AbstractThe outgassing properties of the lower hybrid current drive (LHCD) antenna during r.f. operation were studied on the JT-60U tokamak. The antenna is not equipped with a differential pumping system to evacuate the outgassing of the antenna. The pressure rise in the antenna during r.f. operation strongly depended on the r.f. power when r.f. conditioning was started. However, the outgassing rate was quickly reduced after several shots, and showed weak r.f. power dependence after sufficient r.f. conditioning. Finally, the outgassing rate of the LHCD antenna was in the range of approximately 10−6 Pa m3 s−1 m−2 after four weeks operation at the working temperature of 250 °C. Then a power density of up to 25 MW m−2 was successfully injected into the plasma without the differential pumping system at the antenna on JT-60U.