On the use of magnetic buckets for ion beam profile tailoring

Magnetic multipole plasma confinement geometries employing permanent magnet "buckets" are used extensively for a range of laboratory plasma applicatio ns. Among the several consequences for plasma confinement is the important result that the plasma can acquire a more-or-less flat density profile, whi ch when embodied in an ion source, can also lead to a flat profile for the extracted ion beam. For many applications a uniform ion beam current densit y profile is quite advantageous, for example, for carrying out large-area i on implantation. There are, however, inherent limitations on the extent to which this approach to beam "homogenization" can be utilized, and even for a perfectly flat profile in the immediate postextraction region, the beam w ill evolve toward Gaussian as it propagates downstream. Here we describe th e rare-earth permanent magnet bucket that we have incorporated into our bro ad-beam vacuum arc ion source, and its effect on the beam profile at the ex tractor and downstream. The experimental results are compared with a simple model for the beam profile evolution with axial distance. We find that the beam loses memory of its initially flat profile and relaxes to a more-or-l ess Gaussian shape in a relatively short axial distance similar to w/4 thet a, where w is the initial width of the flat beam profile and theta is the b eamlet divergence half angle. (C) 2000 American Institute of Physics. [S003 4-6748(00)56302-3].