Improved ion extraction from an electron cyclotron resonance ion source by a metal-dielectric-extraction electrode

Abstract

The goal of the experiment was to study the influence and the physics of the boundary region ͑between the plasma and the extraction potential͒ with direct impact on the source ion-beam output. A specially processed high-emissive metal-dielectric structure was installed on the extraction electrode of the Frankfurt 14 GHz electron cyclotron resonance ion source ͑ECRIS͒, forming a so-called metal-dielectric-͑MD͒ extraction electrode. The emissive layer of the disk faced the plasma; its inner hole was about the size of the normal extraction hole of the ECRIS. The output of the ECRIS in the presence of the MD electrode was compared with the outputs for the standard configuration ͑overall stainless-steel plasma chamber͒ and with the same plasma chamber with the radial wall covered by a highly electron emissive MD liner that raise the plasma electron density and temperature. The charge state distributions of the argon ions extracted from the source show an important increase of the ion beam for the high charge states as compared to the standard situation whereas the low charge states are less reduced than in the case of the presence of a MD liner. Due to the special position of the dielectric layer, the MD electrode introduces a new effect, which is connected to its property of becoming a positively charged surface under electron and ion bombardment. The MD electrode creates a quasiconfinement of the peripheral ions in the extraction, those ions that are normally lost to a conducting extraction electrode.

The influence of ambipolarity on plasma confinement and on the performance of electron cyclotron resonance ion sources

“…6 The effect observed with the MD electrode therefore indicates that the second of the above mechanisms plays now an essential role: the change of the ambipolar nature of the plasma by cutting very effectively the flux lines for the compensating wall currents.…”