Understanding Plasma-initiated Secondary Arc Discharges
Open AccessThis work describes the physics of secondary arc discharges produced by a seed plasma, and their potential applications as propulsion devices for spacecraft. Secondary arc discharges occur when plasma comes in contact with a second cathode, which is at a much lower electrical potential than the cathode or the anode of the plasma source, the device that produces the seed plasma. Depending on the bias voltage of the secondary cathode, the discharge currents can reach values from \(20~A\) to \(325~A\) for a bias voltage of \(-100~V\) and \(-500~V\), respectively. Two prototypes were designed, built, and tested in order to gain insight into the physics of the secondary arc discharge. The insight gained from these experiments allowed the author to develop a physical model based on electron emission. This model obviates the dependence of the field electron emission and the work function of the secondary cathode, particularly in the presence of chemisorbed oxygen, i.e. an oxide layer. The discharges have been characterized by a series of methods such as quadruple Langmuir probe measurements, as well as time of flight and total ion current measurements. The quadruple Langmuir probe measurements showed that the yttrium plasma density was in the order of magnitude of \(10^{17}~m^{-3}\) at a distance of approximately \(110~mm\) downstream from the thruster's exit plane, with electron temperatures averaging around \(3~eV\). Initially, the discharges were very repeatable, but as the oxide layer on the cathodes was eroded away by successive discharges, it became clear that the time between the trigger discharge and the secondary discharge increased gradually until the cathode's surface was fully eroded. Since the process is affected by the chemisorbed oxygen, it was necessary to understand the time scale in which the process occurs. The oxygen monolayer formation time is in the order of \(30~s\) at a base pressure of approximately \(6.7\times 10^{-5}~Pa\), which was the pressure inside the vacuum chamber where the experiments wre performed. This time scale is larger than the duration between pulses of \(4~s\) that was used, and therefore, was not sufficiently fast to enable the production of an oxygen layer that was thick enough to allow for additional secondary discharges. Using the knowledge gained from the experimental data and the physical model, the author proposes three potential applications for the technology. The first application has the prospect of solving a fundamental problem with vacuum arc thrusters, namely the issue with cathode feeding systems by using a seed plasma to initiate the main cathodic arc discharge instead of the "trigger-less" method. The second application is a propulsion system with the ability to inject additional plasma into the discharge channel, thus increasing the overall mass flow, and thus, thrust. This application would allow spacecraft operators to perform a variety of orbital maneuvers using a single engine. And lastly, the third application would enable spacecraft to operate in a so-called "air-breathing" mode at very low orbital altitudes of around \(100~km\) and \(120~km\). The mass flow of oxygen obtained through a special intake at the front of the thruster would oxidize the cathode, turning the surface into a metal oxide. The oxide layer changes the electronic structure of the metal and allows for the secondary discharges to occur. Furthermore, the mass gained through oxidation can be eroded by the arc discharge and therefore, the spacecraft obtains additional fuel to burn passively due to oxidation. Calculations performed in this work show that these applications are plausible, but further work is needed in order to characterize every aspect of the discharge and the operation of such propulsion systems.
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