High-density cascade arc plasma sources for application to plasma windows for virtual vacuum interfaces

Abstract:  We have developed two cascade arc plasma sources for application to plasma windows for virtual vacuum interfaces. For windowless vacuum–atmosphere separation enabling us to carry out an electron beam welding under atmospheric air condition, a compact arc discharge source having a channel diameter of 3 mm is fabricated, and an atmospheric Ar thermal plasma is generated. On the other hand, to generate a highly charged 238U65+ ion beam with a high intensity and high energy of 10.8 MeV/u in heavy ion accelerators, a He gas charge stripper target has been employed in the Radioactive-Isotope Beam Factory (RIBF) at RIKEN. Using a cascade arc plasma as a virtual window creates an effective vacuum interface to isolate the high pressure He gas cell from the lower pressure vacuum stages. Therefore, we also developed a larger diameter cascade arc discharge apparatus (channel diameter: 8 mm) for application to an alternative differential pumping system. The performances of the two cascade arcs as plasma windows are investigated. The 3-mm arc discharge generates a steep pressure gradient of Ar 100 kPa to 100 Pa through the discharge channel, while the 8-mm discharge apparatus isolates the high-pressure side at 7 kPa from the lower pressure of 54 Pa. Emission spectroscopy of visible and vacuum UV radiation reveals the characteristics of the Ar and He plasmas. Spectral analysis yields a plasma temperature of around 1 eV in both discharges. Stark broadenings of the H- and Ar I lines give an electron density of 6.5*1016 cm-3 for Ar 60 A with a gas flow rate of 1.0 L/min, and 4.7*1013 cm-3 under a He 100-A and 0.45-L/min condition.