Computer Simulation of Ion Beam Extraction by Finite Element Method

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Abstract
A simulation code of the ion beam extraction from a plasma is described. It treats 2-D problems, and the finite element method (FEM) is adopted for the computation of potential distribution with good accuracy. In accel-decel type one-stage extraction, the relations among the divergence angle and extracting conditions, and the optimum extracting conditions, are obtained. By accel-accel-decel type two-stage extraction, an ion beam of higher energy is extracted with small divergence angle. Improvement with precel is also examined. But in this case a large bias voltage can not be applied because of the large heat load of the positive electrode and electrical breakdown. One method which avoids these shortcomings is to insert one more electrode between the plasma and the positive electrode. The characteristics of this configuration are examined.

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