Design and RF measurements of a 5 GHz 500 kW window for the ITER LHCD system

摘要

CEA/IRFM is conducting R\&D efforts in order to validate the critical RFcomponents of the 5 GHz ITER LHCD system, which is expected to transmit 20 MWof RF power to the plasma. Two 5 GHz 500 kW BeO pill-box type window prototypeshave been manufactured in 2012 by the PMB Company, in close collaboration withCEA/IRFM. Both windows have been validated at low power, showing good agreementbetween measured and modeling, with a return loss better than 32 dB and aninsertion loss below 0.05 dB. This paper reports on the window RF design andthe low power measurements. The high power tests up to 500kW have been carriedout in March 2013 in collaboration with NFRI. Results of these tests are alsoreported. In the current ITER LHCD design, 20 MW Continuous Wave (CW) ofRadio-Frequency power at 5 GHz are expected to be generated and transmitted tothe plasma. In order to separate the vacuum vessel pressure from the cryostatwaveguide pressure, forty eight 5 GHz 500kW CW windows are to be assembled onthe waveguides at the equatorial port flange. For nuclear safety reasons, fortyeight additional windows could be located in the cryostat section, to separateand monitor the cryostat waveguide pressure from the exterior transmission linepressure. These windows are identified as being one of the main criticalcomponents for the ITER LHCD system since first ITER LHCD studies [1] [2] [3]or more recently [4] [5] , and clearly require an important R\&D effort. Inthis context and even if the LHCD system is not part of the constructionbaseline, the CEA/IRFM is conducting a R\&D effort in order to validate adesign and the performances of these RF windows. In order to begin theassessment of this need, two 5 GHz 500 kW/5 s pill-box type windows prototypeshave been manufactured in 2012 by the PMB Company in close collaboration withthe CEA/IRFM [6]. The section 2 of this paper reports the RF and mechanicaldesign of a 5 GHz window. Some features of the mechanical design and theexperimental RF measurements at low power are reported in section 3. High powerresults, made in collaboration with NFRI, are detailed in section 4. Thedevelopment of CW windows is discussed in the conclusion. 2-RF AND MECHANICALDESIGN The proposed 5 GHz RF window is based on a pill-box design [2] , i.e. aceramic brazed in portion of a circular waveguide, connected on either side toa rectangular waveguide section. Typical design rules of thumb of such deviceare circular section diameter about the same size of the diagonal of therectangular waveguide (cf. FIGURE 1). Without taking into account the ceramic,the circular section length is approximately half a guided wavelength of thecircular TE 11 mode, in order for the device to act as a half-wave transformer.Once optimized, taking into account the ceramic, matching is correct only for anarrow band of frequency and is very sensitive to the device dimensions and theceramic relative permittivity. The heat losses in the ceramic, which have to beextracted by an active water cooling, depends on the inside electric fieldtopology and of ceramic dielectric loss (loss tangent). Undesirable modes dueto parasitic resonances can be excited in the ceramic volume, raising theelectric field and