Hot spot phenomena on Tore Supra ICRF antennas investigated by optical diagnostics

A systematic study of the thermal behaviour of ion cyclotron range of frequencies (ICRF) antenna front faces was undertaken on the Tore Supra tokamak over two years of plasma operation, by means of infrared (IR) and visible light cameras. Among the variety of edge-antenna interaction phenomena observed experimentally, the present paper focuses on the most deleterious effect for ICRH operation, a non-resonant radio-frequency (RF) process causing hot spots on the Faraday screen and bursts of metallic impurities. The hot spots phenomenology (time history, location) is presented. Their magnitude is characterized by semi-quantitative indicators, defined from the IR and visible light films. This makes possible a parametric study of the antenna–plasma interaction, over a wide range of experimental configurations. The roles of the local RF electric field and of the edge density regimes in the spurious process are outlined. The observed hot spot behaviour is compatible with the build-up of RF sheaths. Some ways of reducing the problems are suggested.

[1]  N. Hershkowitz,et al.  RF generated voltage on the Faraday screen of an ICRF antenna and its effect on the Phaedrus-T edge plasma , 1996 .

[2]  F. W. Perkins,et al.  Radiofrequency sheaths and impurity generation by ICRF antennas , 1989 .

[3]  L. Colas,et al.  Analysis of energy flux deposition and sheath transmission factors during ergodic divertor operation on Tore Supra , 2001 .

[4]  Sylvain Brémond,et al.  RF-sheath physics assessment of Tore Supra ICRF antenna designs , 2002 .

[5]  F. Louche,et al.  Modeling of the JET-EP ICRH antenna , 2002 .

[6]  J. Jacquinot,et al.  Radio‐frequency‐sheath‐driven edge plasma convection and interaction with the H mode , 1993 .

[7]  J. Jacquinot,et al.  The modification of the plasma edge and impurity production by antenna phasing during ICRF heating on JET , 1988 .

[8]  D. D'Ippolito,et al.  Faraday screen sheaths and impurity production during ion cyclotron heating , 1990 .

[9]  P. Ghendrih,et al.  REVIEW ARTICLE: Theoretical and experimental investigations of stochastic boundaries in tokamaks , 1996 .

[10]  P. M. Ryan,et al.  ICRF/Edge Interaction Guidelines for ICRF Antenna Design and Initial ICRF/Edge Interaction Experiments on the Tore Supra Tokamak , 1996 .

[11]  L. Colas,et al.  Edge plasma density convection during ion cyclotron resonance heating on Tore Supra , 2002 .

[12]  R. Guirlet,et al.  Plasma edge characterisation and control in ergodic divertor experiments on Tore Supra , 1999 .

[13]  A. Becoulet,et al.  Tore Supra steady-state power and particle injection: the ‘CIMES’ project , 2001 .

[14]  S. Heuraux,et al.  Numerical modeling of the coupling of an ICRH antenna with a plasma with self-consistent antenna currents , 2002 .

[15]  B. Beaumont,et al.  Tore Supra ICRH antennas for long pulse operation , 1997, 17th IEEE/NPSS Symposium Fusion Engineering (Cat. No.97CH36131).

[16]  G. Oost,et al.  The interaction between waves in the ion cyclotron range of frequencies and the plasma boundary , 1993 .

[17]  B. Beaumont,et al.  Interaction of ICRF power and edge plasma in Tore Supra ergodic divertor configuration , 2000 .

[18]  D. D'Ippolito,et al.  Three-dimensional analysis of antenna sheaths , 1996 .

[19]  J. Jacquinot,et al.  Assessment of beryllium Faraday screens on the JET ICRF antennas , 1992 .

[20]  Y. Peysson,et al.  Progress towards high-power lower hybrid current drive in TORE SUPRA , 2000 .

[21]  Sternberg,et al.  Dynamic model of the electrode sheaths in symmetrically driven rf discharges. , 1990, Physical review. A, Atomic, molecular, and optical physics.

[22]  J. Jacquinot,et al.  Impurity release from the ICRF antenna screens in JET , 1991 .