Thrust performance and plasma generation of microwave rocket with microwave beam space transmission system

Summary form only given. Experiments of microwave beamed energy propulsion using 1 MW-class, 170 GHz gyrotron and a microwave beam space transmission system comprising of a transmission mirror system and a receiving system are described¹. The transmission mirror system was designed to expand the radius of an incident beam from 20.4 to 120 mm to suppress beam divergence due to diffraction. Two types of receiving system, a mirror type and a tapered-tube type, were developed. The receiving systems were equipped on the thruster to focus the expanded beam and guide it into the thruster. The characteristics of the transmission mirror system and the two receiving systems were examined using a low-power 170 GHz generator. In the high-power experiments using gyrotron, plasma propagation in the thruster was successfully observed through an acrylic window by fast-framing camera using the transmission mirror system and both receiving systems. Additionally, the pressure histories in the thruster were measured, and the propagation velocity of the shock wave as well as the thrust impulse were deduced. As a result, thrust impulse using the mirror-type receiving system is higher than that of the tapered-tube type. Moreover, it was also found that plasma exhausted from the thruster remains for several milliseconds, and the microwave beam was cut off by remaining plasma at high pulse repetition frequency, in the order of 200 Hz. The phenomenon has important role to optimize the thruster design and operation parameter of the gyrotron for the microwave rocket.