Generation of stationary high-density cascade arc plasmas and its application to plasma windows

In order to realize plasma windows for virtual vacuum interfaces, we have developed cascade arc plasma sources. For windowless vacuum–atmosphere separation, a compact arc discharge source having a channel diameter of 3 mm is fabricated, and an atmospheric Ar thermal plasma is generated. For an alternative differential pumping system, separating low- and high-pressure vacuum chambers, a larger arc device with an 8-mm diameter is also constructed, producing a high-density He plasma. The performances of the two cascade arcs as plasma windows are investigated by variations of pressure gradient between high- and low-pressure chambers and by UV/visible emission spectroscopy. The 3-mm arc discharge generates a steep pressure gradient of Ar 100 kPa–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 determines the electron density and temperature of Ar and He plasmas, yielding the temperature of ~1 eV (Boltzmann plot) in both discharges and electron density of 1.5×10¹⁶ cm^-3 for Ar 35 A with a gas flow rate of 5.0 l/min and 1.5×10¹⁴ cm^-3 under a He 100-A and 0.5-l/min condition by means of Stark broadenings of the H-b and Ar I lines.