Sustainer Discharge Research Articles

Nitrogen Vibrational Population Measurements in the Plenum of a Hypersonic Wind Tunnel

Picosecond coherent anti-Stokes Raman scattering is used for measurement of nitrogen vibrational distribution function in the plenum of a highly nonequilibrium Mach 5 wind-tunnel incorporating a high-pressure pulser– sustainer discharge. First-level vibrational temperatures of the order of 2000K are achieved in the 300 torr non-selfsustained plasma discharge generated by a high E=n ( 300 Td) nanosecond-pulsed discharge, which provides ionization in combination with an orthogonal low E=n ( 10 Td) dc sustainer discharge, which efficiently loads the nitrogen vibrational mode. It is also shown that operation with the nanosecond-pulsed plasma alone results in significant vibrational energy loading, withTv N2 of the order of 1100K.Downstream injection ofCO2, NO, andH2 results in vibrational relaxation, demonstrating the ability to further tailor the vibrational energy content of the flow. N2-NO vibration–vibration andN2-H2 vibration–translation rates inferred from these data agreewell with previous literature results to within the uncertainty in rotational-translational temperature.

Effect of iodine dissociation in an auxiliary discharge on gain in a pulser-sustainer discharge excited oxygen–iodine laser

The paper presents the results of iodine vapour dissociation measurements in a high voltage, nanosecond pulse duration, repetitively pulsed discharge, used as an auxiliary (‘side’) discharge in an electric discharge excited oxygen–iodine laser. The side discharge, sustained in a high-pressure iodine vapour/helium mixture remained stable in the entire range of experimental conditions. Iodine dissociation fraction generated in the side discharge and measured in the laser cavity is up to 50%. However, the experiments showed that additional iodine dissociation generated in the side discharge only moderately increases laser gain, by 10–15%. Parametric gain optimization by varying main discharge pressure, O2 and NO fractions in the flow, I2 flow rate, pulsed discharge frequency and sustainer discharge power, with the side discharge in operation produces gain up to 0.08% cm−1. Two parameters that critically affect gain are the energy loading per molecule in the discharge and the NO flow rate controlling the O atom concentration in the flow. In particular, operation at the main discharge pressure of 60 Torr results in significantly higher gain than at 100 Torr, 0.080% cm−1 versus 0.043% cm−1, due to higher discharge energy loading per molecule at the lower pressure. Laser output power measured at the gain optimized conditions is 1.4 W.

Gain and output power measurements in an electrically excited oxygen–iodine laser with a scaled discharge

Singlet delta oxygen (SDO) yield, small signal gain, and output power have been measured in a scaled electric discharge excited oxygen–iodine laser. Two different types of discharges have been used for SDO generation in O2–He–NO flows at pressures up to 90 Torr, crossed nanosecond pulser/dc sustainer discharge and capacitively coupled transverse RF discharge. The total flow rate through the laser cavity with a 10 cm gain path is approximately 0.5 mole s−1, with steady-state run time at a near-design Mach number of M = 2.9 of up to 5 s. The results demonstrate that SDO yields and flow temperatures obtained using the pulser-sustainer and the RF discharges are close. Gain and static temperature in the supersonic cavity remain nearly constant, γ = 0.10–0.12% cm−1 and T = 125–140 K, over the axial distance of approximately 10 cm. The highest gain measured is 0.122% cm−1 at T = 140 K. Positive gain measured in the supersonic inviscid core extends over approximately one half to one third of the cavity height, with absorption measured in the boundary layers near top and bottom walls of the cavity. Laser power has been measured using (i) two 99.9% mirrors on both sides of the resonator, 2.5 W, and (ii) 99.9% mirror on one side and 99% mirror on the other side, 3.1 W. Gain downstream of the resonator is moderately reduced during lasing (by up to 20–30%) and remains nearly independent of the axial distance, by up to 10 cm. This suggests that only a small fraction of power available for lasing is coupled out, and that additional power may be coupled in a second resonator. Preliminary laser power measurements using two transverse resonators operating at the same time (both using 99.9–99% mirror combinations) demonstrated lasing at both axial locations, with the total power of 3.8 W.

Repetitively Pulsed Nonequilibrium Plasmas for Magnetohydrodynamic Flow Control and Plasma-Assisted Combustion

This paper demonstrates significant potential of the use of high-voltage, nanosecond pulse duration, high pulse repetition rate discharges for aerospace applications. The present results demonstrate key advantages of these discharges: 1) stability at high pressures, high flow Mach numbers, and high-energy loadings by the sustainer discharge, 2) high-energy fractions going to ionization and molecular dissociation, and 3) targeted energy addition capability provided by independent control of the reduced electric field of the direct current sustainer discharge. These unique capabilities make possible the generation of stable, volume-filling, low-temperature plasmas and their use for high-speed flow control, nonthermal flow ignition, and gasdynamic lasers. In particular, the crossed pulsersustainerdischargewasusedformagnetohydrodynamic flowcontrolincoldM � 3 flows,providing firstevidenceof cold supersonic flow deceleration by Lorentz force. The pulsed discharge (without sustainer) was used to produce plasma chemical fuel oxidation, ignition, and flameholding in premixed hydrocarbon–air flows, in a wide range of equivalence ratios and flow velocities and at low plasma temperatures, 150–300 � C. Finally, the pulser-sustainer discharge was used to generate singlet oxygen in an electric discharge excited oxygen–iodine laser. Laser gain and output power are measured in the M � 3 supersonic cavity.

Numerical Simulation of a Crossed Pulser–Sustainer Discharge in Transverse Magnetic Field

Modeling of a crossed pulser-sustainer discharge in the presence of a transverse magnetic field is used to obtain insight into the kinetics of a low-temperature magnetohydrodynamics (MHD) nitrogen plasma. Ionization in the discharge is produced by repetitive high-voltage nanosecond-duration pulses, while the low-voltage DC sustainer discharge couples power to the plasma between pulses. Model predictions are compared with the experimental sustainer discharge current, both without and with magnetic field, showing a satisfactory agreement. At low DC voltages (below the cathode voltage fall UC), the calculations show that the entire sustainer discharge, including the cathode layer, remains nonself-sustained and that the electric field does not penetrate into the plasma, which results in a very low sustainer current. This demonstrates that MHD power generation in low-temperature plasmas with MHD open voltages that are below the cathode fall is not feasible. At higher voltages (UDC > UC), when the cathode layer becomes self-sustained, the sustainer current grows approximately linear with the DC voltage. This occurs because the applied DC field does not produce any additional ionization in the plasma that is outside the cathode layer. In the presence of a magnetic field, the sustainer current is significantly lower than the current without a magnetic field, which is due to the Hall effect. At UDC > UC, the model predicts a significant electron and ion drift in the EtimesB direction due to the Lorentz force. Charge separation that is induced by the disparity in electron and ion mobilities also causes the entire plasma to shift in the EtimesB direction, which is approximately at the ion slip velocity. The estimated neutral flow velocity change due to a collisional momentum transfer from the charged to the neutral species in the plasma is consistent with the prediction of quasi-1-D phenomenological MHD equations, as well as with the previous experimental results in a low-temperature MHD wind tunnel.

Effect of nitric oxide on gain and output power of a non-self-sustained electric discharge pumped oxygen-iodine laser

This letter discusses the effect of nitric oxide on gain and output power of a pulser-sustainer discharge excited oxygen iodine laser. Adding small amounts of NO to the laser mixture (a few hundreds of ppm) considerably increases gain and output power due to (i) O atom titration and resultant slower I* atom quenching and (ii) improved stability of the dc sustainer discharge, which allows stable operation at significantly higher discharge powers. Gain on the 1315nm iodine atom transition and laser power in the M=3 transverse laser cavity are 0.049%∕cm and 1.24W, at a flow temperature of T=100K.

Design and operation of a supersonic flow cavity for a non-self-sustained electric discharge pumped oxygen–iodine laser

The paper presents results of a high-pressure, non-self-sustained crossed discharge–M = 3 supersonic laser cavity operation. A stable and diffuse pulser–sustainer discharge in O2–He flows is generated at pressures of up to P0 = 120 Torr and discharge powers of up to 2.1 kW. The reduced electric field in the dc sustainer discharge ranges from 0.6 × 10−16 to 1.2 × 10−16 V cm2. Singlet delta oxygen (SDO) yield in the discharge, up to 5.0–5.7% at the flow temperatures of 400-420 K, was inferred from the integrated intensity of the (0, 0) band of the O2(a 1Δ → X 3Σ) infrared emission spectra calibrated using a blackbody source. The yield increases with the discharge power and remains nearly independent of the O2 fraction in the mixture (in the 10–20% range). Static pressure and temperature measurements in the supersonic cavity show that a steady-state M = 3 flow in the cavity can be sustained for up to 20 s, at the flow temperature of T = 120 ± 15 K. The results suggest that the measured SDO yield exceeds the threshold yield at the cavity temperature by up to a factor of 2.5. PLIF iodine vapour visualization in the supersonic cavity, which showed the presence of large-scale structures, suggests the need to improve iodine vapour mixing with the main oxygen–helium flow.

Novel long-pulse TE CO 2 laser excited by pulser–sustainer discharge

A novel long-pulse TE CO 2 laser with UV-preionization is presented. With an active volume of 1.17 l and gas pressure of 30 kPa, the laser can discharge stably with low pulser energy and high sustainer energy. Various long-pulse discharges such as 12, 20 or 25 μs are demonstrated. At discharge pulse width of 23.9 μs, maximum output laser energy of 6.8 J is obtained at an efficiency of 9.0%.

Low-temperature supersonic boundary layer control using repetitively pulsed magnetohydrodynamic forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short-pulse duration, high-voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization results show that as the Reynolds number increases, the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using laser differential interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0∼200–250Torr. A crossed discharge (pulser+dc sustainer) in M=3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0A in nitrogen and up to 0.8A in air. The electrical conductivity and the Hall parameter in these flows are inferred from the current voltage characteristics of the sustainer discharge. LDI measurements detected the MHD effect on the ionized boundary layer density fluctuations at these conditions. Retarding Lorentz force applied to M=3 nitrogen, air, and N2–He flows produces an increase of the density fluctuation intensity by up to 2dB (about 25%), compared to the accelerating force of the same magnitude. The effect is demonstrated for two possible combinations of the magnetic field and current directions producing the same Lorentz force direction (both for accelerating and retarding force).

Singlet oxygen generation in a high pressure non-self-sustained electric discharge

This paper presents results of singlet oxygen generation experiments in a high-pressure, non-self-sustained crossed discharge. The discharge consists of a high-voltage, short pulse duration, high repetition rate pulsed discharge, which produces ionization in the flow, and a low-voltage dc discharge which sustains current in a decaying plasma between the pulses. The sustainer voltage can be independently varied to maximize the energy input into electron impact excitation of singlet delta oxygen (SDO). The results demonstrate operation of a stable and diffuse crossed discharge in O2–He mixtures at static pressures of at least up to P0 = 380 Torr and sustainer discharge powers of at least up to 1200 W, achieved at P0 = 120 Torr. The reduced electric field in the positive column of the sustainer discharge varies from E/N = 0.3 × 10−16 to 0.65 × 10−16 V cm2, which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O2(a 1Δ) excitation. Measurements of visible emission spectra O2(b 1Σ → X 3Σ) in the discharge afterglow show the O2(b 1Σ) concentration to increase with the sustainer discharge power and to decrease as the O2 fraction in the flow is increased. Rotational temperatures inferred from these spectra in 10% O2–90% He flows at P0 = 120 Torr and mass flow rates of are 365–465 K. SDO yield at these conditions, 1.7% to 4.4%, was inferred from the integrated intensity of the (0, 0) band of the O2(a 1Δ → X 3Σ) infrared emission spectra calibrated using a blackbody source. The yield remains nearly constant in the discharge afterglow, up to at least 15 cm distance from the discharge. Kinetic modelling calculations using a quasi-one-dimensional nonequilibrium pulser–sustainer discharge model coupled with the Boltzmann equation for plasma electrons predict gas temperature rise in the discharge in satisfactory agreement with the experimental measurements. However, the model overpredicts the O2(a 1Δ) yield by a factor of 2–2.5, which suggests that the model's description of nonequilibrium O2–He plasma kinetics at high pressures is not quite adequate.

Improvement in the performance of an Ar-Xe laser using magnetic spiker sustainer discharge excitation

In an X-ray preionized Ar-Xe laser using a magnetic-spiker sustainer discharge circuit with a magnetic pulse compressor a parametric investigation of the laser output versus gas composition and electrical excitation was performed. A considerable improvement in the performance of the discharge excited Ar-Xe laser was demonstrated. An output energy of 436 mJ was obtained from an active volume of 0.26 l at a total gas pressure of 1.8 bar containing 1% Xe in Ar with an overall efficiency of 2.1%.

Two methods of operation of a CO2transversely excited waveguide laser

The operation of a CO2 transversely excited (TE) waveguide laser using capacitor discharge excitation and pulser sustainer excitation results in specific output energy densities of 85 mJ m-3 Pa-1 and 113 mJ m-3 Pa-1 respectively. The 3.7% efficiency achieved for the laser when operated in the pulser-sustainer mode is the highest reported for a CO2 TE waveguide laser. Assessment of the waveguide losses results in a correct prediction for the polarisation of the laser output. Consideration of the sustainer discharge allows the arcing limit of the discharge and the form of the sustainer current to be determined.

Electron-beam-stabilized discharge in a flowing medium - Numerical calculations

A numerical computational method for the calculation of the bulk characteristics of an electron beam sustainer discharge, in the presence of a flowing or stationary medium, is presented. The sustainer electric field is taken perpendicular to the flow direction, as it is commonly found in a large class of high-energy electrically pumped lasers. A detailed discussion of the underlining assumptions of the computational method is given. Typical results of the numerical calculations are presented in detail.

Sustainer enhancement of the VUV fluorescence in high-pressure xenon

A low-energy (20-60 kV), high-repetition-rate (∼60-Hz) electron-beam system is used to excite xenon at pressures up to 600 psi. The salient features of the temporal and spatial measurements of the VUV fluorescence, corresponding to the1Σ g +-3,1 \Sigma_{u}+ transitions in xenon, are presented. Experimentally, it is found that in a regime where more than 20 percent of the electron-beam energy incident on the gas is converted to VUV fluorescence, the maximum incremental efficiency due to the addition of a sustainer discharge is about 7 percent. A kinetic model is described which attributes the low sustainer efficiencies to electron interactions with the excited state populations.

Use of electron backscattering for smoothing the discharge in electron-beam-controlled lasers: Computations

The anode material in an electron-beam-controlled gas laser strongly influences the uniformity of the sustainer discharge, because the backscattering of the beam electrons from the anode is different for different atomic numbers. As a result of switching from aluminum to, for instance, uranium, there is a 60% increase in the ionization near the anode, according to Monte Carlo computations for a typical planar laser configuration. This effect can cause a 30–60% drop in the local electric field, depending upon the field dependence of the electron-ion recombination coefficient. As an example for studying this effect, we have considered a 150-keV beam passing through an 0.8-mil Ti window into a 26-cm-wide planar cavity containing a 3:2:1 mixture of He, CO2, and N2 at 1 atm and 293 K, with Eave=4.5 kV/cm. Window power loading, electron attenuation, electron energy distributions, and sustainer field distributions were calculated for the two different anode materials.