Current-voltage characteristics of the droplet shield plate (aluminum, 100mmφ) in the shielded vacuum arc apparatus was measured in order to probe the possibility of ion control. Arc current was 50 A and pres-sure was 0.5 Pa with Ar or N₂ gas flow. At negative voltage region, the current was almost constant at around 1A. However, even at lower positive voltage region, the current rapidly increased more than 10 A. Such large current was found to be a part of arc current, resulting the shield plate act as an anode. These re-sults indicate that the higher positive voltage cannot be applied to the shield plate and thus it is difficult to reflect the ions with few tens electronvolts, which are emitted from cathode.
Next, in order to clarify the disturbance of the droplet shield to the plasma, the plasma parameters were measured by using Langmuir probe. It was found that the electron and ion densities drastically decreased behind the shield plate. Furthermore, the plasma parameters were derived from the current-voltage char-acteristics of the shield plate, assuming the plate was a large electrostatic probe, and compared with those obtained from Langmuir probe measurement. The result indicates that the shield plate could be used for rough evaluation of electron energy and ion density.