Construction of a He cascade arc plasma window device for an alternative differential pumping system*

A cascade arc plasma window device with a channel diameter of 8 mm has been fabricated for a differential pumping system in He gas cell ion beam stripper facility. The required pressure difference is 7 kPa to 1 Pa through 20-cm plasma window channel without large vacuum pumps. Stationary He plasma up to a current of 100 A was generated as the plasma window, and the performance of the pressure difference was investigated by measuring the pressures of the upstream and downstream sections. The pressure ratio through the plasma window was ~230 at a 100-A discharge of 0.05 L/min. Our cascade arc source demonstrated the pressure difference from 7 kPa to 27 Pa, and has a high potential for alternative differential pumping to separate the gas cell from the downstream chamber. To characterize the He plasma at the anode electrode, emission from vacuum UV to visible wavelengths were observed. Spectral analysis of He I continuum spectra and H-b Stark broadening showed that the plasma temperature was ~0.9 eV and density was 3.4×10¹³ cm^-3 at a 100-A discharge of 0.05 L/min.