The experiments described here were performed at a 5 GHz electron‐cyclotron‐resonance ion source with a magnetic hexapole. The discharge was run in argon. By launching a fast magnetoacoustic wave from the high‐field side (end on) we heated hydrogen ions—present in traces from the residual gas—at their fundamental ion cyclotron resonance (ICR) frequency of 2.2 MHz with 5 W only. As a consequence the currents of Arq+ions extracted from the source were significantly increased forq≥5 while the currents of lower charged argon ions were decreased, indicating an enhanced plasma confinement. However, a reduction of the hydrogen content in the residual gas resulted in a reversal of the effect of ICR heating. ©1996 American Institute of Physics.