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Surface Microwave Discharge at High Pressures of Air

A.F.Aleksandrov1, V.M.Shibkov2, L.V.Shibkova3

Department of Physics, Moscow State University, 119992, Moscow, Russia

Surface microwave discharge excited on the outer surface of a dielectric antenna athigh values of air pressure has been investigated. The transverse and longitudinaldimensions and propagation velocities of the discharge have been measured asfunctions of the air pressure and the power and duration of the exciting microwavepulse. The spatial distributions and time evolution of the gas temperature, andelectron density have been determined.

In previous our works [1-8] for search of optimum ways of creation of non-equilibrium

plasma in a supersonic gas stream we had been offered new type of the super high frequency

discharge, namely, the microwave discharge which is created by a surface wave on a dielectric

body, streamline by a supersonic stream of air and a propane-air mixture. The basic properties of

such discharge have been investigated in detail in a range of pressure 1 mTorr - 50 Torr. It was

shown, that the general view of a surface microwave discharge is transformed at change of air

pressure. At low air pressure (р < 1 Torr) the transversal size of the discharge increases rapidly

with decreasing air pressure and reaches of ~10 cm at air pressure p ~ 1 mTorr. The degree of

ionization of the discharge plasma can exceed 10% under these conditions. The spatial

distribution of the electron density was found to depend strongly on the air pressure. At average

pressure (р =1-50 Torr) the discharge represents a thin homogeneous plasma layer.

In the given work the microwave discharge created on an external surface of the quartz

antenna at atmospheric pressure of air is considered. Experimental installation includes vacuum

chamber, magnetron generator, system for input of the microwave energy in the chamber and

diagnostic system.

The microwave source is a pulsed magnetron generator operating in the centimeter

wavelength range. The parameters of the magnetron generator are as follows: the wavelength is

λ = 2.4 cm, the pulsed microwave power is Wp < 70 kW, the pulse duration is τ = (5-100)·10-6 s,

the period-to-pulse duration ratio is Q=1000, and the mean microwave power less than 100 W.

The vacuum system allowed us to perform experiments in a wide pressure range, from 10 Torr

up to 760 Torr.

1 Professor, Department of Physics, Moscow State University, 119992, Moscow, Russia, alex@ph-elec.phys.msu.ru.2 Professor, Department of Physics, Moscow State University, 119992, Moscow, Russia, shibkov@phys.msu.ru3 Professor, Department of Physics, Moscow State University, 119992, Moscow, Russia, shibkov@ph-elec.phys.msu.ru

47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition5 - 8 January 2009, Orlando, Florida

AIAA 2009-490

Copyright © 2009 by Moscow State University. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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In experiments the time and spatial distributions of the main parameters of a surface

microwave discharge, namely, general view of the discharge, spectrum of plasma radiation, gas

temperature, electron density, shadow photos of area of the discharge existence, position of the

shock waves generated by the discharge, and dynamics of development of the discharge were

investigated.

The surface microwave discharge at air pressure more than 100 Torr represents the

complex system consisting of thin branching plasma channels. Channels diameter changes from

0.1 mm up to 1 mm. The general view of a surface microwave discharge is submitted in Fig. 1 at

atmospheric air pressure. The transversal size of the channel decreases with increasing of

pressure. It reaches value of ~0.1-0.2 mm at p = 760 Torr depending on microwave pulse

duration and power. It is connected to development of ionize-overheat instability.

Fig. 1. General view photo of the microwave discharge on an external surface of the quartz

antenna of rectangular section 9.5х19 mm (at the left - front view, on the right - side view)

p = 760 Torr, τ = 5 µs, W = 55 kW. The surface wave is distributed from below upwards.

Let's consider dynamics of development of a surface microwave discharge at air pressure

of p = 760 Torr. Experiments were carried out at various values of pulsed microwave power and

pulse duration. The general view of the discharge registered during the various moments of time

at the different microwave power are submitted in Fig. 2-4. Process develops in time so, that

plasma arises in separate areas of space on a border of contact of a wide wall of a metallic

waveguide with the dielectric antenna where there is an initial breakdown of gas. From these

areas the thin plasma channels extending basically in a longitudinal direction start to develop.

Eventually there is a branching channels, and speed of their distribution along the antenna

decreases.

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τ = 5 µs 10 µs 30 µs 60 µs 100 µs

Fig. 2. Evolution of the microwave discharge on an external surface of the quartz antenna at

p = 760 Torr, W = 22 kW, and different pulse duration. The surface wave is distributed from

below upwards.

τ = 5 µs 10 µs 30 µs 60 µs 100 µs

Fig. 3. Evolution of the microwave discharge on an external surface of the quartz antenna at

p = 760 Torr, W = 41 kW, and different pulse duration. The surface wave is distributed from

below upwards.

τ = 5 µs 7 µs 20 µs 50 µs 75 µs

Fig. 4. Evolution of the microwave discharge on an external surface of the quartz antenna at

p = 760 Torr, W = 78 kW, and different pulse duration. The surface wave is distributed from

below upwards.

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In Fig. 5 one can see investigation results of the surface microwave discharge length

temporary dependence at air pressure р = 760 Torr and different values of microwave power.

One can see that linear sizes of the surface microwave discharge grow with increase of input

microwave power (for the fixed time moment). At that variation of the discharge longitudinal

size in the beginning of microwave pulse takes place much faster that at its quasi-stationary stage

of existence at all the values of microwave power. One has to pay attention to the fact that we

managed to realize the discharge at atmospheric pressure at pulsed microwave power 20 kW, i.e.

at electric field strength amplitude 2.8 kV/cm. It is much smaller than the electric field amplitude

necessary for atmospheric air breakdown. Time dependence of longitudinal velocity of the

surface microwave discharge at p = 760 torr and W = 100 kW is submitted in Fig. 6.

Fig. 5. Surface microwave discharge length vs time at air pressure р = 750 Torr and different

values of pulsed microwave power Wp, kW: 1 − 20; 2 − 30; 3 − 40; 4 − 55; 5 − 70; 6 − 100.

Fig. 6. Time dependence of longitudinal velocity of the surface microwave discharge at

p = 760 Torr, W = 100 kW.

0 20 40 60 80 1000

2

4

6

8 65

4

3

2

1

L,cm

t, µs

0 10 20 30 4010

4

105

106

υ,cm

/s

t, µs

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Longitudinal propagation velocity of surface microwave discharge along the antenna

varies significantly during the microwave pulse (see Fig. 6). In the initial stage of the discharge

the longitudinal propagation velocity reaches some kilometers per second and monotonously

decreases with increasing air pressure (see Fig. 7). Received results indicate that surface

microwave discharge can be realized in supersonic gas flows.

p, Torr: 20 30 40 50 100 150 300 500 750

Fig. 7. General view photo of the surface microwave discharge at different values of air pressure

at τ = 5 µs, W = 78 kW. The surface wave is distributed from below upwards.

In Fig. 8 one can see discharge propagation longitudinal velocity dependence on air

pressure at different pulsed microwave power. Velocity is averaged over first 10 µs of the

discharge existence.

Fig. 8. Surface microwave discharge propagation velocity vs air pressure at different microwave

pulsed power W, kW: 1 − 24; 2 − 55; 3 − 78.

100

101

102

103

105

106

107

3

2

1

υ,cm

/s

p, Torr

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Fig. 9. Dependence of propagation velocity of a surface microwave discharge on the reduced

electric field at different microwave power W, kW: 1 − 24; 2 − 55; 3 − 78.

Fig. 10. Dependence of reduced propagation velocity υ/p of a surface microwave discharge on

W/(p2+219).

Gas is quickly heated up in boundary area because of the electric field is located in a thin

layer near surface of the antenna under conditions of a surface microwave discharge (Fig. 11). It

results in thermal explosion near to a surface of the antenna. Therefore formation of the

discharge is accompanied by generation of shock waves. Shock wave velocity reaches ~1 km/s

near surface of the antenna and during shock wave moving its velocity quickly decreases up to

420 m/s. At late stages in the area of the discharge existence the zone of the lowered density of

neutral gas (cavity) is formed.

101

102

103

104

105

106

107

3 2 1

υ,cm

/s

E/N, Td

10-2 10-1 100 101 102 103102

103

104

105

106

107

3

2

1

υ/p,

cms-1

Tor

r-1

W/(p2+219), W/Torr2

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The obtained results show that a microwave discharge initiated on the outer surface of a

dielectric antenna can be used to develop new types of plasma sources for various technological

applications.

Fig. 11. Gas temperature vs time under conditions of surface microwave discharge at

p = 760 Torr, τ = 100 µs, W, kW: 1 – 42; 2 – 65.

The work was partially supported by the Russian Foundation of Basic Research (grant

#08-02-01251), Russian Academy of Science (P-09 program) and CRDF Project # RUP-1514-

MO-06.

References

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No.6, p.775.

4. Shibkov V.M., Aleksandrov A.F., et al. //Plasma Physics Reports, 2005, v.31. No.9, p.795.

5. Shibkov V.M., Ershov A.P., Chernikov V.A., Shibkova L.V. //Technical Physics, 2005,

v.75 No.4, p.455.

6. Shibkov V.M., Dvinin S.A., Ershov A.P., Shibkova L.V. //Technical Physics, 2005, v.75,

No.4, p.462.

7. Shibkov V.M., Aleksandrov A.F., et al. //Moscow University Physics Bulletin, 2004, v.59,

No.5, p.64.

8. Shibkov V.M., Vinogradov D.A., et.al. //Moscow University Physics Bulletin, 2000, v.55,

No.6, p.80.

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