This paper discusses the possibility of producing high concentrations of O2(a1Δg) states at pressures up to atmospheric in
rare-gas/oxygen/NO mixtures by using micro-plasmas. Micro-plasmas refer to electric discharges created in very small
geometries which have been proven able to operate in DC mode at high pressure and high power loading without
undergoing any glow to arc transition. The so-called Micro Cathode Sustained Discharge (MCSD), which is a three-electrode
configuration using a Micro Hollow Cathode Discharge (MHCD) as a plasma cathode, can be operated as a
non-self-sustained discharge with low values of the reduced electric field and of the gas temperature. As a result, these
MCSDs can efficiently generate large amounts of singlet delta oxygen. In Ar/O2/NO mixtures, at an oxygen partial
pressure of 10 mbar, high values of O2(a1Δg) number density (1.5 1016 cm-3) and of the production yield (6.7 %) can be
simultaneously obtained. For lower O2 partial pressure, yields higher than 10 % have been measured. In He/O2/NO
mixtures, O2(a1Δg) number densities around 1016 cm-3 were achieved at atmospheric pressure for flow rates in the range
5-30 ln/mn, which could give rise to new applications.
Considered is the method of isothermal calorimeter intended for the measurement of singlet oxygen (SO) concentrations in oxygen flow. The method is distinguished for its simplicity and does not require expensive equipment. However, by now no calculation methods were available that would grant the necessary properties of the calorimeter at designed conditions. In this our work such calculation method is developed for calorimeters. Compared are the concentrations of SO produced in electric discharge generators as measured by calorimetric and optical methods.
A few years ago, it has been demonstrated that the use of hydrocarbons such as C2H4 or C6H12 as hydrogen donor allows realizing high-volume discharges in SF6 based non-chain HF/DF gas mixture. Although high energy, short pulses and high repetition rates have been obtained, for some uses the specific output energy, the efficiency and, to a less extent, the optical quality of all these lasers remain insufficient. IT appears now that the only way to increase these values is to use chain reaction. But chain reaction is known to be delicate to control, especially when working at high repetition rate. This paper will describe recent work directed towards a better understanding of the discharge mechanisms and an increase of the efficiency and of the specific output energy of HF/DF pulsed lasers using chain reaction in discharge initiated repetitive and non-repetitive experiments. High volume discharge cells compatible with repetition rate have been realized. Discharge initiated repetitive chain reaction ash been studied up to 10Hz with large gap. Efficiencies as high as 70% have been obtained in single pulse experiments. Computer modeling allows now predicting satisfactorily the performances of discharge-initiated chain reaction HF/DF lasers as a function of electric circuit characteristics and gas composition.
The efficient repetitively pulsed (10 Hz) HF chemical laser initiated by barrier electric discharge with electrode gap 10 cm was realised. In mono-pulse mode specific output energy 3 and 23 J/l, technical efficiency (η) 3.4 and 26%, for non-chain and chain process, correspondingly, were obtained. In the repetitively pulsed (RP) mode of the laser operation at 10 Hz on the depleted fluorine-hydrogen mixture (20% F2, 5% H2) the mean laser power of 43 W was obtained (specific output energy E/V ~ 10 J/l, η=11.3%). The computational prediction for laser operation in repetitively pulsed mode at active length of about 0.5 m has shown the possibility of achievement of the specific laser energy about 15 J/l and technical efficiency up to 20%. Output laser specific energy ~ 14 J/l under RPCL conditions at length of active medium 0.37 m in mono-pulse mode was obtained in a good agreement with numerical prediction.
KEYWORDS: Electrons, Gas lasers, Solid state lasers, Switches, Capacitors, Solid state electronics, Electrodes, Solid state physics, High power lasers, Dielectrics
The phototriggering has been proposed many years ago as a way to improve the reliability of excimer lasers. It is now a mature technique that is used in conjunction with the most recent solid-state devices to make an industrial high average power excimer laser. After a brief historical introduction we here analyze the main features and scientific aspects of this discharge technique and give very recent results.
A few years ago, it has been demonstrated that the use of hydrocarbons such as C2H4 or C6H12 as hydrogen donor allows realizing high-volume discharges in SF6 based non-chain HF/DF gas mixture. However, for some uses, the specific output energy, the efficiency and, to a less extent, the optical quality of all these lasers remain insufficient. It appears now that the only way to increase these values is to use chain reaction. But chain reaction is known to be delicate to control, especially when working at high repetition rate. This paper will describe recent work directed towards a better understanding of the discharge mechanisms and an increase of the efficiency and of the specific output energy of HF/DF pulsed lasers by using chain reaction in discharge initiated repetitive and non-repetitive experiments. High volume discharge cells compatible with repetition rate have been realized. Discharge initiated repetitive chain reaction has been studied up to 10Hz with large gap. Efficiencies as high as 70% have been obtained in single pulse experiments. Computer modeling allows now predicting satisfactorily the performances of discharge-initiated chain reaction HF/DF lasers as a function of electric circuit characteristics and gas composition.
Dynamics of development of single hot spot discharge in SF6 gas and in the mixture SF6/C2H6 and of its contraction has been studied experimentally. Visualization of discharge development has been performed by a fast shutter CCD camera. Discharge properties have been studied in a wide range of current densities and input energies. It has been found that a single hot spot discharge has a cup- like structure. The diameter of the cathode hot spot is 0.04 cm, while the diameter of the plasma near the anode reach a value of about 3 cm. An increase of the discharge current causes an enlargement of the discharge cross section. The hypothesis of discharge widening is presented and discussed. It has been shown that discharges in the mixture are more homogeneous and stable. Single hot spot allows to realize a discharge current of 2 - 3 kA at input energies of about 1 - 3 J without distortion of discharge homogeneity. When the discharge with three hot spots separated by 5 mm has been investigated, the formation of inhomogeneity from one hot spot occurs at 700 A. Development of a channel with enhanced conductivity from single hot spot in SF6 gas has been also obtained experimentally in case with advanced preionization.
Results of the investigation of the discharge parameters, active medium homogeneity and stability, and laser performance are reported for a 2.5x2.5x50 cm3 HF phototriggered laser working either in chain and non-chain mode. Using a Ne/F2/H2/O2=93/5/1.5/0.5 mixture, a specific output laser energy of 9 J/l was obtained with an electrical efficiency of 38%. Compared to the performance ofnon-chain reaction HF lasers, these data correspond to an increase ofthe efficiency by a factor of 8. However, due to the development of discharge instabilities the output laser energy is not higher than that obtained with the non-chain laser mixtures.
The influence of the X-rays preionization on the performance of a 312 cm3 active volume phototriggered HF laser has been studied for Ne/SF6/C2H6 and SF6/C2H6 mixtures at a fixed SF6 partial pressure (78 mbars). Parameters are the Xrays dose (0.1 -10 mR), the pumping current pulse duration (60-155 ns), the electric charge deposited in the medium (< 3 mC), and the ethane partial pressure (< 10 mbars). It is shown that, without ethane in the mixture, arcs develop close to the discharge peak current as soon as the pumping pulse duration exceeds 100 ns at a fixed deposited charge value, in the whole range of X-rays dose values studied. An efficient discharge stabilisation effect can be obtained by addition of ethane, even without X-rays preionization and with a pumping pulse duration value as high as 155 ns. Nevertheless a low ethane concentration in the mixture is required in order to optimise the laser performance. In such conditions an arc free discharge, and correlatively high output laser energy and efficiency values, can be obtained only with use of the preionization. Optimum laser performance can be achieved with a low X-rays dose.
Conditions allowing a controlled phototriggered operating mode of a 6 cm gap HF/DF laser have been studied. A detailed investigation of the discharge quality and laser performance have been performed through the extensive use of intensified gated CCD camera recordings correlated to the measurements of current and voltage waveforms, and output laser energy.
A large volume, self-sustained discharge, non-chain reaction, HF/DF laser has been built and tested. It was successfully operated at 12 Hz repetition rate and 20 J per pulse output energy (HF) in accordance with numerical model.
The non-chain reaction HF laser energized by an x-ray photo- triggered discharge has been studied for Ne/SF6/C2H6 and Ne/SF6/H2 gas mixtures. For an active volume of 312 cm3, a high specific output energy of 10 J/l has been reached with an electric efficiency of 4.7% for the mixture with ethane, whereas only 6 J/l is obtained with an efficiency of 3.1% for the mixture with hydrogen. It is shown that discharge instabilities are responsible for the laser emission disruption in the case of the H2-mixture. However, addition of C2H6 to Ne/SF6, in place of H2, induces the discharge stabilization. The higher laser performance obtained with the C2H6-mixture, compared with the performance of the H2-mixture, is a direct consequence of this stabilization effect.
The non-chain reaction HF laser energized by an x-ray photo- triggered discharge has been studied for Ne/SF6/C2H6 and Ne/SF6H2 gas mixtures. For an active volume of 312 cm3, a high specific output laser energy of 10 J/l has been reached with an electric efficiency of 4.7% for the mixture with ethane, whereas only 6 J/l is obtained with an efficiency of 3.1% for the mixture with hydrogen. The study of the discharge development shows that the total production of HF molecules weakly depends on the gas mixture type, so that the plasma kinetic is not responsible for the difference observed between the laser performances obtained with the two mixtures. Time and spatially resolved measurements of the plasma fluorescence shows that discharge instabilities occur in the Ne/SF6 discharge, which are responsible for the HF laser emission disruption when hydrogen is added. But addition of ethane at about 3% of the total pressure induces the discharge stabilization, and allows the laser emission to occur in an homogeneous medium. As a result, this work clearly establishes the physical reason of the observed laser performance improvement when hydrogen is replaced by some hydrocarbons in discharge pumped HF laser.
X-ray photo-triggered discharges have been used to energetize Ne/SF6/H2 and Ne/SF6/C2H6 gas mixtures. For a discharge volume of 312 cm3 an output energy as high as 3.0 J has been obtained in the ethane mixture, while the best laser performance achieved in the hydrogen mixture is only 1.8 J. The physical reasons which could explain these differences, i.e. different kinetic pathways leading to the formation of the HF molecule, or the onset of discharge instabilities, have been investigated. It is shown, experimentally and theoretically, that the production of atomic fluorine weakly depends on the gas mixture type. A time resolved imagery of the interelectrodes space emphasized that the Ne/SF6/H2 active medium degenerates into a spatially inhomogeneous plasma, whereas a very stable homogeneous discharge is obtained in the Ne/SF6/C2H6 mixture. As a result the onset of a discharge instability is responsible for the laser emission collapse at low hydrogen partial pressure and high initial applied electric field values.
A theoretical model has been developed to study the XeCl phototriggered lasers developed in the frame of the Eureka EU205 program: one of small active volume studied at LPGP, and a larger one at LASERDOT. Over a large range of experimental parameters, a pretty good agreement is obtained for the evolution of the electric parameters and the laser output characteristics -- energy and power. At moderate pressures (2 - 3 bars), and high energy loading (400 - 600 J/l), the temporal evolution of the laser power exhibits the successive emission of several laser pulses. This feature is also predicted by the model, but the calculations largely overestimate the energy included in the later pulses. The physical reasons for this disagreement have been established on the LPGP device, for which optical spectroscopic studies have shown the development of discharge instabilities.
An X-ray photoswitched XeC1 laser is described with an emphasis on
the physical processes with determine its behaviour. A short pulse
duration, 5 ns, has been used to phototrigger a 50 cm3 active volume
laser head. To gain insight into the avalanche processes, the breakdown
delay times have been measured and compared to the theoretical
values obtained from a code assuming a local field approximation. A
specific laser energy higher than 6 J/L with an effiency of 2 % is
reported. The duration of the laser pulse is 30 ns leading to a specific
extracted power higher than 200 MW/L.
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