Adam Hicks

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 Vc m 2 , which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O2(a 1 �) excitation. Measurements of visible emission spectra O2(b 1 � → X 3 �) in the discharge afterglow show the O2(b 1 �) 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 ˙ m = 0.73–2.2 g s −1 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 1 � → X 3 �) 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 1 �) 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. (Some figures in this article are in colour only in the electronic version)

Continuous wave operation of a non-self-sustained electric discharge pumped oxygen-iodine laser

This letter discusses operation of an electric discharge excited oxygen-iodine laser using a high-pressure, non-self-sustained pulser-sustainer discharge. Small signal gain on the 1315 nm iodine atom transition and the laser output power are measured in the M = 3 supersonic cavity downstream of the discharge section. In a 15% O 2 – 85 % He mixture, at a discharge pressure of 60 torr and discharge power of 1.5 kW , the highest gain measured in the M = 3 cavity is 0.022 % ∕ cm , at the flow temperature of T = 100 ± 10 K . At these conditions, the laser output power is 0.28 W .

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.

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, ? = 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.

Nonthermal Ignition of Premixed Hydrocarbon-Air Flows by Nonequilibrium Radio Frequency Plasma

Results are presented of nonequilibrium rf plasma-assisted combustion experiments in premixed air-fuel flows. The experiments have been conducted in methane-air, ethylene-air, and CO-air mixtures. The results show that large volume ignition by the uniform and diffuse rf plasma can be achieved at significantly higher flow velocities (up to u = 25 m/s) and lower pressures (P = 60-130 torr) compared to both a spark discharge and a dc arc discharge. The experiments also demonstrated flame stabilization by the rf plasma, without the use of any physical obstacle flameholders. Fourier transform infrared (FTIR) absorption spectra of combustion products show that a significant fraction of the fuel (up to 80%) burns in the test section. Temperature measurements in the diffuse rf discharge using FTIR emission spectra show that the flow temperature in the plasma before ignition (T = 250-550°C at P = 60-120 torr) is considerably lower than the autoignition temperatures for ethylene-air mixtures at these pressures (T = 600-700°C). Visible emission spectroscopy measurements in C 2 H 4 -air flows in the rf discharge detected presence of radical species such as CH, C 2 , and OH, as well as O atoms. In CO-air flows, O and H atoms have been detected in the rf plasma region and CO 2 emission (carbon monoxide flame bands) in the flame downstream of the rf plasma.

Measurement of Flow Conductivity and Density Fluctuations in Supersonic Nonequilibrium Magnetohydrodynamic Flows

A new blowdown nonequilibrium plasma magnetohydrodynamic (MHD) supersonic wind tunnel operated at complete steady state has been developed and tested at Ohio State. The wind tunnel can be operated at Mach numbers up to M = 3-4 and mass flow rates of up to 45 g/s at a stagnation pressure of 1 atm

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.

Scaling of an Electric Discharge Excited Oxygen-Iodine Laser

Electric discharge excited oxygen-iodine laser apparatus has been successfully scaled to increase the electric discharge volume and power, the laser mixture flow rate, and the gain path in the M=3 laser cavity. Singlet delta oxygen (SDO) generator discharge power has been increased up to at least 4.5 kW, laser mixture flow rate up to approximately 0.5 mole/sec, and gain path up to 10 cm. The steady-state run time of the new scaled-up laser apparatus at these conditions is up to 10 sec. Two different discharge configurations have been used to generate singlet delta oxygen, crossed nanosecond pulser / transverse DC sustainer discharge and capacitively coupled transverse RF discharge. Flow temperature downstream of the discharge, singlet delta oxygen yield, and laser gain have been measured in a wide range of discharge powers, nitric oxide mole fractions in the main oxygen-helium flow, and oxygen percentage in the mixture, at discharge pressures ranging from 60 to 86 torr. The results demonstrate that SDO yield increases with the discharge power for both discharge configurations, although highest yields achieved so far remain rather low, 3.6-3.7%, due to fairly low energy loading per oxygen molecule in the discharge. Small signal gain measured in the M=3 cavity of the new laser apparatus is up to 0.116%/cm (2.3% gain per double pass), at the flow temperature of T=125 K. Laser gain remains steady during operation and decreases along the cavity, although the flow temperature along the cavity remains nearly constant. Iodine vapor flow rate critically affects gain when all other discharge and flow parameters are kept the same. The optimum iodine flow rate appears to increase with the discharge power, which suggests that greater amounts of iodine vapor in the flow are needed to optimize gain at higher powers.

Progress in Development of a Non-Self-Sustained Electric Discharge Pumped Oxygen-Iodine Laser 1

The present paper discusses results of experiments on singlet oxygen generation in a highpressure, non-self-sustained crossed discharge. The discharge consists of completely overlapping high-voltage (15-20 kV), short pulse duration (10-20 nsec), high repetition rate (up to 50 kHz) repetitively pulsed discharge, which produces ionization in the flow, and a low-voltage DC sustainer discharge which sustains current in a decaying plasma between the pulses. The sustainer discharge voltage can be independently varied to choose the reduced electric field value such as to maximize the sustainer energy input into electron impact excitation of singlet delta oxygen. The results demonstrate operation of a stable and diffuse crossed discharge in a 10% O2 – 90% He mixture at pressures of 60-120 torr (mass flow rates of 1.5-3.0 g/sec) and sustainer discharge powers of up to 1.6-2.2 kW. The reduced electric field in the positive column of the sustainer discharge in O2-He flows ranges from 0.6·10 -16 V·cm 2 to 1.2·10 -16 V·cm 2 . Singlet delta oxygen yield at these conditions, 1.7% to 5.7% at the flow temperatures ranging from 330 K to 430 K, was inferred from the integrated intensity of the (0,0) band of the O2(a 1 ∆→X 3 Σ) infrared emission spectra calibrated using a blackbody source. The results show that the O2(a 1 ∆) yield increases nearly proportionally to the sustainer discharge power and remain nearly independent of the O2 fraction in the mixture (up to 20%). Static pressure measurements in a M=3 supersonic test section downstream of the crossed discharge section show that a steady-state M=2.5-3.0 flow in the test section can be sustained for up to 10 to 25 seconds. These results also show that the measured singlet delta oxygen yield would exceed the threshold yield at the flow temperatures achieved in the supersonic section, 100-110 K, by about a factor of three. This suggests that yields measured in the present experiments may be sufficient for achieving positive gain in the supersonic laser cavity.

Development of a Non-Self-Sustained Electric Discharge Pumped Oxygen-Iodine Laser 1

The paper presents results of singlet oxygen generation experiments in a high-pressure, non-selfsustained crossed discharge. The discharge consists of 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. 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 V·cm 2 to E/N=0.65·10 -16 V·cm 2 , which is significantly lower than E/N in self-sustained discharges and close to the theoretically predicted optimum value for O2(a 1 ∆) excitation. Measurements of visible emission spectra O2(b 1 Σ→X 3 Σ) in the discharge afterglow show the O2(b 1 Σ) 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 m& =0.73 g/sec to 2.2 g/sec are 365 K to 465 K. Singlet delta oxygen yield at these conditions, 1.7% to 4.4%, was inferred from the integrated intensity of the (0,0) band of the O2(a 1 ∆→X 3 Σ) 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 modeling 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 1 ∆) 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.