Propagation of an ionization front in the beam channel was observed after plasma was generated using a $170\mathrm{GHz}$ millimeter-wave beam in the atmosphere. The propagation velocity of the ionization front was found to be supersonic when the millimeter-wave power density was greater than $75\mathrm{kW}{\mathrm{cm}}^{-2}$ . The momentum coupling coefficient $C_m$ , a ratio of the propulsive impulse to the input energy, was measured using conical and cylindrical thruster models. A $C_m$ value greater than $350\mathrm{N}{\mathrm{MW}}^{-1}$ was recorded when the ionization front propagated with supersonic velocity.

A thrust stand has been developed to accurately measure thrust produced by two types of microthrusters—a liquid propellant pulsed plasma thruster (LP-PPT) and a diode laser ablation microthruster. The impulse of LP-PPT ranged from 20 to $\text{80\hspace{0.5em}μ}\mathrm{Ns}$ . The diode laser microthruster, which is a new type of microthruster, produces much lower impulse range of $\text{1–10\hspace{0.5em}μ}\mathrm{Ns}$ for about $\text{1\hspace{0.5em}}\mathrm{s}$ . The mechanical noise induced from the background vibrations becomes a crucial problem for precise estimate of thrust particularly in low impulse measurements. A data analysis method to reduce the effect of mechanical noise is proposed by introducing an additional term in a fitting function. It was verified that the analysis method used in our experimental conditions reduced variance caused by noise down to one-third that of a normal fitting method. The accuracy of the thrust stand is $\text{2.1\hspace{0.5em}μ}\mathrm{Ns}$ in the case of the LP-PPT and $\text{0.7\hspace{0.5em}μ}\mathrm{Ns}$ in the case of the diode laser microthruster.

To extend the lifetime of commercial and scientific satellites in low Earth orbit (LEO) and below (100-250 km of altitude) recent years showed an increased activity in the field of air-breathing electric propulsion as well as beamed-energy propulsion systems. However, preliminary studies showed that the propellant flow necessary for electrostatic propulsion at these altitudes exceeds the mass intake possible within reasonable limits, and that electrode erosion due to oxygen flow might limit the lifetime of eventual thruster systems. The pulsed plasma thruster (PPT), however, can be successfully operated with smaller mass intake and at relatively low power. This makes it an interesting candidate for air-breathing application in LEO and its feasibility is investigated within this paper. An analysis of such an air-breathing PPT system shows that for altitudes between 150 and 250 km, drag compensation is at least partially feasible assuming a thrust-to-power ratio of 30 mN/kW and a specific impulse of 5000 s. Further, to avoid electrode erosion, inductively heated electrothermal plasma generator technology is discussed to derive a possible propulsion system that can handle gaseous propellant without unfavorable side effects. Current technology can be used to create an estimated 4.4 mN of thrust per 1 mg/s of mass flow rate, which is sufficient to compensate the drag for small satellites in altitudes between 150 and 250 km.

Plasma acceleration processes in an ablative pulsed plasma thruster (APPT) were investigated. APPTs are space propulsion options suitable for microspacecraft, and have recently attracted much attention because of their low electric power requirements and simple, compact propellant system. The plasma acceleration mechanism, however, has not been well understood. In the present work, emission spectroscopy, high speed photography, and magnetic field measurements are conducted inside the electrode channel of an APPT with rectangular geometry. The successive images of neutral particles and ions give us a comprehensive understanding of their behavior under electromagnetic acceleration. The magnetic field profile clarifies the location where the electromagnetic force takes effect. As a result, it is shown that high density, ablated neutral gas stays near the propellant surface, and only a fraction of the neutrals is converted into plasma and electromagnetically accelerated, leaving the residual neutrals behind.