Configurations for rotating electric and magnetic fields for mass separation

Rotating electric and magnetic fields, parallel to each other and both perpendicular to a steady magnetic field, generate a ponderomotive force on ions along the magnetic field. The direction of the force depends on whether the ion cyclotron frequency is larger or smaller than the fields rotation frequency. Thus, by tuning either the rotation frequency or the intensity of the steady magnetic field, ions of different mass could be pushed in opposite directions. This process could be used for mass separation, an important process for a variety of societal needs.¹ For example, Li⁶, crucial for generating tritium for fusion, has to be separated from Li⁷, the majority isotope. Planar standing electromagnetic waves of an appropriate frequency exert such a ponderomotive force. Indeed, we have recently studied the trajectory of an ion under the forces by such waves.² However, exciting such waves in a plasma (like the shear Alfven wave) is difficult, and the wave magnetic field is weak. In this talk we examine configurations of biased electrodes and current-carrying coils that generate vacuum electric and magnetic fields. Two pairs of coils and two pairs of electrodes generate rotating electric and magnetic field at a central point between the two pairs. However, away from that central point, the fields vary and are not uniform. Such gradients in the electric and magnetic fields could generate undesirable drifts. We show an ideal configuration that generates rotating electric and magnetic fields that are uniform across a cylinder cross section. We then compare the trajectories of ions in the two fields configurations in order to examine how the non-uniformities affect those trajectories.

1. 1. S. Zweben, R. Gueroult, and N. J. Fisch, “Plasma mass separation”, Phys. Plasmas 25, 090901 (2018).
   
   2. A. Fruchtman and G. Makrinich, “Ion dynamics in standing electromagnetic wave near the cyclotron resonance”, Phys. Plasmas 31, 043502 (2024).