Surface enhancement of molecular ion H₂⁺ yield in a 2.45-GHz electron-cyclotron resonance ion source

High current hydrogen molecular ion beam is obtained with a specially designed stainless steel liner permanent magnet 2.45-GHz electron-cyclotron resonance (ECR) ion source (PMECR II) at Peking University (PKU). To further understand the physics of the hydrogen generation process inside a plasma chamber, theoretical and experimental investigations on the liner material of the plasma chamber in different running conditions are carried out. Several kinds of materials, stainless steel (SS), tantalum (Ta), quartz, and aluminum (Al) are selected in our study. Experimental results show that stainless steel and tantalum are much better than others in H₂⁺ generation. During the experiment, an increasing trend in H₂⁺ fraction is observed with stainless steel liner after O₂ discharge inside the ion source. Surface analyses show that the roughness change on the surface after O₂ discharge may be responsible for this phenomenon. After these studies, the pure current of H₂⁺ ions can reach 42.3 mA with a fraction of 52.9%. More details are presented in this paper.