Abstract The effect of electron cyclotron resonance on the electron flow is experimentally examined in the magnetic nozzle of a radio-frequency plasma source powered at 13.56 MHz under a series of operating conditions. Measurements of the electron energy probability function show that the bulk of electrons is effectively heated when the external magnetic field meets the cyclotron resonance condition in the proximity of the antenna. A careful tuning of the magnetic field topology inside the plasma source leads to a gain in electron density and temperature up to about 20% and 40%, respectively.