Theory and Practice in ICRF Antennas for Long Pulse Operation
- Association Euratom-CEA, CEA/DSM/DRFC, Centre de Cadarache, 13108 Saint-Paul lez Durance (France)
- LPMIA, Unite du CNRS 7040, Universite Henri Poincare, Nancy 1, BP 239, 54506 Vandoeuvre Cedex (France)
Long plasma discharges on the Tore Supra (TS) tokamak were extended in 2004 towards higher powers and plasma densities by combined Lower Hybrid (LH) and Ion Cyclotron Range of Frequencies (ICRF) waves. RF pulses of 20sx8MW and 60sx4MW were produced. TS is equipped with 3 ICRF antennas, whose front faces are ready for CW operation. This paper reports on their behaviour over high power long pulses, as observed with infrared (IR) thermography and calorimetric measurements. Edge parasitic losses, although modest, are concentrated on a small surface and can raise surface temperatures close to operational limits. A complex hot spot pattern was revealed with at least 3 physical processes involved : convected power, electron acceleration in the LH near field, and a RF-specific phenomenon compatible with RF sheaths. LH coupling was also perturbed in the antenna shadow. This was attributed to RF-induced DC ExB0 convection. This motivated sheath modelling in two directions. First, the 2D topology of RF potentials was investigated in relation with the RF current distribution over the antenna, via a Green's function formalism and full-wave calculation using the ICANT code. In front of phased arrays of straps, convective cells were interpreted using the RF current profiles of strip line theory. Another class of convective cells, specific to antenna box corners, was evidenced for the first time. Within 1D sheath models assuming independent flux tubes, RF and rectified DC potentials are proportional. 2D fluid models couple nearby flux tubes via transverse polarisation currents. Unexpectedly this does not necessarily smooth RF potential maps. Peak DC potentials can even be enhanced. The experience gained on TS and the numerical tools are valuable for designing steady state high power antennas for next step devices. General rules to reduce RF potentials as well as concrete design options are discussed.
- OSTI ID:
- 20726298
- Journal Information:
- AIP Conference Proceedings, Vol. 787, Issue 1; Conference: 16. topical conference on radio frequency power in plasmas, Park City, UT (United States), 11-13 Apr 2005; Other Information: DOI: 10.1063/1.2098214; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-243X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ANTENNAS
CALORIMETRY
CONVECTION
COUPLING
DISTRIBUTION
ELECTRIC CURRENTS
ELECTRONS
GREEN FUNCTION
ICR HEATING
ION CYCLOTRON-RESONANCE
LOWER HYBRID CURRENT DRIVE
LOWER HYBRID HEATING
OPERATION
PLASMA
PLASMA DENSITY
PLASMA SIMULATION
POLARIZATION
POTENTIALS
PULSES
RF SYSTEMS
STEADY-STATE CONDITIONS
THERMOGRAPHY
TORE SUPRA TOKAMAK