23D02
Propagating Velocity Control of Atmospheric Millimeter Wave Plasmaand Its Application to Microwave Rocket
大気圧ミリ波プラズマの伝播速度制御とそのマイクロ波ロケットへの応用
Toshikazu Yamaguchi, Tensei Takeichi, Masafumi Fukunari, Kimiya Komurasaki,Yasuhisa Oda1, Ken Kajiwara1, Koji Takahashi1 and Keishi Sakamoto1
山口敏和, 武市天聖，福成雅史，小紫公也，小田靖久1，梶原 健1，高橋幸司1，坂本慶司1
Department of Advanced Energy, The University of Tokyo
5-1-5 Kashiwa-no-ha, Kashiwa, Chiba 277-8561, Japan東京大学大学院・先端エネルギー工学専攻 〒277-8561 千葉県柏市柏の葉5-1-5
1Plasma Heating Technology Group, Japan Atomic Energy Agency801-1 Mukoyama, Naka, Ibaraki 311-0193, Japan
日本原子力研究開発機構・加熱工学研究グループ 〒311-0193 茨城県那珂市向山801-1
The propagating velocity of millimeter-wave supported detonation is one of the key parameters for thrust
performance of Microwave Rocket and it is faster than that of laser supported detonation (LSD), resulting
in lower heating rate in the ionized region and lower pressure behind the detonation wave than LSD. In theexperiments, thrust performance was enhanced by reducing the propagating velocity of ionization front,
which is achieved by converting the millimeter-wave beam from the Gaussian profile into a flat-top profile.

1. Introduction and Propagating Structure
inside Microwave Rocket
A Microwave Rocket is a future low-costlauncher to the space, as shown in Fig. 1.
Propulsive energy is repetitively supplied by pulsed
millimeter wave beams irradiated from the groundand the atmospheric air is used as a propellant, sothat it can be propelled without any energy sources
or propellants on-board [1]. Microwave Rocket
consists of a reflector to ignite and a cylindricaltube to keep high pressure inside. Figure 2 shows
the thrust generation model. The closed end of thecylindrical tube has a beam-focusing reflector
called “thrust wall” and the other end serves as anentrance for the millimeter wave beam, from which
also air is exhausted and refilled after the end of athrust generation cycle [2].
A propagating structure inside a Microwave
Rocket is millimeter-wave supported detonation,
consisting of shock wave and plasma as millimeterwave absorption layer, as shown in Fig. 3. The

propagating velocity of millimeter-wave supported
detonation is one of the key parameters for thrust
performance of Microwave Rocket and in previousstudy it was faster than that of laser supported
detonation (LSD) at the same power density [3,4].
This faster velocity can lead lower heating rate in
the ionized region and lower pressure behind thedetonation wave than LSD.
Therefore the objective of this study is to reducethe detonation wave velocity and to enhance thethrust performance of Microwave Rocket.

Fig.2. Thrust generation cycle of Microwave Rocket

Fig.1. Schematic figure of Microwave Rocket

Fig.3. Propagating structure model of millimeter-wave
supported detonation

2. Experimental Setup
170GHz gyrotron at JAEA was applied to amillimeter wave beam generator, the specificationsof which is shown in Table I [5]. A Gaussian profile
beam was converted into a flat-top profile byquasi-optical phase correcting mirrors and theprofile was shown in Fig. 4. Experiments were donein the condition of 200kW power. The pressure on
the parabolic reflector was measured by ahigh-speed pressure gauge (603B, Kistler Co., Ltd.),
and the propagating velocity of the ionization front
was detected by the home-movie camera images
with changing of the beam pulse duration.
Table I. Specifications of the JAEA’s gyroton
Parameters
ValuesMillimeter wave frequency
170GHz (1.73mm)
Output power at the peak
0.2-1MW
Pulse duration time
0.1ms to 1000s
Beam profile
Gaussian (HE11)
Beam diameter
40.8mm (at waist)
Electrical efficiency
50-60%

Fig.4. Beam profiles of Gaussian and flat-top

3. Results and Discussions
Figure 5 shows plasma structures with two
different beam profiles, and the Plateau pressure at
the thrust wall was shown in Fig. 6. Table II showsthese results with analyzed thrust performancevalues. As a result, the propagating velocity of theionization front of the flat-top beam was around ahalf of that of the Gaussian beam due to the lower
peak power density, and this slower velocity
affected the pressure induced inside the thruster.
Table II. Results of Gaussian and flat-top beam
Gaussian
Flat-top
Peak power density
33kW/cm2 16kW/cm2
Propagating velocity of140m/s
68m/s
ionization front
Plateau pressure
7.8kPa14.4kPa
Thrust impulse
13.2mNs
28.4mNs
Cm = thrust / input power 93N/MW
199N/MWFig.5. Plasma structures with different beam profilesFig.6. Plateau pressure increment by the flat-top profile

While the propagation was slow, the fluid inside thethruster could obtain high enthalpy from the beam,thus it generated the higher plateau pressure andhigher thrust impulse. The thrust impulse wasestimated by the integral of the pressure history at
the thrust wall.
4. Conclusion
To reduce the detonation wave velocity and toenhance the thrust performance, a Gaussian profile
beam was converted into a flat-top profile byquasi-optical phase correcting mirrors. As a result,thrust performance was enhanced by reducing thepropagating velocity of ionization front.
Acknowledgments
This work was partially supported by Grant-inAid for JSPS (Japan Society for the Promotion ofScience) Fellows (DC1, 2010-).
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