Experimental Study of Plasma Decay for Enhancing Pulse Repetition Frequency in Microwave Rocket

Abstract

In Microwave Rocket, which uses beamed energy propulsion, a phenomenon has been observed by which plasma remains inside the thruster for some long period of time. A difficulty with this plasma retention is that it delays the next millimeter-wave pulse irradiation, thereby decreasing the pulse repetition frequency and reducing the time-averaged thrust of the rocket. Enhancing the millimeter-wave pulse repetition frequency of Microwave Rocket demands a better understanding of the phenomenon of plasma remaining inside a thruster. For this study, the plasma decay time of 28 GHz atmospheric millimeter-wave discharges was investigated experimentally using analysis of images taken using a high-speed camera. Findings indicated that the plasma decay time increased rapidly to over 1 ms with the decrease of millimeter-wave power density at 100 kPa atmospheric pressure. Moreover, the plasma decay time at 20 kPa showed no such drastic increase at low power density. The vibrational and translational–rotational temperatures measured in the earlier study and the zero-dimensional non-equilibrium analysis performed for this study suggest that once thermal equilibrium is achieved at the plasma front, the plasma might remain for a long period of time up to 10 ms.