Discharge characteristics of steady-state high-density plasma source based on cascade arc discharge with hollow cathode

Abstract

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.

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

“…In addition, an RL circuit (inductance: L = 0.6 mH, resistance: R = 0.3 Ω, relaxation time: 2 ms) was serially connected between the anode and cathode to stabilize the plasma. 13) The plasma produced between the cathode and anode flowed into a vacuum expansion section due to adiabatic free expansion. The expansion chamber (0.2 m 3 ) was exhausted by two sets of mechanical booster pumps (at 1200 l min −1 ) and a rotary pump (at 605 m 3 h −1 ) that maintained pressures well below 100 Pa during operation.…”

“…For the realization of a vacuum interface to isolate a vacuum (1 Pa) from the atmosphere (100 kPa), high gas temperature above 10 000 K and high viscosity in the discharge channel attributed to the higher gas temperature play an important role to create the pressure separation through the discharge channel. Therefore, we have fabricated a hollow cathode electrode that incorporates a gas heater and a 3 mm channel diameter cascade arc argon plasma source to generate a dense thermal plasma of >10 16 cm −3 and a gas temperature of around 11 000 K. [11][12][13] On the other hand, Ikoma et al has proposed the use of a PW with a larger channel diameter in a heavy ion accelerator, where an intense 238 U 65+ ion beam is generated by a He gas charge stripper target. 14) The He charge stripper, with a tube length of 0.5 m and a 6 mm orifice for the beam transport, maintained a pressure of 7 kPa.…”