The electron-cyclotron resonance (ECR) ion source was conceived by Richard Geller as a means of producing the high ion currents needed for fusion applications. Later it was discovered that ECR sources were capable of providing highly charged ions as well as high currents of 1⁺ ions. The basic operating principle of the ECR source is to confine the plasma in vacuum chamber surrounded by an axial magnetic field produced by solenoid magnets. Electrons in the plasma follow circular orbits around the magnetic field lines. The time required for an electron to travel around the circumference of the circle is a proportional to the charge-to-mass ratio of the electron and the strength of the magnetic field. Therefore electrons always make a specific number of orbits per second–the cyclotron frequency, approximately 2.79 GHz per kiloGauss. Introducing radiofrequency (rf) power at the cyclotron frequency causes the electrons to speed up, i.e. the electrons gain energy from the rf fields. These electrons collide with residual gas atoms in the vacuum chamber. When the energy of an electron exceeds twice the binding energy of an atom, that atom can be ionized in the collision. The result is a positively charged ion and two electrons. These two electrons absorb more energy from the rf field and ionize more atoms. The number of electrons and ions increases rapidly after the rf power is turned on, resulting in a plasma comprised of positively charged atoms and negatively charged electrons.

In general, ECR sources come in two varieties: 1) high currents of low charge-state ions and 2) low currents of highly charged ions. The characteristics of these two varieties are generally incompatible. Highly charged ions require excellent plasma confinement and relatively low vacuum pressure (to limit charge-sharing collisions that reduce the average ionization number). Low charge-state sources are essentially plasma “buckets” that require relatively little confinement and operate at higher pressures to improve the probability of ionizing each atom in the bucket at least once. 

ECR sources have also been designed as charge-breeders to ionize radioactive nuclei produced in nuclear reactions. Nuclear physicists want to study nuclei far from the “island of stability” in the chart of nuclides. None of these nuclei are stable and most decay within a few seconds of being produced, so the only way to study them is to be very efficient at collecting and accelerating them in a beam.