A Concept of Plasma Source for Producing an Impurity-Free, Low-Energy Ion Beam by Using a String-Plasma

An ion cyclotron resonance scheme can be utilized to separate different masses in a string-plasma. A string-plasma is a magnetized filamentary plasma whose radius is equal to or less than the ion Larmor radius. We have recently succeeded in producing such a plasma by utilizing a TP-D discharge apparatus. Our analysis has revealed that the movements of the majority of ion guiding centers in a steady-state string-plasma are restricted in the vicinity of the axis of the plasma column. This situation can be viewed that the ions are trapped in a radial electric-potential-well which is a result of the charge separation between an electron “string” confined along an axial magnetic field and the ions themselves gyrating around this electron string. One can consider the string-plasma as a “boundary-like” plasma that still preserves an overall charge neutrality.The peculiar properties of this string-plasma can be useful for various interesting applications especially in materials science. In particular, a mass separation technique using a string-plasma is described. A set of rf electrodes are placed in such a way that they can produce a rotating (in the direction of ion gyration) electric field in the string-plasma, resulting in the efficient selective energyzation of resonant ions (whose cyclotron frequency matches the rf frequency). The essential point is that resonant and nonresonant ions can be physically well separated since all ions are gyrating around the same axis, one of the important features of the string-plasma. This allows one to easily separate different mass species using a simple device. An impurity species can be selected as a resonant species. Applying the ion-cyclotron-resonance mass-separation technique several times along the string-plasma, undesirable ion species can be eliminated from the plasma. Consequently, it becomes possible to produce an impurity-free, low-energy ion beam or a plasma stream.