Investigation of fringe plasma parameters on a high power rf driven ion source^a)

It has been observed that there are differences between the uncompensated Langmuir probes installed in the upper and lower areas of the rf driven $H−$ sources at IPP Garching. The two probes often had substantially different floating potentials or ion saturation currents. In an effort to understand the reasons for these differences a Langmuir probe analysis system was used on the probes to collect the full current voltage characteristic. The results show what is likely the formation of an ion-ion plasma. The paper shows the effect of beam extraction and the presence of caesium on the probe characteristics.