Low charge-state (i.e. 1⁺) ECR sources have become one of the workhorses of industrial plasma processing. These sources typically operate at low frequency (2.45 GHz) and tend to have large plasma volumes for producing large area beams. They typically produce multiple milliAmperes of 1⁺ ions (such as boron, argon, nitrogen, and oxygen) for ion implantation, plasma etching, and surface treatment of materials. These sources exceed the critical electron density limit by operating at high vacuum pressures where rf energy can be absorbed through other plasma mechanisms. Low charge-state ECR source are also beginning to have an impact as sources of very high currents (100 mA to multiAmperes) of hydrogen ions.  A good example of a 1+ ECR sources is the 110 mA proton source developed at the Los Alamos National Laboratory (LANL) for the Low Energy Demonstration Accelerator (LEDA), Accelerator Production of Tritium (APT) accelerator, and the Accelerator Transmutation of Waste (ATW) systems.

The optimal magnetic field profile of a low charge-state ECR source is quite distinct from the classical magnetic bottle profile of high charge-state ECR sources.  This difference in field profile reflects the different mode of coupling the rf energy into the plasma.  The electron density achievable in a plasma is a function of the excitation frequency–the classical “plasma frequency.”  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.

Fortunately the plasma in an ECR source is strongly “magnetized” and therefore has other mechanisms available to absorb the rf power.   As a result, the plasma in a low charge-state ECR source can be pushed to densities far greater than the classical limit imposed by the plasma frequency.   High charge-state ECR sources seek to limit the number of collisions between plasma and neutral gas atoms that share electrons and therefore decrease the average charge state in the plasma.  Low charge-state ECR sources operate at higher gas pressures and utilize these collisions to produce a larger number of ions with low charge state.