Development of a large-bore plasma window with an indirectly heated hollow cathode

For plasma window (PW) applications, we developed the cascade arc discharge device with an indirectly heated hollow cathode. The 8-mm channel diameter hollow cathode made of a lanthanum hexaboride (LaB $6$⁠) was heated by the C/C composite heater surrounding the cathode to increase the thermionic electron emission. The PW developed successfully separated 2.4 kPa and 16 Pa, and the pressure separation capability was sustained for over 1 h. H- $β$ Stark broadening measurement and the Thomson scattering measurement showed that the electron density and temperature inside the channel reached $10 19– 10 20 m − 3$ and 2.0 eV, respectively. The power balance analysis on the electron thermal energy revealed that the neutral density and temperature inside the channel were as high as $10 23 m − 3$ and 4000 K, respectively. The relation between the pressure separation capability and the neutral temperature showed that the flow inside the channel of the PW had the molecular flow feature. The SEM-EDX analysis on the LaB $6$ cathode showed that boron diffused to the molybdenum (Mo) shaft during plasma operation, which supported the LaB $6$ cathode. Mo shaft became brittle after more than 50 h of operation, exhibiting the necessity of buffer material between the LaB $6$ cathode and Mo shaft for long-duration operation.