Plasma Structure Observed in a 94 GHz Microwave Rocket

In a propellant-free microwave rocket which consists of a shock wave propagation path part and a millimeter-wave focusing part, an atmospheric discharge occurs at the millimeter-wave focal point at the cone apex, exciting the plasma heated by the propagating millimeter-wave, and then the next ionization region is generated. At the same time, a millimeter-wave-supported shock wave is formed behind the plasma, leading to high pressure behind the wave. Finally, the rocket obtains thrust.

The shock wave directly affects the propulsion performance of microwave rockets and is given energy depending on the cone angle of a sphere that spreads isotropically in all directions, thus the cone angle is directly related to the propulsion performance. Therefore, this study mainly reports the effect of cone angle on the propulsion performance by observing the thrust and a plasma structure. In the experiment, the microwave source was a gyrotron, which was a high-power microwave oscillation source being considered as a rocket base. The rocket body was a translucent cylinder in order to observe the internal plasma structure, and the effect of the apex part can be easily observed. Moreover, a microwave beam energy transmitted from the ground launch base is assumed to propagate several tens of kilometers above the ground. Thus, in this study, it was chosen a frequency of 94 GHz because it had high linearity and atmospheric attenuation was lower than that of a terahertz wave.