Recent research on high-pressure hybrid radiofrequency/arc (RF/DC) generator of singlet oxygen for DOIL is presented.
A stable hybrid RF/DC discharge with a diffusive arc root near the anode side was demonstrated experimentally under
well defined conditions. A minimization of anode erosion and a non-equilibrium state of the arc plasma were achieved in
this way. The RF discharge is used for the ignition of the DC arc and its sustainment. The hybrid discharge can be
characterized as an RF sustained (i.e. non-self sustained in the pure DC mode) DC arc between a conventional glowing
tungsten cone cathode and a cooled aluminum cylindrical anode with a diffusive arc mode. The diffusive mode of the arc
is assisted by a plasma anode formed inside the aluminum anode channel due to the radiofrequency. The generation of
singlet oxygen was achieved by a laterally symmetric injection of neutral oxygen in a mixture of O2+He+NO into the
Ar+He plasma jet of the hybrid RF/DC plasmatron. An overview of basic characteristics and CFD modeling of some key
gas flow phenomena are presented. The research is directed to experimental investigation and theoretical modeling of the
hybrid plasma. In the case of its successful completion it will be used for pumping a discharge oxygen-iodine laser.
Latest advances in development of a hybrid RF/DC plasma jet generator of O2(1Δ) for a discharge oxygen-iodine laser
are presented. This novel apparatus is based on a fast mixing of hybrid Ar+He plasma jet of DC electric arc sustained by
an RF discharge with an injected neutral O2+He+NO stream. Calculations of singlet oxygen yield have shown that only
non-equilibrium plasma with a high content of heated electrons (~3 eV) and a gas temperature of ~2500 K appears to be
promising for an achievable yield of singlet oxygen up to ~42 %. A stable high-pressure hybrid RF/DC plasmatron with
a diffusive arc mode near the anode side (assisted by a plasma anode) was demonstrated experimentally in the mixture
Ar:He = ~9.84:5 mmol/s at a power level of 610 W (RF power 490 W and DC arc power 120 W) and at a pressure of
165 Torr. The specific energy of the plasma jet was 41 J/mmol. A generation of singlet oxygen was performed by a
laterally symmetric injection of neutral mixture O2:He:NO = 1.5:2.44:0.22 mmol/s into the plasma jet of the hybrid
RF/DC plasmatron. The estimated yield of singlet oxygen O2(1Δ) was ~5 % at a pressure of 10.5 Torr.
Recent advances in the RF atomic iodine generator for oxygen-iodine lasers are presented. The generator is based on the
RF discharge dissociation of a suitable iodine donor immediately before its injection to the flow of singlet oxygen. The
discharge is ignited directly in the iodine injector, and the configuration is ready for the laser operation. The dissociation
fraction was derived from the atomic iodine number density measured at a presupposed position of laser resonator. The
dissociation fraction and the fraction of RF power spent on the dissociation (discharge dissociation efficiency) were
measured for the following donors: CH3I, CF3I and HI. A significant improvement of the discharge stability was
achieved by increasing the cross-sectional area of the exit injection holes and employing a tangential inlet of working gas
into the discharge chamber. The flow rates 0.15 mmol/s and 0.19 mmol/s of produced atomic iodine were achieved using
the HI and CF3I, respectively. The atomic iodine number density in the supersonic flow attained 4.22 × 1014 cm-3. The
dissociation efficiency was substantially better for HI than for studied organic iodides.
A cw/pulsed radiofrequency discharge coupled by electrodes in coaxial arrangement was used to dissociate iodine atoms
from CH3I or CF3I molecules diluted in a carrier gas (a mixture of Ar and He). The discharge chamber was arranged
directly inside an iodine injector (made of aluminum) to minimize the recombination of generated atomic iodine and
enabling an increased assistance of UV light for a photo-dissociation enhancement of I atoms production. The effluent of
the discharge chamber/iodine injector was injected into the flow of N2 downstream the nozzle throat. Measurements of I
atoms concentration distribution at different distances from the injection and in two directions across cavity were done
by means of absorption measurements at the wavelength of 1315 nm. Dependences of atomic iodine concentration on
main RF discharge parameters and flow mixing conditions were measured. This novel method could be an alternative to
the chemical generation of atomic iodine and also an efficient alternative to other electric discharge methods of I atoms
generation for chemical oxygen-iodine laser (COIL) and discharge oxygen-iodine laser (DOIL).
The initial stage in development of hybrid plasma jet generator of singlet oxygen O2(1Δ) for a discharge oxygen-iodine
laser (DOIL) is presented. This novel type of generator is based on a fast mixing of hybrid argon plasma jet of DC
electric arc and RF discharge with a neutral molecular oxygen stream. Arc plasma jets have a much higher density of
electrons than RF plasma jets to compensate the electro-negativity of oxygen and they can be operated at a higher
pressure for an efficient supersonic expansion cooling in DOIL. An RF discharge is applied to the DC arc plasma jet at
the hollow anode of the plasmatron to switch it from a hot-spot mode to a diffusive mode. That is advantageous with the
use of aluminum anode, which has a lower melting point and a significantly lower rate of O2(1Δ) quenching compared to
a standard copper anode in a gas plasmatron. An enhanced non-equilibrium in the plasma jet caused by the RF discharge
and neutral oxygen injection is desirable for an efficient O2(1Δ) generation. Preliminary calculations on the equilibrium
composition of O2-Ar mixture suggest keeping the arc temperature well down to prevent an excessive dissociation of the
molecular oxygen.
Generation of singlet oxygen and atomic iodine for operation of the chemical or discharge oxygen-iodine laser
(COIL/DOIL) is described, employing novel methods and device configurations proposed in our laboratory. A
centrifugal spray generator of singlet oxygen was developed, based on the conventional reaction between chlorine and
basic hydrogen peroxide. Recent results of theoretical and experimental investigation of the generator parameters are
presented. A new conception of the discharge generator of singlet oxygen was initiated, based on a combined DC arc jet
and RF discharge techniques. Principle of the generator currently developed and constructed is described. A new device
configuration was designed for the alternative method of atomic iodine generation using a radiofrequency discharge
decomposition of iodine compounds like CH3I or CF3I. Some recent experimental results of this research are also
presented.
A new RF plasma jet generator (DSOG-5) of singlet oxygen has been developed for use in an oxygen-iodine laser. The plasma jet was produced in Al nozzles, which were fed by the radio-frequency (100 MHz) power of up to 200 W. The usual mode of operation was an energy transfer from Ar plasma jet to a neutral O2 gas stream. The yield of singlet delta oxygen was up to 24%. Iodine molecules were dissociated by 200 MHz RF discharge with the power of 60 W prior to injection into the mixing zone of laser. The pre-dissociation enhancement was up to 22% of iodine spontaneous emission intensity. Both the DSOG-5 and the RF iodine pre-dissociation were tested in laser experiments in a transverse flow Discharge Oxygen-Iodine Laser (DOIL). The effluent of DSOG-5 was cooled by liquid nitrogen to temperatures in the range 120-300 K. There was a temperature dependent loss of singlet delta oxygen on the walls. The singlet delta oxygen yield and the atomic iodine luminescence at the wavelength of 1315 nm were measured. The highest luminescence was achieved at pressures of ~1 Torr with the yield of 10-20%. Laser oscillations have not been achieved.
A decade has passed, since Discharge Oxygen Iodine Laser (DOIL) research started in our laboratory. Singlet delta oxygen production tests were carried out using RF discharge singlet oxygen generator of the first version (DSOG-1) in 1993. The maximum yield of 4.2% was achieved by DSOG-1. Efforts for improving RF-DSOG have been continuously carrying on and DSOG-5 is now under operation. The DSOG-5 consists of a jet nozzle having a diameter of 3 mm, an injector quarts nozzle of 2 mm diameter set inside the jet nozzle at coaxial position and a mixing slit nozzle with the height of 0.2 mm set surrounding the jet nozzle exit. High electric field is laid on inside surface of the jet nozzle by 200 W RF power source. The singlet delta oxygen is produced by energy transfer from Argon plasma which is produced in the jet nozzle. It is important for achieving high yield to have a good mixing of oxygen, blown from the slit nozzle and the quarts nozzle, with the Argon plasma. The yield of 24% was recorded when oxygen gas 110 sccm was mixed with 700 sccm Argon gas at the pressure 0.6 torr and the RF power 196 W. With a new laser system reinforced by a discharge iodine dissociation and a laser gas cooling device, oscillation tests were carried out in conjunction with the DSOG-5. Performance of the system was confirmed by an emission from excited iodine atoms which energy was transferred from the singlet delta oxygen. It is obvious that the new system gives a progress for the DOIL oscillation.
A new RF plasma jet generator (DSOG-4) of singlet delta oxygen has been developed for use in an oxygen-iodine laser. Two different modes of operation were studied: (1) chilling of the plasma jet by a neutral gas stream and (2) an energy transfer from plasma jet to a neutral gas stream. The plasma jet was produced in an Al cylindrical nozzle, having the cross section of 3 mm2. The chilling mode used mixtures O2:NO to produce the plasma jet, which was subsequently chilled by He, injected at the nozzle exit. The energy transfer mode used mixtures He:NO to produce the plasma jet, which was mixed with a neutral stream of O2, allowing thus energy transfer to oxygen molecules with enhanced selectivity. The RF frequency was 99.9 MHz and the RF power was up to 200 W. Both the modes of operation were tested in a transverse flow Discharge Oxygen-Iodine Laser (DOIL). The singlet delta oxygen yield and the atomic iodine luminescence at the wavelength 1315 nm were measured. The energy transfer mode proved to be an effective alternative of the classic chilling mode. It enables new generating schemes, which may bypass some of the classic limitations in oxygen discharges.
An RF plasma jet generator of singlet delta oxygen has been developed for use in an oxygen-iodine laser. The plasma jet was produced in an Al cylindrical nozzle, having the inner diameter of 3 mm and the length of 8 mm. The discharge was done in the gas mixture O2:N2:NO=200:20:10 sccm and then it was chilled reactively by the mixture Ar:NO2=200:10 sccm, which was injected into the plasma jet at the nozzle exit. The RF frequency was 99.9 MHz and the RF power was 200 W. The O2(?1Æg) relative yield of 32 % was achieved at the pressure of 0.43 Torr. The current device DSOG-3 was tested in a discharge oxygen-iodine laser (DOlL). The reaction scheme of DOlL is the same as in COIL, except of the generator of singlet delta oxygen. The pressure inside the laser was 0.6-0.9 Torr and the mixtures O2:NO=200: 100 sccm and Ar:NO=2100: 100 sccm were used. The iodine flow rate was 0.3 mmol/min. Both types axial and transverse subsonic flow lasers were examined. The latter had better performance. The CW output power was 3 nW at the laser wavelength of 13 15 nm, when the yield of O2(?1Æg) was 25%. The RF DOlL has been demonstrated experimentally.
The trial for world first oscillation of oxygen iodine laser using high frequency discharge was conducted. Maximum excited oxygen efficiency was recorded up to 21% by the microwave (MW) discharge and 32% by the radio frequency (RF) discharge. The highest efficiency of 32% singlet oxygen was achieved by producing plasma jet through the hollow cathode of RF discharge. Laser oscillation test was carried out connecting with RF discharge singlet oxygen generator (DSOG) to a resonator which was arranged longitudinal to the gas flow. Spontaneous emission of its wavelength 1.315 micrometer from iodine electronic transition was detected by the spectra-analyzer from laser output mirror. The laser oscillation was confirmed by detecting an amplification of the emission when laser mirrors were aligned.
Rf oxygen plasma jets were studied experimentally as an alternative source of molecular singlet delta oxygen for an oxygen-iodine laser. The relative yield of singlet delta oxygen was measured under a wide variety of experimental conditions. The rf frequency range was 27.2 - 99.9 MHz and the rf power was up to 200 W. The oxygen output pressure was 0.05 - 0.40 torr and the oxygen flow rate was 195 - 1000 sccm. High purity oxygen or its mixtures with Ar, N2, NO and Hg at the pumping velocity of 250 m3/h were used. The plasma jet was produced in nozzles, having the inner diameter of 1 - 6 mm and the length of 1 - 16 mm. The nozzle materials Al, Ti, Ta and W gave significantly better results than Pt and Ni. The dependence of singlet delta oxygen production on the radiofrequency was increasing monotonously. Other dependencies were not monotonous and exhibited an optimum. The cw mode of operation gave usually better results than a pulsed mode. The most effective admixture was N2, which gave the highest enhancement. This resulted in the relative yield of singlet delta oxygen exceeding 15%.
A hollow electrode fast flow oxygen RF discharge was examined experimentally as an alternative source of molecular singlet delta oxygen. The relative yield of singlet delta oxygen was measured under the following experimental conditions: the RF frequency 13.66 MHz, the RF power up to 500 W, the oxygen output pressure 0.5 - 2.5 Torr, the oxygen flow rate 0.1 - 2.5 Nl/min, the hollow electrode inner diameter 1 - 4 mm, the hollow electrode length 1 - 4 mm, the gas composition--99.5% and 99.995% oxygen, respectively. The singlet delta oxygen yield was increasing with the RF power and decreasing with the pressure. The dependence of the yield on the flow rate and the geometrical dimensions was not monotonous and exhibited an optimum. In such cases, the singlet delta oxygen yield up to 16% was achieved.
This paper brings experimental results on a repetitively pulsed Chemical Oxygen-Iodine Laser (COIL) using a gain modulation or switching by means of an externally applied magnetic field, based on the Zeeman effect. A peak-to-average laser power enhancement factor and a power conservation factor were evaluated from these experiments in dependence on a magnetic field intensity and a length of magnetic pulses, as well as their shape. A critical view on the pulsed method used is presented.
KEYWORDS: Magnetism, Iodine, Chemical oxygen iodine lasers, Chemical lasers, Resonators, Laser damage threshold, Modulation, Laser resonators, Zeeman effect, Chemical species
A magnetic quenching of generation in a chemical oxygen-iodine laser (COIL) has been studied experimentally. This work gives experimental data on a quenching threshold magnetic field in dependence on a resonator output coupling and an iodine concentration in the laser active zone, respectively.
A magnetic quenching of generation in a chemical oxygen-iodine laser (COIL) has been studied experimentally. This work gives data on a quenching threshold magnetic field in dependence on a resonator output coupling and an iodine concentration in the laser active zone, respectively. It was found that the threshold value of magnetic field was a decreasing function of the output coupling and an increasing function of the iodine molar flow rate. The normalized gain ((alpha) th/(alpha) ) is a decreasing function of the threshold field.
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