ISSN 1063780X, Plasma Physics Reports, 2011, Vol. 37, No. 11, pp. 965–971. © Pleiades Publishing, Ltd., 2011. Original Russian Text © S.I. Gritsinin, P.A. Gushchin, A.M. Davydov, I.A. Kossyi, M.S. Kotelev, 2011, published in Fizika Plazmy, 2011, Vol. 37, No. 11, pp. 1034–1040.
PLASMA DIAGNOSTICS
Coaxial Microwave Plasma Source S. I. Gritsinina, P. A. Gushchinb, A. M. Davydova, I. A. Kossyia, and M. S. Kotelevb a
Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia b Gubkin State University of Oil and Gas, Leninskii pr. 65, Moscow, 11917 Russia Received March 25, 2011
Abstract—Physical principles underlying the operation of a pulsed coaxial microwave plasma source (micro wave plasmatron) are considered. The design and parameters of the device are described, and results of exper imental studies of the characteristics of the generated plasma are presented. The possibility of application of this type of plasmatron in gasdischarge physics is discussed. DOI: 10.1134/S1063780X11100059
A microwave plasmatron [1], or a microwave spark plug (MSP), designed and fabricated at the Prokhorov General Physics Institute, Russian Academy of Sci ences, and the Baranov Central Institute of Aviation Motors (CIAM) was tested as an alternative to stan dard spark plugs used to ignite fuel/air mixtures in avi ation propulsion engines [2–4]. A schematic and a photograph of the MSP are shown in Figs. 1 and 2, respectively. The plasmatron is a coaxial waveguide transporting microwave energy from magnetrons used in domestic microwave ovens to the discharge system located at the waveguide output. The microwave energy is transported at the funda mental mode of the waveguide. The discharge system is a 20mmdiameter disk insert made of a radiotrans parent material (quartz). The disk diameter is equal to the inner diameter of the outer electrode. The 6mm diameter central electrode is inserted through an aper ture made at the center of the disk. A special design of the discharge system ensures tight contact between the central electrode and the quartz disk.
In the absence of matching and adjusting elements in the MSP waveguide (which simplifies its design and makes it appreciably cheaper), the geometry of the device (the length of the coaxial waveguide, the radial dimensions of its elements, etc.) plays an important role. The plasmatron version presented in Figs. 1 and 2 was used as an igniter of a kerosene/air flow at the CIAM test bench [2–4]. The experiments were carried out with a model frontal device of a turbojet combus tion chamber (Fig. 4) at different ignition and stabili zation conditions. The MSP was placed at the exit of the model frontal device, which formed a fuel/air flow with a Mach number of Mair = 0.1–0.35 and tempera
The magnetron operates at a frequency of f = 2.45 GHz in a singlepulse mode. The pulsed micro wave power is up to 1.6 kW, and the amplitude of the electric field in the running wave is up to 1.3 kV/cm. The pulse duration varies from 5 μs to 1 ms. A typical oscillogram of the magnetron anode current is shown in Fig. 3. When operating with atmosphericpressure gas mixtures, the supply of microwave power to the plas matron is accompanied by the development at its end of several discharge channels adjacent to the quartz disc surface and bridging the discharge gap. For the plasmatron to operate efficiently, the discharge system should satisfy a number of requirements. An impor tant requirement that ensures generation of plasmoids at the output of the coaxial waveguide is that the cen tral electrode be in tight contact with the quartz disk. If there is no such contact, the MSP does not operate. 965
Δr
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5 Fig. 1. Schematic of the MSP coaxial microwave plasma tron: (1) outer electrode, (2) quartz disk, (3) inner elec trode, (4) discharge channels, and (5) microwave radia tion.
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Fig. 2. Photograph of the MSP.
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Fig. 3. Oscillogram of the magnetron anode current (the time scale is 25 µs/div).
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Fig. 4. Scheme of the experiment carried out at the CIAM test bench imitating an aviation turbojet engine: (1) air nozzle, (2) frontal device model, (3) swirler, (4) whirled air flow, (5) standard spark plug, (6) ignition zone, (7) tangen tial kerosene supply, (8) MSP, and (9) microwave dis charge.
ture of Tair = 280–565 K, the kerosene/air content in the mixture being φ = 0.3–1.5. The results of experi ments on the mixture ignition with the help of the MSP were compared with those obtained using a stan dard aviation spark plug. It was found that, with the use of the MSP, the parameter range corresponding to stable ignition extended toward leaner fuelair mix tures and higher Mach numbers Mair. The relatively simple MSP, which demonstrated its competitive ability [2–4] as compared to standard spark plugs, was designed in such a way that the fol lowing two physical phenomena observed in micro wave discharges in highpressure gases were imple mented. These are a lowthreshold microwave dis charge (microwave spark) at the contact between a metal and a dielectric surface [5–7] and thermal–ion ization instability, which reaches a strongly nonlinear stage in microwave discharges [8, 9]. The MSP was designed so as to employ specific features of micro wave discharges for generating hot dense plasmoids. Figure 5 shows a scheme of the experiment in which the formation of a constricted discharge at the MSP output was studied and the parameters of the generated plasmoids were determined. The coaxial plasmatron operated in atmospheric pressure air. The end region of the coaxial waveguide was photographed at different durations of the micro wave pulse. The optical spectrum of the discharge was recorded using an AvaSpec 2048FT4RM and an AVSHR2000 spectrographs with spectral resolutions of 0.3 and 0.15 nm, respectively. The exposure time was longer than the microwave pulse, i.e., the time integrated spectrum was recorded. Due to the use of an optical fiber at the entrance of the spectrograph, the spatial resolution in the discharge region was better than δ ≈ 3 mm. This made it possible to separately study the spectral characteristics of radiation emitted from the electrode regions and the electrode gap. Figure 6 shows framecamera photographs of microwave discharges at the output of the coaxial plas matron. The circles show the positions of the inner and outer conductors of the coaxial waveguide. The photographs were taken at different instants and dem onstrate different stages of a discharge excited by a 100μslong microwave pulse. Figure 6a presents a photograph taken 5–10 μs after the beginning of the microwave pulse. This time interval corresponds to the initial stage of the discharge, in which a finite number of microsparks are initiated at the contact between the central electrode and the quartz disk. Figure 6b pre sents a photograph taken 15–20 μs after the beginning of the microwave pulse. This time interval corresponds to the second stage of the discharge, in which several initial microsparks become sources of highly con stricted bright filaments surrounded by a photoplasma halo. The filaments propagate radially along the quartz surface (from the central to the outer elec trode). Figure 6c presents a photograph taken at PLASMA PHYSICS REPORTS
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mined from the CN(B–X) molecular band by calcu lating the emission spectrum of CN at different tem peratures by using the LIFBASE software [10] and comparing the calculated and measured spectra. It was found that the calculated CN spectrum was close to the measured one only when both the vibrational and rotational temperatures exceeded 6000 K. On the other hand, analysis of the (0, 2) and (0, 3) bands of the second positive system of nitrogen shows that, while the vibrational temperature is indeed about 6000 K, the rotational temperature (which can be identified with the translational temperature) is as low as 1000 K. This discrepancy can be explained by the fact that the CN radiation is emitted from the inner region of the gasdischarge channel, where the tem perature and the degree of gas dissociation are high, while the radiation of molecular nitrogen apparently comes from the region adjacent to the channel, where the energy release and gas heating are not so high, but the electron energy is sufficient to efficiently excite vibronic levels of nitrogen molecules.
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We succeeded to record the Hα line of atomic hydrogen emitted from constricted discharge regions. A typical profile of this line is shown in Fig. 9. The observed substantial broadening of the Hα line can be attributed to the Stark mechanism [11], provided that the electric field is as high as 40 kV/cm (see [11, 12]). However, the microwave field in the discharge gap at the output from the coaxial line is as low as E r ≤ 1 kV/cm; hence, it cannot be responsible for the observed broadening. Apparently, such high electric fields are of the Holtsmark nature, the electron density corresponding to the observed broadening being ne ≈ 5 × 1016 cm–3 [11, 12].
Fig. 5. Scheme of the experiment: (1) magnetron (f = 2.45 GHz), (2) power supply (highvoltage pulsed genera tor), (3) coaxial waveguide, (4) discharge initiator, (5) optical fiber, and (6) optical spectrograph.
100 μs. This time corresponds to the third (final) stage of the discharge, in which the transverse size and brightness of the filaments increase substantially and the discharge passes into the socalled “arc” stage. Finally, Fig. 6d presents a timeintegrated photograph of the discharge. Figure 7 shows a typical spectrum of optical radia tion emitted from the electrode region. This spectrum is dominated by atomic and ionic lines of the electrode material. Figure 8 shows a typical spectrum of optical radia tion emitted from the electrode gap. It is dominated by the spectral bands of N2 and CN molecules. The gas temperature in different discharge regions was deter
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The results of these measurements do not contra dict the physical model underlying the operating prin ciple of the MSP. This model takes into account elec tricdischarge phenomena characteristic of the micro wave frequency range and determining the specific features of the discharge parameters at the output of
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Fig. 6. Photographs of the discharge taken at (a) 5–10, (b) 15–20, and (c) 100 µs after the beginning of the microwave pulse and (d) timeintegrated photograph of the discharge. PLASMA PHYSICS REPORTS
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Fig. 7. Spectrum of optical radiation emitted from the electrode region.
the coaxial waveguide. As was mentioned above, these phenomena include the processes occurring on the contacts between metal and dielectric surfaces irradi ated by microwaves. The tight contact between the central electrode and the dielectric insert at the output of the waveguide is one of the main specific features of the MSP design that differ it from analogous micro wave plasma sources described, e.g., in [13–16]. It is on these contacts that microscopic plasma objects (microsparks) are generated. This is clearly seen in Fig. 6a, which demonstrates the initial stage of plasma generation. In our plasmatron, the discharge in air is excited in a paradoxical situation in which the reduced electric field η ≡ E 0 / nm (where E 0 is the amplitude of the microwave electric field and nm is the gas molecule density) in the electrode gap is substantially lower than the threshold reduced field ηthr required for the onset of a selfsustained discharge (ηthr ≈ 10 −15 V cm2 [17]). Indeed, for E 0 ≅ 1.3 kV/cm nm ≅ 2. 5 × 1019 cm–3, we have
η ≈ 5 × 10 −17 V cm 2 Ⰶ ηthr .
(1)
Air breakdown at such low reduced electric fields is initiated by microsparks formed near the central elec trode of the coaxial waveguide at its output from the quartz disk. Although these microscopic plasma objects absorb only a small fraction of the incident
microwave power, it is efficiently converted into UV radiation (see [6, 7]), which, in turn, ionizes gas in the electrode gap at the waveguide output. In this stage, the gas discharge is not yet selfsustained. However, in contrast to most nonselfsustained discharges described in the literature (see [17–19]), our discharge is sustained in the absence of external ionization sources (such as UV radiation or an electron beam). The microwave electric field serves simultaneously as an energy source for both ionization of a highpressure gas medium (through the UV radiation of microsparks) and plasma heating. (Nonselfsus tained microwave discharges of this type underlie the operating principle of the microwave rocket engines described in [6].) The stage of a nonselfsustained discharge in our coaxial plasmatron lasts for 10–15 μs. Then, thermal– ionization instability develops, as a result of which sev eral discharge channels (or filaments) form (see Fig. 6b). Thermal–ionization instability develops in all types of highpressure gas discharges excited by quasi dc and highfrequency fields [17, 20]. However, only in microwave discharges, this instability reaches its nonlinear stage (see [8, 9]). In this stage, a highly con stricted discharge stretched along the electric field of the electromagnetic wave reaches a state characterized by a very high electron density and high electron and ion gas temperatures. This stage of a microwave dis PLASMA PHYSICS REPORTS
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charge cannot be identified with any known type of gas discharge. Here, this stage will be called “a microwave arc,” although we realize that the parameters of this new type of gas discharge differ substantially from those of conventional arcs (i.e., those operating between the electrodes in unidirectional fields), which PLASMA PHYSICS REPORTS
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have been actively studied and found wide application in different fields [21, 22]. The energy ε released per unit volume in a micro wave arc can be estimated from the relationship
ε ≈ σ E 02 τ,
(2)
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microwave energy is supplied to the discharge region is such a way that the discharge is attached to the output window of the coaxial plasmatron during the entire microwave pulse (~100 µs). 6000 Another specific feature of the MSP is that the plasmatron simultaneously generates several discharge Hα 4000 channels (in contrast to conventional arcs in dc or quasidc fields, in which a single discharge channel forms in the electrode gap). 2000 Other specific features of the MSP, such as high efficiency of microwave energy input in the discharge 0 in highpressure gases and its ability to simultaneously 655 656 657 658 generate several arc channels, make this type of plas λ, nm matron very promising for use as an efficient UV source, the main element of plasmachemical reactors, Fig. 9. Typical profile of the Hα line emitted from the elec etc. It should be specially emphasized that the MSP trode gap. can be used as an alternative to standard spark plugs in aviation engines. 2 In constructing an adequate physical model of the ne e where σ = is the plasma conductivity in MSP, it may be useful to employ the results of the m ω2 + ν 2eff recent work [23], in which the development of a “glo the discharge channel, τ is the arc lifetime, ω is the bal” microwave discharge initiated by a hot spot in air circular frequency of microwave radiation, and νeff is was analyzed theoretically. the effective electron–neutral collision frequency. Substituting ne ≈ 1016 cm–3, ν eff ≈ 5 × 109 p (Тоrr) ≈ ACKNOWLEDGMENTS 1012 s–1, τ = 100 μs, and E 0 ≈ 1 kV/cm into Eq. (2), we We are grateful to V.A. Vinogradov and 4 obtain ε ≈ 10 J/cm3. Yu.M. Shikhman for helpful discussions. This work This energy is substantially higher than the ε values was supported in part by the Presidium of the Russian typical of conventional microwave discharges and gas Academy of Sciences under the program “Fundamen tal Problems in Mechanics” (program no. P11), the discharge physics as a whole. International Science and Technology Centre (project The final stage of thermal–ionization instability is no. 3833r), and the Russian Federal target program illustrated in Fig. 6c. According to our measurements, “Scientific and Education Personnel of Innovative the electron density in the discharge channels at the Russia” for 2009–2013. output of the coaxial plasmatron is as high as ne ≈ 5 × 1016 cm–3 and the gas temperature in the dis REFERENCES charge region is Tg ≥ 6000 K. These values agree in 1. V. A. Vinogradov, I. A. Kossyi, S. I. Gritsinin, et al., RF order of magnitude with those predicted in [9] and Patent No. 2342811, 2007. measured in [8]. 2. A. M. Davydov, S. I. Gritsinin, I. A. Kossyi, et al., IEEE Note that in the experiments carried out in [8, 18], Trans. Plasma Sci. 36, 2909 (2008). where thermal–ionization instability in highpressure 3. V. A. Vinogradov, Yu. M. Shikhman, I. A. Kossyi, in discharges excited by converging microwave beams Proceedings of the 47th AIAA Aerospace Sciences Meeting was investigated, the duration of the phase of active and Exhibition, Orlando, FL, 2009, Report AIAA 2009 energy release in the plasma of discharge channels (fil 494. aments) was usually no longer than ~1 μs. This time is 4. I. A. Kossyi, S. I. Gritsinin, A. M. Davydov, et al., in limited by absorption of microwave energy in new fil Proceedings of the 35th IEEE International Conference aments that appear at a certain distance from the on Plasma Science, Karlsruhe, 2008, p. 282. already formed plasma channels (an ionization wave 5. G. M. Batanov, E. F. Bol’shakov, A. A. Dorofeyuk, propagating oppositely to microwave radiation [18]). et al., J. Phys. D 29, 1641 (1996). Therefore, it may be supposed that, in the previous 6. G. M. Batanov, S. I. Gritsinin, and I. A. Kossyi, J. Phys. experiments [8, 18], the plasma parameters of the dis D 35, 2687 (2002). charge channels did not reach their maximum values. From this point of view, an advantage of the MSP as 7. G. M. Batanov, N. K. Berezhetskaya, I. A. Kossyi, compared to the previously studied gasdischarge sys et al., Eur. Phys. J. Appl. Phys. 26, 11 (2004). tems is that its design allows one to increase the dura 8. V. G. Avetisov, S. I. Gritsinin, A. G. Kim, et al., Pis’ma tion of the active phase during which energy is depos Zh. Eksp. Teor. Fiz. 51, 306 (1990) [JETP Lett. 51, 348 ited in the discharge channels. This is possible because (1990)]. 8000
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Translated by E.V. Voronov