Dielectric window for reactor like ICRF vacuum transmission line

The dielectric window of a 35-60 MHz ion cyclotron resonance frequency (ICR F) transmission line is designed for reactor conditions in due consideratio n of irradiation, dielectric heating, fabrication issues, and remote handli ng. Electric field, temperature distribution, and thermal stresses are eval uated for beryllia and alumina dielectrics using finite element codes. The analysis is made for two annular ceramic septa joining coaxial water-cooled conductors having maximum operating 50 kV peak rf voltage and 30 Omega cha racteristic impedance. For unfavourable irradiation conditions ( > 10(23) n /m(2) neutron fluence), alumina is found to be heated excessively to about 1000 degreesC with unacceptable stresses. For moderately irradiated berylli a ( < 10(22) n/m(2)) or for unirradiated alumina of specific grades (tan de lta comparable or less than 5 x 10(-4)), the temperature and the thermal st resses are found to stay acceptable provided niobium. or titanium is used a s a conductor. Beryllia (BeO) is chosen as a candidate for the window ceram ic because of its better thermal conductivity and smaller thermal expansion mismatch with the conductor material helping also the manufacturing proces s. The characteristics for the ceramic/metal joints are estimated for candi date conductor and ceramic (alumina) materials. Vacuum brazing using active filler materials provides sufficiently good conditions for heat conduction across the joint, and the joints appear to be tight enough. Suitable windo w location on the transmission line is investigated by calculating neutron fluences at the window using an experimental reactor like configuration wit h 2500 h total burn time (1500 MW). (C) 2001 Elsevier Science B.V. All righ ts reserved.