(EX/P6-12) Interactions of RF Antennas with the Edge Plasma in Tore Supra Steady-State Discharges

M. Goniche1), L. Colas1), A. Ekedahl1), M. Chantant1), V. Petržílka2), V. Basiuk1), Ph. Bibet1), L. Delpech1), L.-G. Eriksson1), J. Gunn1), S. Heuraux3), E. Joffrin1), F. Kazarian1), G. Lombard1), C. Lowry1), L. Millon1), P. Mollard1), P. Moreau1), C. Portafaix1), M. Prou1), H. Roche1), J.M. Travère1)
 
1) Association Euratom-CEA, Saint Paul-lez-Durance, France
2) Association EURATOM/IPP.CR, Prague, Czech Republic
3) LPMIA,UMR 7040 CNRS,BP 239 F-54506 Vandoeuvre cedex, France

Abstract.  High frequency antennas operating in the Ion Cyclotron Range of Frequency (ICRF) or the Lower Hybrid Range of Frequency (LHRF) are exposed to high heat flux deposition from the plasma. The gap between the plasma and the last closed flux surface has to be as large as possible while still ensuring good wave coupling. In ITER it is foreseen to have the antennas embedded in the first wall. This should guarantee low enough heat flux from the convected/conducted power. However specific phenomena arising from the intense oscillating electric field in front of the antennas, leading to the acceleration of ions and/or electrons, are up to now not fully taken into account. The involved mechanisms have to be investigated in order to ensure safe and controlled operation in future fusion devices. On Tore Supra, the total area of the three ICRF antennas and two LHRF launchers are monitored by an infrared (IR) system. The IR data is complemented by calorimetric measurements of the energy extracted by the cooling loops on the antennas and their side limiters. The long pulse capability of Tore Supra allows to assess the plasma-antenna interaction in steady-state plasmas. Both fixed and reciprocating Langmuir probes are used for studying the local density perturbations. In particular, a detailed 2D mapping of the scrape-off layer perturbation in the vicinity of an ICRF antenna has been performed for the first time, using a reciprocating Langmuir probe. The detailed analysis of the IR images of the ICRF antennas yield that at least four different phenomena can arise during combined ICRF-LHRF heating: i) The formation and rectification of an RF sheath. ii) Fast electrons (1 keV) accelerated by Landau damping in the near-field of the LHRF antenna. iii) The conducted/convected power from the plasma. iv) Interaction by fast ions (500 keV) created by ICRF in the hydrogen minority heating scheme. This identification of heat flux deposition sources has been integrated in the real-time control system of Tore Supra in order to optimize the RF power output without deleterious effects. The different scalings can be used for the design of the ITER RF antennas.

Full paper available (PDF)