Design and demonstration of micro-scale vacuum cathode arc thruster with inductive energy storage circuit

Highlights

• An inductor storage power system was used for generating the pulsed plasma.

• A micro VACT with inductive energy storage circuit was designed and tested.

• A battery was used to reduce the energy consumption of the entire VAT unit.

• VAT prototype proposed in this study achieved a single impulse of 2.3 μNs.

• A specific impulse is 2360 s, and a thrust efficiency is approximately 10%.

Abstract

This study focused on the development of a vacuum cathode arc thruster (VAT), particularly on its design, manufacturing process, and demonstration. Characteristically, the proposed thruster does not require any additional propellant feeding system as the cathode electrode is simultaneously used as a propellant. In the ignition system, tiny spots are coated on the cathode surface to induce plasma flow. Such a setup has the advantages of simplicity, low price, small size, and low weight and is suitable for microsatellites. Moreover, a “trigger-less” method with an inductor storage power system was used for generating the pulsed plasma. This discharge method can significantly reduce input power. Thrust is mainly caused by high exhaust velocities of metal ions in the plasma flow, making ion density, ion velocity, and ion charge influential parameters. A battery was used instead of a power supply system to reduce the energy consumption of the entire VAT processing unit. The energy required for a single pulse was estimated to be 0.266 J, by measuring the change between the discharge current and the voltage. The ion current was measured using an ion detector and was 3.55 A, and the ion velocity was 23150 m/s. In the theoretical analysis, the VAT prototype proposed in this study achieved a single impulse of 4.31 μNs, a specific impulse of 1571 s, and a thrust efficiency of approximately 12.5%.

Introduction

There are two primary systems in the space propulsion technology—chemical propulsion (CP) and the electric propulsion (EP). In chemical propulsion, thrust is produced by severe chemical reactions and considerable exothermicity. Thrust in EP is produced by the reaction force generated when plasma is expelled by means of propellant acceleration. For EP the propellant's enthalpy is driven by external electric energy. Compared with the conventional CP, EP has a higher specific impulse, but lower thrust. If the specific impulse ($I_{sp}$) is higher, the fuel consumption is comparably much lower. Correspondingly, CP is inevitable to offset Earth's gravity. However, once the rocket successfully enters space, the EP is more useful than chemical propulsion in the long-hauling operation. Plasma comprises a group of electrons, ions, radicals, and neutrals (if the plasma is not fully ionized). As the mass of an electron smaller than that of an ion, the effect of an electron can be neglected during the propulsion process. In general, the EP mechanism can be categorized into three types based on the ion acceleration mechanism as electrothermal, electrostatic, and electromagnetic [1]. Moreover, it is not practical to scale existing EP systems such as ion and Hall thrusters to the range of 1–10 W. The unavoidable flow controls and plumbing and the overhead mass of the propellant storage tank reduce the overall efficiency of ion thrusters at the aforementioned power levels. Therefore, these types of propulsion systems are not appropriate for the use in cube satellites.

To enable microsatellites to achieve capabilities such as attitude control, position maintenance, and orbital transformation, new types of micro- and nano-thrusters should be developed to provide a wide range of impulse bits with the corresponding values spanning from nano-Newton-seconds to micro-Newton-seconds. Thus, pulsed plasma thrusters (PPTs) [[39], [40]] and vacuum cathode arc thrusters (VATs) [41] have proved to be good candidates for fulfilling many missions that require impulse bits approximately in the range of nano-Newton-seconds to milli-Newton-seconds [[2], [3], [4]]. Both types of thrusters can be easily miniaturized with maintaining their power based on the mission requirements. Therefore, they are suitable for microsatellites or CubeSat propulsion systems. Although PPTs can provide impulse bits approximately in the range of micro-Newton-seconds to milli-Newton-seconds, the use of Teflon material causes low fuel efficiency and the requirement of a peak operating voltage of approximately 2 kV. Essentially, the overall efficiency of a very small PPT is approximate 12% [5]. To reduce the restriction of miniaturization in power system, a vacuum cathode arc thruster (VAT) was used in this study. An inductive energy storage device [6] in combination with trigger-less ignition methods [7] was implemented. This configuration presents many benefits, such as a decrease in the size of a thruster, a decrease in the operating voltage required, and no need of an igniter. Most importantly, the VAT is also suitable for use in microsatellites or a CubeSat and can provide impulse bits approximately in the range of nano-Newton-seconds to micro-Newton-seconds.

The vacuum arc plasma source is a good candidate as a thruster because it has the capability of using any conductive metal or alloy as its fuel [7,8], low power consumption, variable thrust capability, and low system mass. The cathode material generates metal plasma, neutral gas, and macroparticles by a vacuum arc discharge from a cathode spot [9,10]. The metal plasma plume moves outward to achieve velocity in the range of 1–3 × 10⁴ m/s for a wide range of materials, that is, from carbon to tungsten. Moreover, the VAT usually operates in the pulse mode with energy in the range of 1–100 W. These characteristics revealed that the VAT is a potential candidate for micro- and nano-propulsion. In summary, the VAT was selected and developed as the main development thruster of a microsatellite in this study.

The earliest VAT literature was published by Dethlefsen in 1968 [11] and Gilmour and Lockwood in 1972 [12]. They tested a variety of cathode materials and successfully used the axial magnetic field to focus the ejected plasma jet. Subsequently, it discovered that when the VAT operates in the pulse mode, high power densities are required within the arc while maintaining low average power and small system size. The study of VATs was restarted from 1998 and continued till 2005 by the collaboration between Alameda Applied Sciences Corporation (AASC), NASA/Caltech Jet Propulsion Laboratory, and the Lawrence Berkeley National Laboratory (LBNL) [[13], [14], [15]]. During that time, some major technological breakthroughs were achieved, such as the development of an inductive energy storage device [6], the combination of the inductive energy storage device and the trigger-less ignition method [16], and the use of a compact magnetic coil for collimating and accelerating plasma [12,17]. In addition, Neumann et al. [18] demonstrated a Mg-fuelled centre-triggered pulsed cathodic arc thruster and it explored higher current centre-triggered pulsed arcs with greater impulse bits, specific impulses and thrust-to-power ratios. In recent years, many scholars have begun to think about how to increase the performance and lifetime of a VAT [19,20]. The applied magnetic field has significantly improved the efficiency and lifetime of a thruster [18,21]. Other study results revealed that the thrust and plasma distribution can be improved by changing the shape of the cathode.

Currently, VAT development focuses on CubeSat propulsion subsystems. This includes studies conducted by the Kyushu Institute of Technology development on HORYU nanosatellites [19], the University of Federal Armed Forces in Germany on the UWE-4 nanosatellite [22], and the US Air Force BRICSat-P CubeSat [23] and George Washington University's participation in the PhoneSat Project at NASA Ames Research Center [24]. This shows that an increasing number of countries are dedicating themselves to the development of VATs. As VATs are still in their preliminary stage of development, some theoretical models of VATs have been published [13,25,26]. In the past twenty years, Statom used some vacuum arc empirical data and a basic energy balance to estimate the ratio of kinetic energy and electrical energy and attempted to understand the phenomenon of plasma [26]. Sekerak and Polk et al. have developed a semiempirical model to more accurately determine the performance of VAT for a wide range of cathode materials [13,25].

In principal, any electrically conductive solid metal, compound, or alloy can be used as a propellant for the VAT due to their high density and high electrical and thermal conductivities. Researchers typically select metals that have high melting points, high boiling points, and relatively low electron work functions to fulfill the basic requirements that are derived from a cathode operation for a VAT. Some experimental results revealed the performance of periodic metal elements when they are applied to a VAT [27]. However, it is still a challenge to determine the most suitable metal. Different metal materials have different thermophysical and electrical properties. These characteristics can even affect the performance of the thruster. However, many factors influence these characteristics, such as the state of the cathode surface, the presence of particles, and the discharge condition of the vacuum arc. Although the VAT is still in the preliminary research stage, this study used copper and aluminum as cathode materials after considering the accessibility and affordability of metal materials. Moreover, studies have indicated that some carbon- and graphite-based materials are effective propellants for the VAT [28,29]. Carbon has high-temperature resistance and sublimes rather than melting, and its resistance decreases with increase in temperature. Graphite-based materials present better erosion rate results and higher amount of ions compared with the carbon-based materials. Therefore, researchers believed that carbon and graphite materials have the potential for application in VATs [28,29]. To apply the propulsion system to microsatellites, the miniaturization of electric propulsion and its auxiliary units are important without jeopardizing their function and thrust performance. Accordingly, this study developed micro function generators and micropulse discharge circuit design integrated with VAT system. Moreover, we tried to use a battery instead of a power supply to generate a pulse discharge. Eventually, experiments have confirmed that this set of VAT prototypes exhibits high stability and repeatability during operation.