We developed a steady-state high-density plasma source by applying a hollow cathode to a cascade arc discharge device. The hollow cathode is made of a thermionic material (LaB6) to facilitate plasma production inside it. The cascade arc discharge device with the hollow cathode produced a stationary plasma with an electron density of about 1016 cm−3. It was found that the plasma source produces a strong pressure gradient between the gas feed and the vacuum chamber. The plasma source separated the atmospheric pressure (100 kPa) and a vacuum (100 Pa) when the discharge was performed with an argon gas flow rate of 5.0 l/min and a discharge current of 40 A. An analysis of the pressure gradient along the plasma source showed that the pressure difference between the gas feed and the vacuum chamber can be well described by the Hagen–Poiseuille flow equation, indicating that the viscosity of the neutral gas is the dominant factor for producing this pressure gradient. A potential profile analysis suggested that the plasma was mainly heated within cylindrical channels whose inner diameter was 3 mm. This feature and the results of the pressure ratio analysis indicated that the temperature, and, thus, viscosity, of the neutral gas increased with the increasing number of intermediate electrodes. The discharge characteristics and shape of the hollow cathode are suitable for plasma window applications.
We have proposed a new method to estimate the optical escape factor (OEF) in high-density helium (He) plasma. Plasma with an electron temperature of ca. 3 eV and density of 5 × 1013 cm−3 was generated by a cascaded arc discharge and rapidly cooled by the introduction of additional He gas, which resulted in a transition from ionizing to recombining plasma. With an increase in the gas pressure, the plasma became optically thick, and the He I forbidden line (spin-exchange intercombination line, 1 1S-2 3P: 59.1 nm) with resonance lines were simultaneously observed using a vacuum ultraviolet spectrometer. Comparison of the intensity ratio of the He I 58.4 nm resonance line to the forbidden emission with those by determined from the collisional-radiative model considering the self-absorption enabled the successful estimation of the OEF. The OEF was decreased with the ambient He gas pressure and was 6.6 × 10−4 and 3.5 × 10−5 for He gas pressures of 1.59 and 20.22 Pa, respectively.
We have developed indirectly heated hollow cathode electrodes and a cascade arc discharge apparatus with different channel diameters to realize plasma windows (PWs) as virtual vacuum interfaces. A compact arc discharge source with a channel diameter of 3 mm is fabricated to realize windowless vacuum–atmosphere separation. An atmospheric high-density Ar thermal plasma is generated, and a PW that produces 100 kPa and 81 Pa separation is demonstrated. An 8 mm channel diameter arc device is also constructed for application as an alternative differential pump with the separation of low- and high-pressure vacuum chambers, and the production of a high-density He plasma. Pressure differences of 2.5 kPa and 16 Pa between PWs are realized. Moreover, vacuum ultraviolet and visible emission spectroscopy reveal the characteristics of expanding plasmas and the plasma parameters.
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