THE EFFECTS ON ION-BEAM OPTICS OF A SPHERICAL-ABERRATION AT THE PLASMA BOUNDARY

Experimental data show that the minimum divergence of a positive helium ion beam does not occur at the beam perveance predicted by linear ion optics. A model is presented that introduces a third-order term into the ion optics equations, in the manner of a spherical aberration to the plasma boundary. Values of the aberration coefficient C are calculated from the data and it is shown that C∝(a/d)-1.9, where a/d is the aspect ratio of the accelerator. The contour of the plasma boundary is plotted and it is shown that the aberration leads to a flattening of the boundary at the extremes of the extraction aperture.