High power pulse-periodical electrochemical HF laser

BULAEV V D，GUSEV V S，F(xiàn)IRSOV K N，KAZANTSEV S Yu，KONONOV I G，LYSENKO S L，MOROSOV Yu B，POZNYSHEV A N

(1．Federal State Unitary Enterprise Kosminov State Scientific Research Test Laser Center( Range) of the Russian Federation《RADUGA》，Raduzhnyi，Vladimir Region，Russia 2．Prokhorov General Physics Institute，Russian Academy of Science，Moscow 119991，Russia)

1 Introduction

Now nonchain electric discharge HF( DF) lasers are unique ecologically safe sources with high peaks and average generation powers at close to the diffraction limit of laser beam divergence in a practically important area of spectrum λ = 2.6/4.2 μm. Such sources represent doubtless interest not only for monitoring an atmosphere［1，2］，but also for carrying out some physical experiments on interaction IR radiation with liquids［3，4］and for optical pumping other gas lasers of IR range［5-7］，including obtaining short pulse［8］and powerful generation in the terahertz region of the spectrum［9］.

The purpose of the present work is to research an opportunity of achievement of the big radiation energy in a pulse-periodic mode of functioning the nonchain electric discharge HF laser with continuous metal electrodes in absence of additional measures on stabilization of Self-sustained Volume Discharge( SVD) .

2 Experiments

Experimental installation looked as follows. The discharge gap of the laser has been formed by two identical flat electrodes from the duralumin，rounded on perimeter in radiusr=2 cm. The size of a flat part of a surface of electrodes made 15 cm × 100 cm.The interelectrode distance could vary in limitsd=10/20 cm，experiments were carried out atd=15 cm. The electrode which was used as the cathode has been subjected to sandblasting with the purpose of formation on its surface small-scale nonuniformity( ～50 μm) ，that，as shown in Ref. ［10］，facilitates development the self initiating volume discharge. Electrodes were placed in to a glass-epoxide tube with internal diameter 80 cm and length 250 cm symmetrically and were relative its axes. Photograph of the electrode system，available through the output window of the discharge chamber is shown in Fig.1.

Fig.1 Discharge chamber with electrode system.

The working mixtures of the laser SF6∶C2H6=20∶1，SF6∶C3H8C4H10=30 ∶1 and SF6∶H2=9 ∶1 with the full pressure about 6/10 kPa were used.Change of working mixtures in volume discharge was provided due to circulation of gas in the closed contour. For this purpose it was applied the special fan block similar described in Ref. ［11］.The gas mixtures were blown along an axis of the discharge chamber，speed of a stream in the discharge zone made 40 m/s.

The resonator of the laser has been formed by a copper mirror with the radius of curvatureR=20 m and a plane-parallel plate from KCl. The mirror was installed in the adjusting unit connected to the flash chamber by sylphon. The plane-parallel plate fastened directly at an end face of the discharge chamber. The radiation energy of the laser was measured in a single pulse mode with the help of a matrix of calorimeters such as E-60 with the size of 18 cm ×18 cm，installed in a direct laser beam on distance of 1 m from a window of the discharge chamber.

The pulse high-voltage generator using for formation SVD consist of 4 identical sections，connected in parallel to electrodes of a discharge gap copper trunks. Sections are placed in the uniform metal case filled SF6at atmospheric pressure. Sections of the generator are collected under two-level Fitch circuit on low inductive condensersC=50 nF with rated voltage 100 kV marking KMKI-100-50. Electric circuit used in high-voltage generator is shown in Fig.2. Controlled spark-gaps［12］are filled with mixtures SF6∶N2=1∶10 with overpressure up to 6 bars.The construction of spark-gaps allows to operate the laser by duration up to 60 s with frequency of following of discharge pulses in 20 Hz without replacement in them of gas. Charging voltageUchin experiments varied in limitsUch= 50/75 kV. In a circuit of spark-gaps，the inductance using for the coordination of a contour of polarity inversion of a voltage on the condenser with a discharge contour was fitted experimentally. The pulse voltage of the discharge was measured by a resistive divider，and a pulse current was measured by a Rogowski coil covering a part of the current carrying trunk. Such quality monitoring at the big trunks width( ～1 m in examined installation) does not give an opportunity of exact measurements of a discharge current，but allows to determine its maximum time position. The system of synchronization provides simultaneous start 8 spark-gaps of the high-voltage generator with accuracy ±10 ns.

Fig.2 Electric circuit of Fitch high-voltage generator( one of 4 section) .

3 Results and discussion

After the coordination of a contour of polarity inversion of the condenser in Fitch generator with a discharge contour steady SVD in mixtures SF6with hydrocarbons has been received in all range of voltage changeUch=50/75 kV at frequency of following of discharge pulses up to 20 Hz. Opportunities of the further increase in frequency of following were limited to speed the change of a working gas mix in a discharge chamber. Fig.3 shows a photograph of SVD in a mixture SF6∶C2H6at the maximum charging voltage inUch=75 kV，illustrating the uniformity of SVD in high edge to strengthen the electric field，which is characteristic for the intervals used here type. Fig.4 shows the distribution of the intensity of the glow discharge plasma to coordinate a plane par-

Fig.3 Photograph of discharge chamber( front view)and SVD in mixtures SF6∶C2H6 =20∶1.

Fig.4 Distribution of intensity glow of discharge plasma to coordinate a plane parallel to the surface of electrodes.

allel to the surface of the electrodes. The distribution reflects the distribution of input power for the interval［14］. Fig. 4 shows that，despite the marginal gain of the electric field，the maximum energy deposition is achieved in the central zone of the gap.Fig.5 shows typical oscillograms of the voltage across the discharge gap and discharge current when the ignition SVD in a mixture of SF6∶C2H6. These oscillograms allow with sufficient accuracy to estimate the duration of the discharge currenttdis≈320 ns. With such a relatively large，the duration of the input electrical energy into the discharge plasma in mixtures of SF6with hydrogen SVD is stable only in the charging voltagesUch≤60 kV. In mixtures of SF6with the same hydrocarbons as follows from the above experimental data，SVD implemented in the whole range ofUchwithout whatever additional measures to stabilize it as a pulse and a pulse-periodic modes of the HF laser. Laser pulse was typical nonchain electric discharge HF laser form［13］，its duration at half amplitude was about 150 ns，the generation began near the current maximum.

Experimentally dependence of radiation energyWgfrom charging voltage is submitted on Fig. 6.Apparently from Fig. 6，the maximal energy of generationWg=67 J is reached on mixtures SF6∶C2H6，atUch=75 kV，that corresponds to electric efficiency ～3% on energy，reserved in condensers.Low efficiency of the laser on mixtureses SF6with hydrogen is caused，apparently，by pumping heterogeneity because of rather big duration of a discharge current. We shall also note，that in the present work the efficiency is lower than that in Ref. ［14］at close characteristics of a discharge gap and identical structures of a mixtures. Probably，the given fact can be connected to the big losses of electric energy in circuits of the Fitch generator.

Fig.6 Dependences of laser generation energy Wg from Uch at use different mixtures of hydrocarbons

4 Conclusions

Thus，by us it is developed and investigated powerful the nonchain electric discharge repetitively pulsed HF laser. The opportunity of realization SVD on mixtures SF6with hydrocarbons in a discharge gap with high edge electric field gain without additional measures of stabilization of the discharge both in pulse，and in a repetitively pulsed mode is shown. Laser generation energyWg= 67 J is received at frequency of pulses following of 20 Hz.

In summary，we shall notice that，for reception of generation on DF as the donor of deuterium can be used C6D12. Stability SVD in mixtures SF6with deuterocarbons，as is known，not worse，than in mixes with hydrocarbons，and energy of generation on DF in identical conditions on pumping in plasma of the discharge makes ～0.8 from energy of generation on HF［15］.

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