Outgassing of lower hybrid antenna modules during high-power long-pulse transmission

Abstract

During high-power, long-pulse transmission from lower-hybrid-range-of-frequency (LHRF) antennas, the waveguide walls outgas as a result of rf-loss-induced heating. If the resulting pressure rise is too high, power transmission will be adversely affected and additional pumping may be required to maintain the pressure at a low enough value. The outgassing rates of waveguides made of various materials (oxygen-free high-conductivity copper, dispersoid copper, copper-coated carbon fiber composite, copper-coated graphite) were measured during rf injection at high power density (50–200MW∕m2) for a duration in the range 100–4700 s. The experiments were performed on a test-bed facility equipped with a 3.7 GHz klystron on multiwaveguide (2 to 18) mock-ups. The effect of the main parameters, namely, the waveguide surface temperature and the initial wall gas loading (“conditioning”), are analyzed in detail. It is concluded that an outgassing rate of 1×10−5(5×10−5)Pam3s−1m−2 at 300 °C (400 °C) can be considered for most materials. The requirement, in terms of additional pumping, for the International Thermonuclear Experimental Reactor LHRF antenna is finally discussed.