Experiments and Modeling of Gas Heating in a Radio-Frequency Plasma Jet

Small plasma sources have been developed for a variety of applications and are often known as plasma jets (e.g. biomedical applications using atmospheric pressure helium or argon plasmas), as micro-plasma sources or arrays [1] (e.g. surface processing using lower operating pressure plasmas), or as micro-thrusters (e.g. electric propulsion using hollow cathode thrusters or neutralizers). At the ANU we have designed a capacitively coupled radiofrequency (13.56 MHz) cylindrical plasma source which currently operates at pressures around a few Torr and at powers ranging from 1-100 Watts with or without a pressure gradient imposed along the 2 cm long cavity [2]. The plasma diameter ranges from 1 to 6 mm before expanding in a larger chamber. Neutral gas heating is measured for a variety of gases (argon, nitrogen, xenon) and cavity diameters using optical emission spectroscopy. The results are combined with electric probe measurements to develop a plasma heating model where the heating is assumed to come from ion-neutral collisions in the maximum of the discharge under the powered electrode. A peaked density on axis is well described by the plasma model [2]. Typically, gas temperatures around 1000K are measured for N₂ and Ar in agreement with theoretical predictions from Fruchtman [3]. The results show that this type of source can be tailored to suit a variety of plasma applications: its effectiveness in dissociating species means that it can be used for remote plasma etching [1] or for surface passivation from atomic nitrogen using N₂. It can also be developed as an electrothermal plasma micro-thruster for space propulsion[2]. All of these applications benefit from the neutral gas heating and from the ability of the source to be developed as an array of sources.

REFERENCES:

[1]S. Dixon, C. Charles, R. Boswell, W. Cox, J. Holland and R. Gottscho, “Interactions between arrayed hollow cathodes”, J. Phys. D: Appl. Phys. 46, 145204, 2013

[2]C. Charles and R.W. Boswell, “Measurement and modeling of a radiofrequency micro-thruster”, Plasma Sources Sci. Technol. 21, 022002, 2012

[3]A. Fruchtman, “Energizing and depletion of neutrals by a collisional plasma”, Plasma Sources Sci. Technol. 21, 022002, 2012