PLASMA POTENTIALS AND PERFORMANCE OF THE ADVANCED ELECTRON-CYCLOTRON-RESONANCE ION-SOURCE

The mean plasma potential was measured on the LBL advanced electron cy clotron resonance (AECR) ion source for a variety of conditions. The m ean potentials for plasmas of oxygen, argon, and argon mixed with oxyg en in the AECR were determined. These plasma potentials are positive w ith respect to the plasma chamber wall and are on the order of tens of volts. Electrons injected into the plasma by an electron gun or from an aluminum oxide wall coating with a very high secondary electron emi ssion reduce the plasma potential as does gas mixing. A lower plasma p otential in the AECR source coincides with enhanced production of high charged state ions indicating longer ion confinement times. The effec t of the extra electrons from external injection or wall coatings is t o lower the average plasma potential and to increase the n(e) tau(i) o f the ECR plasma. With sufficient extra electrons, the need for gas mi xing can be eliminated or reduced to a lower level so the source can o perate at lower neutral pressures. A reduction of the neutral pressure decreases charge exchange between ions and neutrals and enhances the production of high charge state ions. An aluminum oxide coating result s in the lowest plasma potential among the three methods discussed and the best source performance.