A high‐power radio‐frequency plasma source was built and tested for an antenna frequency of 2.45 MHz. A puff valve fed hydrogen gas into a plasma chamber made from 4‐in.‐diam Pyrex tubing. A helical antenna was wound directly around this chamber. The plasma source was surrounded by a magnetic bucket which used a longitudinal line‐cusp geometry. For 20 kW of rf input power, the peak electron density was 2.0×1019m−3, the electron temperature was 2.0 eV, the ion temperature was 0.9 eV, and the atomic hydrogen density was 4.4×1019m−3. For 90 kW of rf input power, the peak electron density was 8.9×1019m−3, the electron temperature was 2.5 eV, the ion temperature was 3.0 eV, and the atomic hydrogen density was 2.3×1019m−3. The plasma source had a typical current efficiency of 6 A/kW. A theoretical model for the plasma loading resistance was developed.