The classic high-charge-state ECR source is comprised of two solenoidal magnet coils separated axially by some distance. The resulting magnetic field profile has a region of lower magnetic field (the valley) between two regions with larger field magnitude (the hills). The field profile resembles the back of a two-humped camel. The magnetic hills tend to confine the helical electron orbits to the valley region–the classic “magnetic bottle” configuration.  Cylindrical multipolar magnetic fields are also used to help confine the electrons in the radial direction.

Several effects conspire to limit the efficiency of the ECR discharge (as the plasma generating process is called). In general, electron orbits are not circular but helical. Electrons are transversely confined to orbit the magnetic field lines, but little prevents electrons from moving longitudinally along these lines. Without some form of longitudinal confinement, electrons can quickly move out of the plasma region. The double-humped magnetic field profile helps confine the electrons into the low-field region between the two “hills.” The confinement is not perfect however and the higher energy electrons tend to “leak” out of both ends. In addition, the resonance condition exists only at those locations where the magnetic field is exactly the resonance value defined by the operating frequency. The typical resonance zone in an ECR source is a thin ellipsoidal surface (essentially an eggshell). The thickness of the shell is defined by the rf bandwidth and the slope of the magnetic field (e.g. the steepness of the hills). Only the electrons that pass through the “shell” absorb energy. Electrons elsewhere in the plasma lose energy to collisions and other plasma processes. To produce a stable plasma, electrons have to absorb enough energy traversing the resonance zone to compensate for losses of energy between transits of the zones and losses of electrons due to imperfect confinement. Also, relativistic effects increase the mass of an electron as its orbital velocity increases. This increasing mass decreases the cyclotron frequency. Hence the higher energy electrons fall out of resonance with the rf power and stop absorbing energy. Finally, although the electrons are relatively well-confined, the ions are only weakly affected by the magnetic field and can “drift” out of the plasma region in both the transverse and longitudinal directions.

The ultimate plasma density achievable in a high-charge-state ECR source is determined primarily by the operating frequency. The classical “plasma frequency” is related to the density of electrons in the plasma. When the electron density exceeds a critical value, rf power cannot be absorbed. This effect is responsible for reflection of low frequency radio waves from the ionosphere that surrounds the earth. Frequencies lower than about 30 MHz are reflected by the ionosphere because the density of electrons in the ionospheric plasma allows absorption and re-emission of rf power at the lower frequencies. This “reflection” of radio waves enables long-range communications at these frequencies (AM and short-wave radio). Frequencies above 30 MHz are transmitted by the ionospheric plasma, limiting FM radio, cell phones, and TV transmissions to line-of-sight. Therefore producing higher plasma densities requires higher operating frequencies and therefore larger magnetic fields. This statement defines the current trend in ECR source development. Most of today’s leading edge ECR sources are operating at or above 14.5 GHz. New sources are being developed at 18 GHz and 28 GHz and future sources are being designed for 35 GHz and higher.

Applications of high charge-state ECR sources are generally limited to nuclear and atomic physics research. The electron confinement in these sources is improved by adding a transverse magnetic field to the longitudinal one. This transverse field is most often a cylindrically symmetric magnet with several magnetic poles facing inward-a multipole magnetic field. The transverse field profile of these magnets is generally characterized by its multipolarity, e.g. quadrupole (4-poles), sextupole (6-poles), octupole (8-poles), etc. The interaction of the longitudinal and transverse magnetic fields significantly improves the confinement of electrons in the plasma and therefore the efficiency of the ECR source. This improved efficiency is critical for the production of highly charged ions such as oxygen 7⁺, argon 18⁺, or bismuth 40⁺ (the number preceding the + sign indicates the number of missing electrons).