Position and Attitude Tolerances of Carbon Nanotube Field Emission Cathode as a Neutralizer in an Ion Engine System

Formation flight using small satellites offers low cost space applications, high resolution earth observation, observation of electromagnetic waves (X-ray, infrared, and so on) emitted from celestial objects or even observation of gravitational waves. The propulsion system requirements of these missions include large total impulse with low propellant and power consumption, high response speed, a 3 digit throttling range, and low thrust noise.1) Offering large total impulse with low propellant and power consumption, an ion engine with field emission cathode is suitable as a main propulsion system. For small satellite applications,2) power consumption is an important factor. A field emission cathode (FEC)3–6) is therefore an attractive candidate for the electron source, since it has lower power consumption than conventional cathodes (such as hollow cathodes, microwave discharge cathodes, or radio-frequency discharge cathodes) and it does not consume propellant. It also does not involve failure-prone parts, such as valves and mass flow controllers. The current density of an FEC assembly is small,7) on the order of several mA/cm2 and that demonstrated for on-orbit use is even smaller,8) about 60 LA/cm2. These features of the FEC provide a robust and compact system, and offer higher specific impulse than conventional systems. Conventional ion engines provide about 20–25% of their propellant to the cathodes,9–12) and this leads to 20–25% degradation of the specific impulse. In our previous work, the neutralization of an ion engine using xenon gas as a propellant with a field emission neutralizer was successfully demonstrated,2) though the electron emission cost was 360W/A, which is higher than that of a conventional hollow cathode (less than 30W/A).13) Small hollow cathodes, whose emission current is in the range of 10–100mA, consume several watts, which means that the electron emission cost of a small hollow cathode is more than 250W/A.14,15) Other remaining challenges in this field include investigation of cathode position and attitude tolerances, evaluation of thrust noise, and extension of the lifespan of the FEC with carbon nanotubes. The aim of the present study is to investigate the dependence of cathode position and attitude on neutralization performance, for the evaluation of the cathode setpoint tolerance. The potential difference between cathode and ground, Vcg, is used here as an indicator of neutralization. If Vcg drops below a critical voltage, the FEC will be sputtered by charged exchanged ions, and this could lead to degradation of emission capacity. For the purposes of the present study, the critical potential difference is taken to be120V, since the potential difference between the plume and the ground is assumed about 30V and the threshold energy of carbon-carbon composite under xenon bombardment is assumed to be about 50 eV.16) This critical value, 120V, a reasonable assumption for the present purposes, should be further investigated in future work.