A cusp filter for altering a multi-species plasma to separate ions of different masses (M.sub.1 and M.sub.2) includes first and second axi-symmetric magnetic fields which are coaxial, have the same magnitude (B), and are oriented back-to-back to establish a null cusp. The null cusp is thus oriented perpendicular to the axis between the magnetic fields. An injector is provided for directing the plasma ions along the axis toward the null cusp to divert the ions (M.sub.1) away from the axis and prevent them from crossing the null cusp, while allowing the ions (M.sub.2) to cross the null cusp and proceed along the axis through the filter. In one embodiment, a cut-off mass, M.sub.c, is determined such that M.sub.1 <M.sub.c <M.sub.2 with M.sub.c =e.sup.2 B.sup.2 r.sup.2 /2W where "e" is the ion charge, "r" is the radial distance of the ion from the axis, and W is its kinetic energy. In another embodiment, ions of selected mass are heated by cyclotron resonance to raise their energies above that of other ions in order to assure their passage through the null cusp. The selected ions then pass through the null cusp for separation from the other ions.
A device for separating the constituents of a multi-constituent material includes a substantially cylindrical plasma chamber and two, axially opposed plasma injectors. The injectors convert the multi-constituent material into a multi-species plasma and inject the multi-species plasma into a core portion of the plasma chamber. Ions in the plasma diffuse from the core portion to an annular volume within the chamber where the ions are separated according to their respective mass to charge ratios. To effect separation, electrodes and coils are provided to establish crossed electric and magnetic fields in the annular volume. With the crossed electric and magnetic fields, low-mass ions in the annular volume are placed on small orbit trajectories and drift axially for capture at the ends of the plasma chamber. High-mass ions in the annular volume are placed on large orbit trajectories for capture at the cylindrical wall of the chamber.
