  The Electron Cyclotron Resonance Ion Source (ECRIS) is nowadays the most
effective device that can feed particle accelerators in a continuous and
reliable way, providing high-current beams of low- and medium-charge-state ions
and relatively intense currents for highly charged ions. The ECRIS is an
important tool for research with ion beams (in surface, atomic, and nuclear
science) while, on the other hand, it implies plasma under extreme conditions
and thus constitutes an object of scientific interest in itself. The
fundamental aspect of the coupling between the electromagnetic wave and the
plasma is hereinafter treated together with some variations to the classical
ECR heating mechanism, with particular attention being paid to the frequency
tuning effect and two-frequency heating. Considerations of electron and ion
dynamics will be presented together with some recent observations connecting
the beam shape with the frequency of the electromagnetic wave feeding the
cavity. The future challenges of higher-charge states, high-charge breeding
efficiency, and high absolute ionization efficiency also call for the
exploration of new heating schemes and synergy between experiments and
modelling. Some results concerning the investigation of innovative mechanisms
of plasma ignition based on upper hybrid resonance will be described.
