Computation of streamwise vorticity in a compressible flow of a winglet nozzle-based COIL device

Chemical oxygen iodine laser (COIL) is a high-power laser with potential applications in both military as well as in the industry. COIL is the only chemical laser based on electronic transition with a wavelength of 1.315 μm, which falls in the near-infrared (IR) range. Thus, COIL beam can also be transported via optical fibers for remote applications such as dismantling of nuclear reactors. The efficiency of a supersonic COIL is essentially a function of mixing specially in systems employing cross-stream injection of the secondary lasing (I₂) flow in supersonic regime into the primary pumping (O₂¹Δ_g) flow. Streamwise vorticity has been proven to be among the most effective manner of enhancing mixing and has been utilized in jet engines for thrust augmentation, noise reduction, supersonic combustion, etc. Therefore, a computational study of the generation of streamwise vorticity in the supersonic flow field of a COIL device employing a winglet nozzle with various delta wing angles of 5°, 10°, and 22.5° has been carried out. The study predicts a typical Mach number of approximately 1.75 for all the winglet geometries. The analysis also confirms that the winglet geometry doubles up both as a nozzle and as a vortex generator. The region of maximum turbulence and fully developed streamwise vortices is observed to occur close to the exit, at x/λ of 0.5, of the winglets making it the most suitable region for secondary flow injection for achieving efficient mixing. The predicted length scale of the scalloped mixer formed by the winglet nozzle is 4λ. Also, the winglet nozzle with 10° lobe angle is most suitable from the point of view of mixing developing cross-stream velocity of 120 m/s with acceptable pressure drop of 0.7 Torr. The winglet geometry with 5° lobe angle is associated with a low cross-stream velocity of 60 m/s, whereas the one with 22.5° lobe angle is associated with a large static and total pressure drop of 1.87 and 9.37 Torr, respectively, making both the geometries unsuitable for COIL systems. The experimental validation shows a close agreement with the computationally predicted values. The studies for the most suitable 10° lobe angle geometry show an observed Mach number of 1.72 with an improved mixing efficiency of 74% due to the occurrence of predicted streamwise vortices in the flow.     

Review of theoretical gain-optimization studies in 10.6µm CO2-N2 gasdynamic lasers

A comprehensive theoretical analysis of optimization of gain in CO₂-N₂ gasdynamic laser employing wedge or conical or hyperbolic nozzles with either H₂O or He as the catalyst is presented. After a review of previous work, the usual governing equations for the steady inviscid quasi-one-dimensional flow in a supersonic nozzle of a gasdynamic laser are used to obtain similar solutions for the various flow quantities, which variables are subsequently used to optimize the small-signal gain on theP(20) line of the (001) → (100) transition of CO₂ at wavelength 10.6μm. The corresponding optimum values like reservoir pressure and temperature and nozzle area ratio also have been predicted and presented in the form of graphs. The analysis predicts that employing of 2D-wedge nozzle results in higher gain values and the CO₂-N₂-H₂O gasdynamic laser employing 2D-wedge nozzle is operationally the best laser system for which the optimum value as high as 3.1 m⁻¹ gain can be obtained.     

Nonequilibrium Ionization by Vibrationally Excited Molecules in Supersonic Flows

The results of experimental investigations on thermal nonequilibrium ionization in CO₂: N₂: He mixtures are presented. Measurements of electron density, n_e, in vibrationally excited nitrogen were made in a supersonic flow with different CO₂ contents as well as in a CO₂: N₂: He = 1 : 5 : 4 mixture laser gas. The mixtures were heated in a shock tube and expanded through a supersonic nozzle. Furthermore, supersonic mixing of N₂ and CO₂ + He was used in some experiments. The measured values of n_e in the plenum chamber and in the supersonic nozzle are reported, and the processes responsible for nonequilibrium ionization in a laser-active medium are discussed.     

A small scale, supersonic chemical oxygen-iodine laser

A supersonic chemical oxygen-iodine laser of 5 cm long active medium has been operated utilizing a simple sparger-type O₂(¹Δ) chemical generator and a medium size pumping system. A grid nozzle was used for iodine injection and supersonic expansion. 9 W of cw laser emission at 1315 nm were obtained in the present experiments. The small size and the simple structure of the laser system and its stable operation for long periods make it a convenient tool for studying parameters important for high-power supersonic iodine lasers.     

Lineshape analysis of stimulated Raman spectra of the near‐nozzle region of free jet expansions

An anomalous lineshape of stimulated Raman spectra obtained from the region very close to the nozzle of supersonic pulsed expansions of nitrogen is presented. High‐resolution Raman spectra of the Q branch of the fundamental vibration mode of N₂ have been recorded from two different nitrogen expansions at T₀ = 295 K and P₀ = 1.5–3.5 bar, the lasers crossing the jet axis in the range z/D = 0.25–1.25, where D is the effective nozzle diameter. The combination of Doppler shifts and strong gradients of density and temperature in the near‐nozzle region yield an inhomogeneous broadening and a double peak structure of the recorded Raman line profiles. The comparison of the experimental results with the simulation of the Raman spectrum from this region provides valuable information about the near‐nozzle flow field. The lineshape described here is different from another reported previously in the literature, which is based on a depletion of the density of free molecules on the axis due to condensation. Copyright © 2009 John Wiley & Sons, Ltd.     

Experiment and modeling of a small-scale,supersonic chemical oxygen-iodine laser

We report on detailed experiment and modeling of a small-scale, supersonic chemical oxygen-iodine laser. The laser has a 5 cm long active medium and utilizes a simple sparger-type O₂(¹ ) chemical generator and a medium-size pumping system. A grid nozzle is used for iodine injection and supersonic expansion. 25 W of cw laser emission at 1.315 µm are obtained in the present experiments. The small size and the simple structure of the laser system and its stable operation for long times make it a convenient tool for studying parameters important for high-power supersonic iodine lasers and for comparison to model calculations. The lasing power is studied as a function of the molar flow rates of the various reagents, and conditions are found for optimal operation. Good agreement is found between the experimental results and calculations based on a simple one-dimensional semi-empirical model, previously developed in our laboratory and modified in the present work. The model is used to predict optimal values for parameters affecting the laser performance that are difficult to examine in the present experimental system.     

Shock associated noise of supersonic jets from convergent-divergent nozzles

Results of experimental and theoretical studies of the characteristics of shock associated noise from imperfectly expanded supersonic jets over an extensive range of underexpanded and overexpanded operating conditions are described. This kind of broadband noise is believed to be generated by the weak but coherent interaction between the downstream propagating large scale turbulent flow structures in the mixing layer of the jet and the nearly periodic shock cell system. Theoretical reasoning based on this mechanism leads to the scaling formula that the intensity of shock associated noise varies as (M_j² ? M_d²)² where M_j and M_d are the fully expanded jet operating Mach number and nozzle design Mach number, respectively. This formula holds for underexpanded as well as overexpanded jet Mach numbers. In addition, a peak frequency formula is also derived from the same model. The noise intensity, directivity and spectra of supersonic jets from a convergent-divergent nozzle of design Mach number 1·67 were measured in an anechoic facility over the Mach number range of 1·1 to 2·0. The effect of jet temperature was investigated by operating the jet at three temperature conditions. These sets of data provide sufficient information for fully assessing the relative importance and characteristics of shock associated noise of supersonic jets from convergent-divergent nozzles. Comparisons between theoretical results and measurements show very favorable agreement.     

Ignition enhancement by addition of NO and NO2 from a N2/O2 plasma torch in a supersonic flow

The effects of NO and NO₂ produced by using a plasma jet (PJ) of a N₂/O₂ mixture on ignition of hydrogen, methane, and ethylene in a supersonic airflow were experimentally and numerically investigated. Numerical analysis of ignition delay time showed that the addition of a small amount of NO or NO₂ drastically reduced ignition delay times of hydrogen and hydrocarbon fuels at a relatively low initial temperature. In particular, NO and NO₂ were more effective than O radicals for ignition of a CH₄/air mixture at 1200 K or lower. These ignition enhancement effects were examined by including the low temperature chemistry. Ignition tests by a N₂/O₂ PJ in a supersonic flow (M = 1.7) for using hydrogen, methane, and ethylene injected downstream of the PJ were conducted. The results showed that the ignitability of the N₂/O₂ PJ is affected by the composition of the feedstock and that pure O₂ is not the optimum condition for downstream fuel injection. This result of ignition tests with downstream fuel injection demonstrated a significant difference in ignition characteristics of the PJ from the ignition tests with upstream fuel injection.     

Supersonic CO2 laser excited by RF discharge in closed cycle

A supersonic gas flow having a Mach number of 2 has been realized in a closed-cycle radio-frequency (RF)-discharge-excited supersonic CO₂ laser system. Stable RF discharge at a high CO₂ gas concentration has become possible using supersonic gas flow and RF discharge generated between dielectric electrodes. As a result, high RF input power density has been obtained. In addition, a high small-signal gain has been obtained in the supersonic section through decreases in gas pressure and gas temperature due to supersonic gas flow.This revised version was published online in August 2005 with a corrected cover date.     

Growth, polarized spectral properties, and 1.5–1.6 μm laser operation of Er:Yb:Sr3Gd2(BO3)4 crystal

An Er:Yb:Sr₃Gd₂(BO₃)₄ crystal was grown by the Czochralski method. The polarized spectral properties of the crystal were investigated, including the polarized absorption and fluorescence spectra and fluorescence decay. The fluorescence quantum efficiency of the upper laser level ⁴I_13/2 of Er³⁺ ions and the efficiency of the energy transfer from Yb³⁺ to Er³⁺ ions were calculated. End-pumped by a diode laser at 970 nm in a hemispherical cavity, a 1.6 W quasi-cw laser at 1.5–1.6 μm with a slope efficiency of 18% and an absorbed pump threshold of 5.9 W was achieved in a 1.8-mm-thick Z-cut Er:Yb:Sr₃Gd₂(BO₃)₄ crystal. This crystal has a flat and broad gain curve at 1.5–1.6 μm and so is also a potential gain medium for tunable and short pulse lasers.     

Condensation of uranium hexafluoride in supersonic Laval nozzle flow

6 carried in a mixture of argon and methane was studied experimentally in a continuously operating supersonic Laval nozzle. The onset of condensation was detected by Rayleigh light scattering. Measurements of static pressure in the nozzle, together with the equations of isentropic flow, permitted the determination of the relation between the pressure of UF₆, P_k, and the temperature, T_c, at the observed onset of condensation. The experiments addressed conditions of condensation onset in the range 80<T_c<160 K and 0.1<P_k<3.0 Torr. We found that there was no influence of the pressure of the carrier gas argon, which ranged from 3.5 to 68 Torr at onset points, on the onset of condensation. The dependence of the supersaturation ratio on temperature at the onset could be expressed by a straight line as deduced by P.P. Wegener and B.J.C. Wu from the classical theory of homogeneous nucleation [Adv. Colloid Interface Sci. 7, 325 (1977)]. These results indicate that supersonic expansion in the nozzle formed single-component droplets of UF₆ in a mixture of UF₆, CH₄, and Ar under the experimental conditions studied herein. Received: 23 December 1997 / Revised version: 7 April 1998     

High resolution study of anion formation in low-energy electron attachment to SF<Subscript>6</Subscript> molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF₆ molecules in helium carrier gas, nozzle temperatures T₀= 300–600 K, stagnation pressures p₀= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF₆ molecules at high energy resolution. Using the standard LPA method, the yield for SF₆^- as well as SF₅^- and F^- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF₆^- and SF₅^- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF₆^- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF₅^- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T₀= 300 K and shifting to lower energies with rising T₀) and a monotonical decrease towards higher energies. SF₅^- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF₆ molecules with rising pressure. The SF₅^-/SF₆^- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF₅^- spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at p₀= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F^- formation is similar to that for SF₆^-, but the F^- signals are three to four orders of magnitude lower than those for SF₆^-; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.     

Infrared diode laser absorption features of N2O and CO2 in a laval nozzle

Design and operation of a pulsed Laval nozzle and the characterization of molecular flow through such a nozzle using IR tunable diode laser (TDL) is the central theme of this work. The results here deal with He diluted N₂O and CO₂ gaseous systems. Boltzmann type plots of the spectral intensity data of both N₂O and CO₂ show non-linear behaviour. We have attempted to understand this non-linear behaviour of Boltzmann plots in terms of (1) instability in the jet and (2) a two-temperature model for the flowing gas, a cold central core and a hot boundary layer close to the nozzle walls. The model based on jet instability represents the data somewhat poorer than the two-temperature model. The parameters derived from fitting our experimental data to the former model could be used to calculate the thermodynamic parameters only through further approximations. Measured absorption line profile of the P(15) line of the v ₂ band of N₂O as a function of axial distance from the nozzle exit gradually shifts from a Lorentzian to a Gaussian type. Velocity distribution of N₂O molecules in a Laval nozzle is determined by differentiating the absorption line profile of the P(15) line (v ₀=576.235 cm^–1) of the v ₂ band of N₂O. Translational temperature of N₂O molecules is determined from the observed spectral profiles.     

Optical gain and upconversion luminescence in LaF3: Er, Yb nanoparticles-doped organic–inorganic hybrid materials waveguide amplifier

A LaF₃: Er, Yb nanoparticle-doped organic–inorganic hybrid materials waveguide amplifier is demonstrated using reactive ion etching. A maximum gain of approximately 6.8 dB is observed in a 20-mm-long waveguide. Under excitation at 976 nm, the waveguides emit a strong green upconversion luminescence. The possible upconversion mechanisms are discussed. The dependence of upconversion emission intensity on excitation power confirms a three-photon process contributes to the upconversion of the emission band 405 nm and two-photon processes for the green and red emission bands. The temperature behavior by the measurement of the fluorescence intensity ratio of the signals at 520 nm and 544 nm as a function of the pump power demonstrates a fast thermalization between the ² H _11/2 and ⁴ S _3/2 levels. The influence of upconversion emission on the gain performance of the waveguide amplifier is analyzed.     

Fabrication and electrical characterization of p-Sb2S3/n-Si heterojunctions for solar cells application

Antimony trisulphide (Sb₂S₃) films were prepared by thermal evaporation technique on n-type single crystal Si substrates to fabricate p-Sb₂S₃/n-Si heterojunctions. The electrical transport properties of the p–Sb₂S₃/n-Si heterojunctions were investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The temperature-dependent I–V characteristics revealed that the forward conduction was determined by multi-step tunnelling current and the activation energy of saturation current was about 0.54 eV. The 1/C²–V plots indicated the junction was abrupt and the junction built-in potential was 0.6 V at room temperature and decreased with increasing temperature. The solar cell parameters have been calculated for the fabricated cell as V_oc = 0.50 V, J_sc = 14.53 mA cm⁻², FF = 0.32 and η = 4.65% under an illumination of 50 mW cm⁻².     

Spectral characterization and energy levels of Cr:Sc2(MoO4)3 crystal

This paper reports the spectral properties and energy levels of Cr³⁺:Sc₂(MoO₄)₃ crystal. The crystal field strength Dq, Racah parameter B and C were calculated to be 1408 cm⁻¹, 608 cm⁻¹ and 3054 cm⁻¹, respectively. The absorption cross sections σ_α of ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ transitions were 3.74×10⁻¹⁹ cm² at 499 nm and 3.21×10⁻¹⁹ cm² at 710 nm, respectively. The emission cross section σ_e was 375×10⁻²⁰ cm² at 880 nm. Cr³⁺:Sc₂(MoO₄)₃ crystal has a broad emission band with a broad FWHM of 176 nm (2179 cm⁻¹). Therefore, Cr³⁺:Sc₂(MoO₄)₃ crystal may be regarded as a potential tunable laser gain medium.     

Effect of neutral pressure on the He plasma flow measurement in a linear device

Axial and azimuthal flow velocities have been measured in a linear plasma device called NAGDIS-II (NAGoya DIvertor Simulator-II), along with plasma density and electron temperature, using a vector Mach probe composed of two Mach probes, one of which is for the axial flow, and the other is for the azimuthal flow. To study the effect of neutral pressure on the deduction of the Mach numbers, the ratio of upstream to downstream currents are measured by changing the neutral pressure for the deduction of flow velocities. Helium plasma was generated with pressure of 2–35 mTorr. Since the ion gyro-radius at the magnetic flux of 300 G is larger than the probe size, an unmagnetized collisionless Mach probe theory was used for the deduction of Mach numbers and their variations. In order to check the range of collisionality, plasma density (n_e = 10¹⁰–10¹¹ cm^?3) and electron temperature (T_e = 2–9 eV) are measured by a single electric probe using a conventional collisionless probe theory. Variations of Mach number, electron temperature and plasma density with collisionless models are to be compared with those using collisional models for different pressures where ionization and ion-neutral collision are included. Mach numbers by the collisionless model are found to be overestimated by 120% for the maximum difference even in weakly collisional plasmas. A clear flow reversal exists in the axial direction with higher pressure plasma, even in the linear machine. Azimuthal flows are also measured simultaneously along with axial flows, yet they seem to be very small in the present cold ion plasma (T_i/T_e << 1).     

Formation of laser jet thrust in the supersonic regime

Basic methods for obtaining laser jet thrust in the supersonic regime corresponding to the supersonic flow in the jet nozzle are analyzed. It is shown that the method based on the interaction of a laser ablative jet with the supersonic flow is promising. In this case, laser thrust is formed due to additional acceleration of the flow behind the ablation region. Numerical simulation of the flow in a parabolic nozzle is employed to demonstrate the possibility of effective formation of laser thrust at a level of 3 × 10^?3 N/W.     

Imaging of the oxygen distribution in an isothermal turbulent free jet using two-color toluene LIF imaging

The results of a novel technique for the quantification of oxygen in an isothermal turbulent free jet using toluene laser induced fluorescence (LIF) are presented. This method relies on the red-shift of the toluene LIF emission spectrum with increasing oxygen concentration. Evaluating the LIF signal ratio from two different wavelength regions simultaneously produces results that depend only on the local oxygen concentration. From calibration data, obtained from repeated tests, the oxygen sensitivity of the two-color LIF technique is best for oxygen partial pressures p_O2 ￡ 120 mbarp_{\mathrm{O}_{2}} \leq 120\mbox{ mbar} in the current setup. Quantified images of oxygen distribution are presented for 40.4, 60.5, 80.5, and 103 mbar p_O2p_{\mathrm{O}_{2}} in the toluene-seeded jet flow that is shielded by a toluene-seeded nitrogen co-flow at atmospheric pressure and temperature. Based on the average oxygen concentration images (obtained from 100 instantaneous oxygen images), the error in accuracy of measuring the oxygen concentration was 0.8, 3.0, 7.7, and 7.3% with a precision of ± 8.6, 5.5, 13.3, and 11.6% for the jet p_O2 = 40.4, 60.5, 80.5, and 103 mbarp_{\mathrm{O}_{2}} = 40.4,\, 60.5,\, 80.5,\mbox{ and }103\mbox{ mbar} cases, respectively. The main jet flow characteristics have been captured by the technique as determined from the measured oxygen distribution images. Centerline profiles of average oxygen concentration, normalized to the value at the nozzle exit, demonstrate self-similar behavior from 5 mm above the nozzle exit. Radial oxygen concentration profiles exhibit a Gaussian-type distribution that broadens with distance above the nozzle exit, in agreement with literature.     

Microhydration of the methylene blue cation in an electrospray ion source

Microhydrated methylene blue cations, MB⁺(H₂O) _n , are produced in an electrospray ion source and their size-distributions are measured as a function of the source temperature. A series of MB⁺(H₂O) _n ions is observed up to n ≃ 60. A striking feature observed in the mass spectra is that the series of hydrated ions starts at n = 4; intensities of n = 1–3 are extremely suppressed. The absence of n = 1–3 ions is well explained by the energetics concerning evaporation processes of water molecules, based on stable structures and the binding energies of MB⁺(H₂O) _n ions calculated by DFT calculations up to n = 5. MB⁺(H₂O) _n ions for n > 4 evaporate a single water molecule sequentially, while MB⁺(H₂O)₄ tends to fragment into MB⁺ and (H₂O)₄ rather than MB⁺(H₂O)₃ and an H₂O molecule. We have observed a clear magic peak at n = 24, which strongly suggests that the MB⁺(H₂O)₂₄ ion is formed by attaching a neutral (H₂O)₂₀ cage onto an MB⁺(H₂O)₄ ion.