FIRSOV K N，KAZANTSEV S Yu，KONONOV I G，KOSSYI I A，TARASOVA N M，ZHANG Lai-ming，XIE Ji-jiang

(1．Prokhorov General Physics Institute，Russion Academy of Sciences，Moscow 119991，Russia;

2．Stake Key Laboratory of Laser Interaction with Matter，Changchun Institute of Optics，F(xiàn)ine Mechanics and Physics，Chinese Academy of Sciences，Changchun 130033，China)

1 Introduction

In recent years，laser ignition has been addressed in many papers［1，2］in connection with its possible practical applications. However，the mechanisms for initiating ignition are not yet completely understood.That is why investigation of the mechanisms for laser ignition of gas mixtures remains a challenging problem. It should be noted that the interest in laser ignition is also stimulated by the fact that laser provides a“pure”modeling of different ignition methods. The present paper is an experimental investigation of the dynamics of gas mixtures ignited by a Laser Spark( LS) or by heating a gas layer with a pulse laser heating.

2 Experimental Technique

A schematic of the experiment is shown in Fig. 1.The cylindrical quartz chamber with an inner diameter of 44 mm was evacuated to a pressure less than 13.3 Pa and was filled with a gas mixture: CH4∶O2=1∶2 at pressures of 18≤p≤40 kPa and mixtures CH4∶O2∶SF6=3∶6∶1(3∶6∶2) at a total pressure of 20(22) kPa. One end of the chamber was equipped with a solid metal flange，and the other end，with a flange having a BaF2window for injecting laser pulses. Different circular diaphragms and calibrated filters were placed in front of the window in order to attenuate laser.

Fig.1 Schematic of the experiment: reactor chamber: a quartz tube with length of 25 -30 cm，lightguides: ( AvaSpec) -spectrograph，P:photodiode，PER-7:streak camera，LS:laser spark，D:circular diaphragm，A:calibrated attenuators of laser light，and L:BaF2( NaCl) lens.

Experiments were carried out with a pulsed electric-discharge CO2laser［3］and a non-chain electrochemical Hydrogen Fluorine( HF) laser［1，4］. The HF laser operated at a wavelength of 2.65 -3 μm，the pulse duration and energy in 140 ns and 4 J，respectively; the CO2laser operated at the P20 line( λ =10.6 μm) ，the pulse energy was 60 J，the pulse duration was 3 μs，and the pulse profile was a typical of Transversely Excited Atmospheric pressure( TEA) CO2lasers［5］.

An LS for igniting gas mixtures was excited at the axis of the cylindrical chamber at a distance of～3 cm from the window by focusing laser by a BaF2lens with the focal lengthF= 10 cm. In the LH method，a small(2 -4 kPa) amount of SF6was added to gas mixtures，which were then heated and ignited by a CO2laser. Sulphur hexafluoride SF6strongly absorbs radiation from such a laser in the P20 line，thereby providing heating of the mixture. An analysis［5］shows that，under our experimental conditions，the energy stored in the vibrational degrees of freedom of an SF6molecule is converted into heat in less than 1 μs. Hence，by the end of the pulse(3 μs at a level of 0.1 of the maximum pulse amplitude) ，the laser energy absorbed by the gas is completely converted into heat( and thermal equilibrium is established) . In order to determine how the equilibrium temperature established by the end of the pulse was distributed along the tube，the dependence of light transmission of the gas mixtures under investigation on the energy density of the incident CO2laser light was determined by a method similar to that used in the Ref. ［5］.

The dynamics of the mixture ignition and burning were analyzed by using an FD-25k photodiode and a PER-7 high-speed photodetector，which made it possible to photograph discharges in the reactor in a continuous sweep mode. The slit of the photodetector was oriented along the optical axis of the cylindrical chamber( theZaxis) . A spectral analysis of the emission from the gas and of the time evolution of the emission from different cross sections along the quartz tube was carried out，respectively，with an AvaSpec-2048 spectrograph( produced by the Avantes company from the Netherlands) and an PEM-106 photomultiplier. The light emitted from the gas in the quartz tube was guided to spectrograph and photomultiplier through lightguides.

3 Results

Fig.2( a) shows a representative photograph of a discharge in the reactor chamber，taken with a PER-7 photoelectric detector，and Fig.2( b) shows an oscillogram of a signal from the photodiode. A CH4∶O2∶SF6=45∶90∶15 gas mixture was ignited by heating by a CO2laser. In the photograph，the horizontal axis is the time axis and the vertical axis is the spatial axis. The characteristic time and spatial scales are indicated in the figures.

Fig.2 ( a) Photograph of the ignition of a gas mixture CH4∶O2∶SF6 =45∶90∶15，taken with a photoelectric detector; ( b) oscillogram of a signal from the photodiode( scale of 2 mV/div，time scale of 1 ms/div) . D=18 mm，Win =2.6 J/cm3.

The dynamic pattern of ignition of gas mixtures by a freely localized laser spark［1］is similar to that shown in Fig.2. The ignition by heating by a CO2laser has the following distinctive features:

( i) the induction time is substantially shorter;( ii) the gas temperature established immediately after the burning of the mixture is substantially higher( the temperature was determined from spectroscopic observations in the same manner as was done in［1］) ; and ( iii) at energy densities slightly above that of the incident CO2laser light，it was observed that the burning rate increased sharply and the chamber exploded.

Fig.3( a) shows a representative photograph of the explosion in the reactor chamber taken with a PER-7 photoelectric detector，and Fig.2( b) shows an oscillogram of a signal from the photodiode.

Fig.3 ( a) Photograph of the explosion of a gas mixture CH4∶O2∶SF6 =45∶90∶15，taken with a photoelectric detector; ( b) oscillogram of a signal from the photodiode( scale of 10 mV/div，time scale of 1 ms/division) . D =18 mm，Win =4. 5 J/cm3.

Fig.4 shows the typical spectrum of radiation from a burning mixture of CH4∶O2∶SF6=45∶90∶15，obtained at different initiation energies. Appropriate spectra，normal ignition and explosion-not qualitatively are different.

Fig.4 The characteristic spectrum of radiation from a burning mixture CH4∶O2∶SF6 =45∶90∶15.The time delay power on the pulse of CO2 laser，initiating ignition，relative to the triggering of the spectrograph is about 0.5 ms，and the time of exposition is about 1 ms.

Presumably，the main reasons for these burning features of gas mixtures in the case of ignition by LH under our experimental conditions are the excitation of shock wave perturbations at the boundaries of the irradiated region and the nonlocal nature of ignition( the gas was ignited in a fairly large volume) . Another possible reason why the ignition by LH is intensified may be the decomposition of SF6molecules by multiphoton dissociation. Fig. 5 shows gas temperature distributions established along the axis of the cylindrical chamber immediately after a CO2laser pulse. The distributions were calculated for two energy densities of incident CO2laser at which the ignition was stable，Win=1.6 J/cm2; the gas mixture was detonated，Win=2.5 J/cm2; and the diameter of the circular diaphragm in front of the input window is 29 mm. It is seen that the gas was heated in a large volume within the chamber，and the notable features are a low ignition temperature and a short induction time(tind≈650 μs) .

Fig.5 Gas temperature distributions in a gas mixture along the cylindrical chamber immediately after a laser pulse，calculated for two energy densities of incident CO2 laser light，Win =1.6 and 2.5 J/cm2.

4 Conclusions

In experiments with laser beams passing through the circular diaphragms that were placed in front of the input window of the chamber，it was found that，for each diameter of the diaphragms，there were two threshold laser energy densities，W1thrandW2thr，above the first of which(W1thr＜Win＜W2thr) the gas mixture was ignited，and above the second(Win＜W2thr) ，it was rapidly burned out and the chamber exploded( detonation regime) . In the second case，however，the combustion wave velocity still remained lower than the detonation wave velocity characteristic of a given gas mixture. It should be noted that，as in the Ref. ［6］，the ignition threshold for gas mixtures depended on the incident laser energy density; i. e.，as the diaphragm diameter was increased，the first threshold energy densityW1thrdecreased approximately in inverse proportion to the diaphragm area.

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