The heating and confinement of electrons in an electron-cyclotron-resonance ion source (ECRIS) plasma is described: the single particle motion in a magnetic beach is presented, showing the role of the rf wave magnetic field in the electron losses. This effect can be approximately modeled by a Fokker–Planck quasilinear equation in velocity space. This modeling is discussed and numerical results are presented. The behavior of the wave close to resonance is analyzed, taking into account the thermal motion of the electrons. The role of the different design parameters (including rf frequency) is shown and the major ingredients for the design of an efficient ECRIS for multiply charged ions production are summarized. ©1998 American Institute of Physics.