 MW/m 2 have been recorded. The use of the field line tracing code MASTOC allows one to link the power deposition on the neutralizer plate to specific regions in front to the grills, thus underlying the importance of the electromagnetic fields there. The edge density is shown to be a key parameter. Not-well-understood local effects arise when the LH is activated, and accurate measurement and analysis of local density and electric field are needed. It has to be noted that a trade-off between the decrease of the edge density at the grill and its coupling capability which drives the electric fields quoted above has to be dealt with; the positioning of the grill has then to be optimized and eventually feedback controlled. In order to increase the operational margin, the spreading of the heat flux was efficiently obtained by a moderate modulation of the divertor current by less than 30%.