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Accelerator-System Solutions for Broad-Beam Ion Sources

1977; American Institute of Aeronautics and Astronautics; Volume: 15; Issue: 7 Linguagem: Inglês

10.2514/3.60745

1533-385X

H. R. Kaufman,

Plasma Diagnostics and Applications

Over 300 space-charge-flow solutions were obtained on a digital computer for an electron-bombardment ion source. Results are presented in both tabular and generalized graphical form for a wide range of geometries and operating conditions. solutions had been obtained by Lathem2'3 and Nudd and Amboss.4 The present study includes over 300 additional solutions, thereby sub- stantially increasing the number of published solutions. The digital computer program of Bogart and Richley5 was used in the present study, together with the plasma sheath condition of Hyman et al.6 and the closure surface technique of Pawlik et al. 7 The use of one computer program and one iteration procedure hopefully avoids some of the scatter that would otherwise be introduced. Although some advantages have been shown for conical hole shapes,l only cylindrical holes were used herein because of their ease of fabrication and widespread use. Method of Solution A screen-hole radius corresponding to 10 mesh squares was used throughout. A larger number of squares would have given greater accuracy, but the required computer time in- creases at least as the second power of the number of squares in a radius for otherwise similar configurations. Inasmuch as well over 100 hr of computer time were used in this study, any substantial increase in the time per solution would have resulted in a reduction in the number of solutions in- vestigated. The upstream plasma sheath was assumed to consist of 10 straight-line segments, with the segments corresponding in location to the 10 mesh squares of radius. This sheath position was iterated to give an ion-current density uniform to better than ± 5%. For the closure surface of the deceleration region, a simple plane surface was used.t This plane was oriented normal to the mean ion-beam direction. The ions have high velocities in the vicinity of the closure surface and are therefore only slightly perturbed by the approximation of the actual complex shape by a plane surface. The procedure for locating the closure surface used the radius from the axis of the beam (from a single hole) within which half the ion-beam current passed. The axial location of the plane closure surface was then selected to give zero electric field at this radius. Most solutions were iterated to find this axial location within one mesh square. The fraction of open screen area was not found