The motion of a charged particle in a crossed rf field and dc magnetic field describes the occurrence of resonance (increase of the energy gained by the particle between collisions) which causes low breakdown voltage, higher plasma intensity, and higher extracted ion current. The resonance in the microwave discharge is essential for plasma electron heating. The development line in both ECRIS’s and RFIS’s includes an increase of the electron population in the discharge to intensify the plasma and increase the extracted ion current; plasma cooling with the use of electron injection or gas mixing increases the lifetime of the ions in the plasma allowing for multiple ionization collision and increase of the highly charged ion species. Adaptation of both electrical and geometrical properties of these sources can lead to obtaining lower beam emittance. ©1996 American Institute of Physics.