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Measurements of Improved ElectricOIL Performance, Gain, and Laser Power

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Recent experiments have led to continued improvements in the hybrid Electric Oxygen-Iodine Laser (ElectricOIL) system that significantly increased the discharge performance, supersonic cavity gain, and laser power output. Experimental investigations of radio-frequency discharges in O 2 /He/NO mixtures in the pressure range of 10-50 Torr and power range of 0.1-2.0 kW have shown that O 2 (a 1 Δ) production is a strong function of geometry, pressure and diluent ratio. The goal of these investigations was maximization of both the yield and flow rate (power flux) of O 2 (a 1 Δ) in order to produce favorable conditions for subsequent gain and lasing in our ElectricOIL system. Numerous measurements of O 2 (a 1 Δ), oxygen atoms, and discharge excited states are made to characterize the discharge. Results with both molecular iodine injection and partially pre-dissociated iodine are presented. A gain of 0.17% cm -1 was measured with a corresponding outcoupled power of 12.3 W. Modeling with the BLAZE-IV model is in good agreement with data.
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Measurements of Improved Hybrid ElectricOIL Discharge
Performance, Gain, and Laser Power
D. L. Carroll,1 J. T. Verdeyen,2 G. F. Benavides,3,4 A. D. Palla,3 and T. H. Field3
CU Aerospace, Champaign, IL 61820
and
J. W. Zimmerman,4 B. S. Woodard,4 and W. C. Solomon5
University of Illinois at Urbana-Champaign, Urbana, IL 61801
Recent experiments have led to continued improvements in the hybrid Electric Oxygen-
Iodine Laser (ElectricOIL) system that significantly increased the discharge performance,
supersonic cavity gain, and laser power output. Experimental investigations of radio-
frequency discharges in O2/He/NO mixtures in the pressure range of 10-50 Torr and power
range of 0.1-2.0 kW have shown that O2(a1Δ) production is a strong function of geometry,
pressure and diluent ratio. The goal of these investigations was maximization of both the
yield and flow rate (power flux) of O2(a1Δ) in order to produce favorable conditions for
subsequent gain and lasing in our ElectricOIL system. Numerous measurements of O2(a1Δ),
oxygen atoms, and discharge excited states are made to characterize the discharge. Results
with both molecular iodine injection and partially pre-dissociated iodine are presented. A
gain of 0.17% cm-1 was measured with a corresponding outcoupled power of 12.3 W.
Modeling with the BLAZE-IV model is in good agreement with data.
I. Introduction
The first demonstration of the electric oxygen-iodine laser (often referred to as ElectricOIL, EOIL, or
DOIL) [Carroll, 2005a] was enabled through an understanding of the importance that oxygen atoms play in the
kinetics of the discharge and post-discharge regions. Oxygen atoms play a positive role in that they rapidly
dissociate the I2 molecule [Atkinson, 1997], but also play a negative role in that they quench the upper laser level
[Azyazov, 2006] and also directly quench the singlet oxygen metastable O2(a1Δ) [denoted hereafter as O2(a)]
through a three-body process [Rakhimova, 2003]. By controlling the atomic oxygen levels through the addition of
small molar fractions of NO or NO2 it is possible to enhance the performance of the system in terms of the O2(a), the
gain, and the laser power [Carroll, 2005a; Carroll, 2005b]. Ionin et al. [Ionin, 2007] provide a comprehensive
topical review of discharge production of O2(a) and ElectricOIL studies. Over the past four years of research and
development, continual improvements in O2(a) production, gain and lasing power have been obtained. In this paper
we discuss recent discharge and configuration improvements that have led to more than a factor of 80x enhancement
in gain and laser power since the initial demonstration of 0.002% cm-1 and 0.16 W, respectively, to 0.17% cm-1 and
12.3 W (with a 5 cm gain length cavity).
II. Transverse Discharge Experiments
Use of the well-calibrated gas laser facility at the University of Illinois allowed advanced concepts to be
economically implemented and compared directly using sophisticated diagnostic techniques. The ability to
implement these critical advanced technologies into the ElectricOIL laser testbed enables the community to
1 Engineering Director, CU Aerospace, 2100 S. Oak St. – Ste 206, Champaign, IL 61820, Associate Fellow AIAA.
2 Senior Scientist, CU Aerospace, 2100 S. Oak St. – Ste 206, Champaign, IL 61820.
3 Staff Engineer, CU Aerospace, 2100 S. Oak St. – Ste 206, Champaign, IL 61820, Member AIAA.
4 Research Assistant, Univ. of Illinois at Urbana-Champaign, 104 S. Wright, Urbana, IL 61801.
5 Prof. Emeritus, Univ. of Illinois at Urbana-Champaign, 104 S. Wright, Urbana, IL 61801, Fellow AIAA.
39th Plasmadynamics and Lasers Conference<BR>
23 - 26 June 2008, Seattle, Washington AIAA 2008-4008
Copyright © 2008 by authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
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understand the highly complex system questions involved and to advance the applicability of this technology. Over
the past two years of research and development we have progressively switched to using a transverse discharge
geometry because of better discharge stability leading to higher yields at higher pressures as compared to
longitudinal discharge configurations [Carroll, 2006].
2.1 Discharge Configurations and Flow System
Several different transverse capacitive RF discharge geometries have been examined (for more details and
other configurations see [Zimmerman, 2008] and [Woodard, 2008]). Four of these configurations are summarized
in Table 1 and illustrated in Fig. 1. All four discharges use 25.4-cm long (10”) copper plates or foil as electrodes.
Fig. 2 illustrates the differences between the “clamshell” and the “parallel plate” discharges. The clamshell
discharge consists of a copper foil fitting around the tube that is clamped against a Teflon bar running the length of
the discharge. The parallel plate discharge has two 2 mm thick copper sheets held by plastic bolts that clamp the
two sheets together. An ENI OEM-25A provided the power for the experiments at 13.56 MHz, and the incident and
reflected powers to the radio-frequency (rf) matching network were measured by a Bird Thruline model 43
wattmeter (rf “System Power” is the difference of the incident and reflected powers). Matching the power to the
discharge was achieved using a traditional PI-matching network with a tapped air-core transformer. Micro-Motion
CMF and Omega FMA mass flow meters were used to measure the flow rates of the gases. Pressures in the flow
tubes were measured with MKS Instruments and Leybold capacitance manometers.
Table 1. Transverse RF Discharge Geometries.
#
Flow Tube, Electrodes
Max. Internal Gap [cm]
Internal Volume [cm3]
Wall thickness [cm]
1
Circular, “Clamshell”
4.9 cm
479
0.25
2
Circular, Parallel Plate
4.9 cm
479
0.25
3
Rectangular, Parallel Plate
3.0 cm
465
0.30
4
Circular, Parallel Plate
2.0 cm
77.4
0.15
Figure 2: Cross-sectional illustrations of clamshell and
parallel plate discharges. Flow is into the paper.
2.2 Diagnostics suite for excited oxygen, and oxygen atom measurements
A Princeton Instruments/Acton Optical Multi-channel Analyzer (OMA-V, 1024-element InGaAs array)
with a 0.3-m monochromator and a 600 g/mm grating blazed at 1 µm was used for measurements at 1268 nm. An
Apogee E47 CCD camera coupled to a Roper Scientific/Acton Research 150-mm monochromator (1) was used to
measure the emission of O2(b) at 762 nm to determine flow temperature, as well as the emissions of excited atomic
oxygen at 777 nm, and excited argon at 750.4 and 751.5 nm. A Santa Barbara Instruments Group CCD with an
Acton 150-mm monochromator (2) was also used to measure O2(b). The broadband emission of NO2* was
measured using a Hamamatsu R955 photomultiplier with a narrowband 580 nm filter and a 50 mm focal length
collection lens; the O-atom concentration was determined from NO2* using the method described by Piper [Piper,
1981]. These optical diagnostics were fiber coupled using either Oriel model #77538 glass fiber bundles or
ThorLabs 600 µm x 5 m multimode fibers.
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2.3 Discharge Behavior at Higher Pressures
The modal behavior of the discharge can be observed qualitatively by visual inspection of the various
discharge glow emissions and the air-glow (O-NO NO2* NO2 + hν) in the various cases with NO added to the
flow. Figure 3 shows photographs of discharge geometry #2 driven at 13.56 MHz with a flow mixture of 10:33:0.15
O2:He:NO at 20 and 50 Torr for a variety of power levels. As the power is increased, the fraction of the plate gap
occupied by the plasma emission is increased. For the 20 Torr case, with power increasing from 0.2 to 1 kW,
transition of the discharge from α to γ-mode is observed (these modes are discussed in more detail in [Raizer, 1995],
[Rakhimova, 2003] and [Zimmerman, 2008a]); when the pressure is increased to 50 Torr, the α-mode is maintained
up to 1 kW. The spatial behavior of the discharge in the smaller 2.0 cm diameter tube is quite different than the
results seen for the 5 cm I.D. tube; as seen in Fig. 4, the visible plasma fills the electrode gap completely for 50 Torr
and 800 W RF. The glow visible downstream of the discharge gap in Figs. 3 and 4 is due to O-NO recombination.
It should be noted that our ability to visibly observe the discharge region has been a significant factor in developing
our understanding of the discharge physics and has enabled us to make systematic enhancements with time.
Figure 3. Photographs of discharge glow in geometry #2 (circular, 4.9 cm diameter) at 13.56 MHz. The flow mixture
was 10:33:0.15 mmol/s O2:He:NO.
Figure 4. Photographs of discharge glow in geometry #4 (circular, 2.0 cm diameter) at 13.56 MHz. The flow mixture
was 10:33:0.15 mmol/s O2:He:NO.
Higher O2(a) yield data of >25% can be obtained at higher diluent ratios, Fig. 5. The O2(a) and oxygen
atom yields are shown versus pressure for 3:60 O2:He at 800 W and 5:100 O2:He at 1200 W. The NO flow rate is
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constant at 0.15 mmol/s, and these data were acquired at the exit of the 4.9 cm clamshell discharge (geometry #1).
The O2(a) yield peaks at 26% at 40 Torr for the 3 mmol/s of oxygen case although the yield is only slightly lower in
the 5 mmol/s of oxygen case. These high helium diluent ratios cause the discharge to produce a large concentration
of oxygen atoms especially at low pressure, but at higher pressures the O atoms rapidly disappear through
recombination and wall reactions. Note that to obtain the most consistent comparison between cases at different
flow and pressure conditions, the measured data presented here in Figs. 5-7 were translated (via a calibration) to be
at a position at the exit of the discharge section (see [Woodard, 2008] for more details).
Figure 5: O2(a) yield and oxygen atom yield vs. pressure
for 3:60 mmol/s O2:He at 800 W and 5:100 mmol/s O2:He
at 1200 W with 0.15 mmol/s NO in a 4.9 cm clamshell
discharge.
In Fig. 6 the O2(a) yield is plotted versus RF system power for 3, 5, and 7 mmol/s of oxygen keeping the
diluent ratio 1:20 O2:He with 0.15 mmol/s NO at 40 Torr using the 2” clamshell discharge. The baseline EOIL case
of 10:33:0.15 O2:He:NO at 20 Torr is also included. Let us now consider the amount of power stored in the O2(a).
Using Eq. 1, the data from Fig. 6 can be converted to Fig. 7.
O2(a)Power W
[ ]
=˙
n
O2×YO2(a)×NA×1.57 ×1019 J
O2(a)molecule
(1)
where NA is Avogadro’s number,
˙
n
O2 is the molar flow rate of oxygen, and Y is the O2(a) yield. While the yields
shown in Fig. 6 for these high diluent ratio cases are higher, the oxygen flow rates are decreased, so as Fig. 7
illustrates, the amount of power in the O2(a) state is not necessarily as high as the baseline case. The conditions for
Fig. 6 and 7 are identical. Clearly, both high yields and high oxygen flow rates are required to produce large O2(a)
powers, and the 7:140 mmol/s O2:He at 40 Torr case produces similar powers to the baseline case which occurs at
half the pressure; these conditions will be explored further in future work.
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Figure 6: O2(a) yield vs. RF system power for 1:20 O2:He
at 40 Torr and ~1:3 O2:He at 20 Torr with 0.15 mmol/s NO
in a 4.9 cm clamshell discharge.
Figure 7: O2(a) Power vs. RF system power for 1:20 O2:He
at 40 Torr and ~1:3 O2:He at 20 Torr with 0.15 mmol/s NO
in a 4.9 cm clamshell discharge.
III. Gain and Laser Enhancement Experiments
As we acquire more understanding of the complex ElectricOIL system, including the species output from
the discharge and the resulting downstream kinetics, we have implemented a logical progression of knowledge into a
3rd generation "Cav3" and then a 4th generation “Cav4” ElectricOIL system to increase the gain and laser power
output levels.
3.1 Third Generation Laser Cavity
As reported by Benavides et al. [Benavides, 2007], a gain of 0.067%/cm was achieved with the Cav3
hardware improvements, and BLAZE-IV modeling [Palla, 2007] is in reasonable agreement with data, Fig. 8. 2"
diameter laser mirrors from AT Films with a reflectivity of approximately 99.997% were used and resulted in laser
power of 4.5 W with the laser cavity gain length held fixed at 5 cm.
Figure 8. BLAZE-IV predictions of gain vs. axial position
through the Cav3 supersonic nozzle. Data taken with the
Cav3 hardware and cold N2 injectors downstream from the
I2 injectors. Flow conditions were O2:He=10:33 at 20 Torr
pressure and 1000 W RF sustainer power.
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3.1.1 Performance with an iodine pre-dissociation discharge
In the prior ElectricOIL experiments we relied upon O atoms to provide the predominant mechanism for
iodine dissociation. However, with the desire to push the system to higher flow pressures there are fewer O atoms
available due to recombination, e.g., see Fig. 5. Thus, to achieve more complete dissociation in ElectricOIL, the use
of an iodine pre-dissociator is of particular importance at higher flow pressures. Carroll and Solomon [Carroll,
2000] first hypothesized and calculated that this technology improvement would be very important for the
ElectricOIL system to maximize the energy transfer of O2(1Δ) into atomic iodine, and consequently significantly
enhance gain and laser power output.
Recently, Benavides et al. [Benavides, 2008] reported the demonstration of a 50% enhancement in gain and
38% enhancement in continuous-wave laser power on the 1315 nm transition of atomic iodine through the addition
of a secondary discharge to pre-dissociate the molecular iodine in an electric oxygen-iodine laser. A block diagram
of that flow tube setup is shown in Fig. 9. The primary rf electric discharge at 13.56 MHz operating between two
transverse electrodes was used as the excitation source for the production of O2(a1Δ). The plasma zone was
approximately 4.9 cm in diameter and 25 cm long. A secondary rf discharge was placed at the exit of the iodine
injection holes using electrodes imbedded in injector blocks fabricated out of the machinable ceramic Macor®.
Fig. 9: Schematic of the
experimental apparatus with
secondary rf discharge.
The flow conditions for these gain and laser power experiments with the I2 pre-dissociator are 10.0 mmol/s
of O2 which is diluted with 50.0 mmol/s of He and 0.05 mmol/s of NO. A secondary stream of 0.045 mmol/s of
I2 with 12.0 mmol/s of secondary He diluent was injected 27.3 cm downstream from the exit of the primary
discharge and run through a 100 W rf secondary discharge. A tertiary flow of 100 mmol/s of cold N2 gas (100 K)
was injected further downstream to lower the temperature. The pressures in the subsonic diagnostic duct and in the
supersonic diagnostic cavity were 30.0 Torr and 3.2 Torr, respectively. Measurements in the subsonic diagnostic
duct from the O2(1Δ) and O2(b1Σ) spectra indicated an O2(1Δ) yield of 13% and a gas temperature of 450 K for
these flow conditions at 700 W of rf power in the primary discharge.
Gain was measured for the above flow conditions at 700 W of primary rf discharge power and 100 W of
secondary rf discharge power for the I2 pre-dissociator. Figure 10 shows the gain at line center which peaks at
0.10% cm-1 with the I2 pre-dissociator secondary discharge. For comparison, the best gain previously observed in
our system (Fig. 8) of 0.067% cm-1 without this secondary discharge is also shown in Fig. 10; the secondary
discharge provides a 50% enhancement in gain as compared to the previous best results. The lineshape indicates a
temperature of 120 K. Note that the flow conditions with and without the secondary discharge are different
between the two cases shown in Fig. 10. The flow conditions without the secondary discharge were
O2:He:NO=10:33:0.15 mmol/s with Pto tal = 20 Torr and a primary rf discharge power of 1000 W, and those with the
secondary discharge were O2:He:NO=10:50:0.05 mmol/s with Ptotal = 30 Torr and a primary rf discharge power of
700 W (as stated above).
It is important to note that the primary discharge was intentionally reduced to 700 W and thus the total
(primary plus secondary) rf discharge power being only 800 W when using the secondary discharge. This reduction
in primary discharge power enabled the 50% enhancement to gain and a 20% reduction in total applied power as
compared to the earlier 1000 W case from [Benavides, 2007]. Therefore, applying power to dissociate the iodine
with the secondary discharge is more energy efficient than applying the power required to produce sufficient O
atoms in the primary discharge to dissociate the iodine downstream.
Figure 11 shows the gain during a sequence of turning different discharges on and off, starting with the
secondary discharge only, followed by both discharges on, followed by data with only the primary discharge, and
then again with both discharges on. As anticipated, there is absorption when only the low power secondary
discharge is running, showing that the molecular I2 is at least partially dissociated. The authors hypothesize that
some of the secondary discharge power may dissociate not only I2, but also O2 through coupling with the rf field,
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followed by the newly created O atoms helping in the I2 dissociation process. A test was run where the O2 flow was
set to zero, and the magnitude of the absorption decreased (but did not disappear), therefore we conclude that this
phenomenon is occurring, but that the secondary discharge is at least partially dissociating the I2 as desired; further
studies of the secondary discharge dynamics and interactions with the primary flow are required. When the primary
discharge is turned on the gain rises quickly to 0.10% cm-1. When the secondary discharge is turned off, the gain
drops to approximately 0.02% cm-1 (note that the gain for these flow conditions without the secondary discharge is
significantly lower than the 0.067% cm-1 gain for the different 20 Torr, 1000 W flow conditions shown in Fig. 10),
and when the secondary discharge is again turned on the gain returns to 0.10% cm-1.
The laser resonator was subsequently installed around the supersonic cavity. For the above 30 Torr flow
conditions and total rf power of 800 W, a total laser output power of 6.2 W was obtained, a 38% improvement to
laser power relative to the results of [Benavides, 2007]. The beam was elliptically shaped with a length of 3.5 cm
(the same as the clear aperture of the mirror mounts) in the flow direction and a height of 1.8 cm.
Fig 10: Gain lineshape in the supersonic cavity as a
function of probe beam scan frequency with and
without a 100 W rf secondary I2 dissociation
discharge.
Fig. 11: Gain in the supersonic cavity as a function
of time with different combinations of the primary
and secondary discharges.
3.2 Fourth Generation Laser Cavity with Multi-Discharge Tube Performance
Zimmerman et al. [Zimmerman, 2008b] recently investigated the use of two parallel primary discharges at
higher total flow rates and pressure plus a secondary discharge to pre-dissociate the molecular iodine (the same used
in [Benavides, 2008], as discussed above) in an electric oxygen-iodine laser. The O2(1Δ) is produced by two parallel
capacitive 13.56 MHz electric discharges sustained in an O2-He-NO gas mixture, and I* is then pumped using
energy transferred from O2(1Δ); the electrode gaps in the primary discharges are transverse to the flow direction, and
the discharges are matched in parallel from a single power supply. A block diagram of the flow tube setup is shown
in Fig. 12. Both of the plasma zones filled the transverse gap and were approximately 1.6 cm diameter and 25 cm
long (the outside diameter of each of these discharge tubes was 1.9 cm). In prior experiments a single 4.9 cm
diameter plasma zone was utilized (discharge tube with outside diameter of 5.1 cm). The change to smaller
diameter discharge tubes was motivated by a detailed series of work summarized in Braginsky et al. [Braginsky,
2007] in which they demonstrated that smaller diameter tubes had substantially increased discharge stability at
higher pressure while maintaining significant O2(1Δ) yields. More information on the performance of the transverse
electric discharge sustained in an O2-He-NO gas mixture used in the experiments presented herein can be found in
Woodard et al. [Woodard, 2008] and Zimmerman et al. [Zimmerman, 2008a]. A secondary rf discharge
[Benavides, 2008] was placed at the exit of the iodine injection holes using electrodes imbedded in injector blocks
fabricated out of the machinable ceramic Macor®.
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Fig. 12: Schematic of the multi-
discharge experimental apparatus.
The flow conditions for these gain and laser power experiments with the dual primary discharges and the I2 pre-
dissociator are 20.0 mmol/s of O2 which is diluted with 66.0 mmol/s of He and 0.08 mmol/s of NO. A secondary
stream of 0.065 mmol/s of I2 with 18.0 mmol/s of secondary He diluent was injected 27.3 cm downstream from
the exit of the primary discharge and run through a 100 W rf secondary discharge. A tertiary flow of 215 mmol/s of
cold N2 gas (100 K) was injected further downstream to lower the temperature. The pressures in the subsonic
diagnostic duct and in the supersonic diagnostic cavity were 43.0 Torr and 4.6 Torr, respectively. Measurements in
the subsonic diagnostic duct from the O2(1Δ) and O2(b1Σ) spectra indicated an O2(1Δ) yield of 11% and a gas
temperature of 415 K for these flow conditions at 1000 W of rf power in each of the two primary discharges (a
total of 2000 W).
Gain was measured for the above flow conditions at a total of 2000 W of primary rf discharge power and 100 W of
secondary rf discharge power for the I2 pre-dissociator. Figure 13 shows the gain at line center which peaks at
0.17% cm-1 with the dual 1.9 cm diameter primary discharges and the I2 pre-dissociator secondary discharge. For
comparison, the best gain previously observed in our system of 0.10% cm-1, using a single 5.1 cm diameter primary
discharge and a little more than half of the flow rates at 30 Torr total pressure (3.2 Torr in the supersonic diagnostic
cavity), is also shown in Fig. 13; the dual discharge provides a 70% [= (0.17-0.10)/0.10] enhancement in gain as
compared to the previous best results. The lineshape indicates a temperature of 125 K.
Fig 13: Gain lineshapes in the supersonic cavity as a
function of probe beam scan frequency with dual
parallel 1.9 cm diameter discharges operating at 43
Torr (4.6 Torr in supersonic cavity) and with a single
5.1 cm diameter discharge operating at 30 Torr (3.2
Torr in supersonic cavity).
The laser resonator was subsequently installed around the 5 cm gain length supersonic cavity. For the above 43 Torr
flow conditions, a total laser output power of 12.3 W was obtained, a 98% improvement to laser power relative to
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the 6.2 W result from [Benavides, 2008]. The beam shape was rectangular with rounded corners and had a length of
3.5 cm in the flow direction and a height of 1.8 cm (the same dimensions as the clear aperture of the mirror
mounts in the flow direction and the height of the nozzle at the center of the beam in the vertical direction).
IV. Concluding Remarks
Over the past four years of research and development, continual improvements in gain and lasing power have been
obtained. The gain has risen from the initial demonstration of 0.002% cm-1 by a factor of 80× to 0.17% cm-1, and
similarly the outcoupled laser power has risen from 0.16 W to 12.3 W (with a 5 cm gain length cavity). We are now
obtaining 30% energy coupling (and for the higher rf power cases more than 200 W of the power) into the desired
O2(a) state, but significant improvements in understanding the role of components of plasma generated “active
oxygen” still need to be made in regards to laser extraction of this energy. While O atoms permit rapid dissociation
of the I2 molecule, they appear to be major problem for energy extraction (as they also act as a quencher) and
alternate I2 dissociation schemes need be investigated.
Geometry and diluent ratio are critical parameters for the production of high O2(a) yields from transverse RF
discharges at moderate pressures (50-100 Torr). The discharge that effectively makes O2(a) at 20 Torr does not fill
the electrode volume as the pressure increases, and this effect leads to decreased yield. By shortening the gap
between the electrodes, the discharge fills the volume at higher pressures and more effectively creates O2(a) at those
pressures. At the same time, increasing the helium diluent in the discharge also increases the O2(a) yield at higher
pressure. Yields over 25% are measured with 1:20 O2:He in the discharge. Future work must exploit both of these
effects to create a discharge capable of producing high yields with high oxygen flow rates to produce a flow with
high O2(a) power.
Applying power to dissociate the iodine with the secondary discharge is more energy efficient than applying excess
power in the primary discharge to produce sufficient O atoms for iodine dissociation downstream. Further
improvements to the iodine dissociator are expected to provide improvements to the gain, laser power, and
additional power savings over the current design. The implementation of a combination of multiple smaller
diameter discharge tubes plus the molecular iodine pre-dissociator has permitted us to expand the flow conditions of
the ElectricOIL device to higher pressures and flow rates. A continued expansion of the operating envelope to
higher flow conditions, pressures, and gain length of the laser cavity, plus further improvements to the iodine
dissociator are expected to provide significant increases to the gain and laser power.
Acknowledgments
This work was supported by the Missile Defense Agency (MDA) through the U.S. Army Space and Missile
Defense Command (USA SMDC), and the Joint Technology Office (JTO) through the Air Force Office of Scientific
Research (AFOSR), and by CU Aerospace internal research and development funds. The authors acknowledge the
contributions of: T. Madden and D.A. Hostutler (AFRL); W.T. Rawlins and S.J. Davis (Physical Sciences Inc.); M.
Heaven (Emory Univ.); G. Perram and G. Hager (Air Force Institute of Tech.); M. Berman (AFOSR); B. Otey
(USA/SMDC); W. Fink (JTO); J. Kotora and D. Podolski (MDA); G.W. Sutton (Sparta Inc.); A. Ionin (P.N.
Lebedev Physics Inst.); and T. Rakhimova and O. Proshina (Lomonosov Moscow State Univ.).
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... A general approach to enhancing performance of any gas laser is to improve the product of gain and the gain length, which is, to some extent, a representation of the power available for extraction. Based on previous experimental efforts [13][14][15], the engineering factors required to further increase the size of the system were well understood. These factors include: discharge size and geometry, radio-frequency (RF) power to O 2 flow ratio, He diluent ratio, control of atomic oxygen (quenching mechanisms), thermal management (heat exchanger, chilled diluent, supersonic expansion), resonator performance characteristics, pressure recovery, and pumping requirements. ...
... In early experiments with the 6 th generation "Cav6" laser cavity [13,15], six 1.6 cm internal diameter discharge tubes were utilized that resulted in substantially increased discharge stability at higher pressure while maintaining significant O 2 (a) yields. To further increase performance beyond the 1.6 cm tube geometry first introduced with ElectricOIL Cav4, a high aspect ratio rectangular quartz discharge channel [13,17] was chosen to simultaneously maintain the good small gap discharge characteristics while at the same time reducing pressure loss across the discharge and improving the electrical efficiency of O 2 (a) production. ...
... The flows exiting the discharge modules enter independent cross-flow heat-exchanger modules which use Syltherm XLT coolant chilled to −30°C by LN 2. Some trade studies used to develop heat-exchangers for ElectricOIL are summarized in [14] and [15]. The design used in Cav7 experiments is altered from the staggered-tube device used for Cav6 (Fig. 2); the new HX having less surface area to reduce O 2 (a) loss, better optimized tube spacing to further reduce pressure loss, and more even coolant distribution to maximize temperature reduction. ...
Article
Continuing experiments with electric oxygen-iodine laser (ElectricOIL) technology have significantly increased laser power output by increasing the product of gain and gain-length, $g_{0}L$. The authors report on progress with recent ElectricOIL devices utilizing a new concentric discharge geometry with improved ${\rm O}_{2}(a^{1}\Delta)$ production at higher discharge operating pressure at higher system flow rates. ${\rm O}_{2}(a^{1}\Delta)$ produced in flowing radio-frequency discharge in ${\rm O}_{2}\hbox{-}{\rm He}\hbox{-}{\rm NO}$ gas mixture is used to pump $I(^{2}P_{1/2})$ by near-resonant energy transfer, and laser power is extracted on the $I(^{2}P_{1/2})\rightarrow I(^{2}P_{3/2})$ transition at 1315 nm. Advancements in heat exchanger design reduce ${\rm O}_{2}(a^{1}\Delta)$ wall loss without sacrificing significant cooling efficiency improving best gain performance from 0.26 to 0.30% ${\rm cm}^{-1}$. Modeling of recent data is presented. By increasing the gain length (system size) by a factor of 3, a 5-fold increase in laser output on the 1315-nm transition of atomic iodine was achieved. Flow conditions with $g_{0}L=0.042$ were used to extract a continuous wave average total laser power of 481 W. A low frequency ${\pm}{11.9\%}$ oscillation in the total power was observed giving a peak outcoupled power of 538 W.
... Additional improvements to the heat exchanger design used between the discharge and laser cavity were also made. The design applied to the new ElectricOIL configuration Cav6 was based on the analysis described in Ref. 13. The thermal power extracted from the flow exiting the discharge was measured by the heating of the cooling water passed through the heat-exchanger. ...
... Recent theoretical investigations have indicated that the combination of short gain length (5.1 cm) with gain < 0.25% cm -1 (which requires highly reflective mirrors for lasing) results in significant diffractive losses inside the laser hardware that thereby reduce the extracted power from the gain medium. 15 To help alleviate this problem the 5.1 cm gain length of the prior fifth generation "Cav5" laser cavity, reported on in Ref. 13, was increased to 7.6 cm in the new sixth generation "Cav6" laser cavity discussed herein. Longer gain length enables lower reflectivity mirrors to be used for the resonator, which reduces the number of passes a photon makes within the resonator, and thereby lowers the amount of energy lost to diffractive spill (or equivalently increases the fraction of power extracted from the gain medium). ...
... The plasma zone filled the transverse gap of 1.6 cm and was 50.8 cm long (the gap between the parallel plate electrodes, the outside dimension of the flow channel, was 2.2 cm, i.e. the quartz tube walls were 0.3 cm thick). In prior experiments 13,14 , single and multiple 1.6 cm internal diameter discharge tubes were utilized that resulted in substantially increased discharge stability at higher pressure while maintaining significant O 2 (a) yields. More information related to the performance of the transverse electric discharge sustained in an O 2 -He-NO gas mixture used in the experiments presented herein can be found in Woodard et al. 17 and Zimmerman et al. 13 . ...
Article
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The authors observed 95% enhancement in continuous-wave laser power on the 1315 nm transition of atomic iodine for only a 50% increase in gain length (5.1 cm to 7.6 cm), flow rates, and discharge input power, making use of a large volume 16-mm gap transverse discharge. A gain of 0.26% cm -1 was obtained and the laser output power was 55 W in a stable resonator with two 5 cm (2-in.) diameter, 0.9970 reflective mirrors. A longer gain length cavity permits use of lower reflectivity resonator mirrors that reduce diffractive spill losses, and thereby extract power from the gain medium more efficiently. The outcoupled power was increased to 92 W by increasing the mirror diameter to 10 cm (4-in.), demonstrating that significantly higher power was available in the electric oxygen-iodine laser gas flow which could be extracted by tailoring the cavity design. Two 4-mirror folded resonator configurations using 5 cm optics were also demonstrated, a stable "Z-resonator" with a z-shaped optical path having six roundtrip passes through the gain medium, and a stable "X X-resonator" having four roundtrip passes through the gain medium. The best measured outcoupled powers for these folded designs were 102 W and 109 W for the Z-resonator and X-resonator, respectively. Continued expansion of the operating envelope to higher flow conditions, pressures, and gain length of the laser cavity, plus the addition of an iodine pre-dissociator discharge are expected to provide significant increases to the gain and laser power. The results presented herein represent more than two orders of magnitude improvement in gain and laser power since the initial demonstration in 2005.
... Ionin et al. 11 and Heaven 12 provide comprehensive topical reviews of discharge production of O 2 (a) and various ElectricOIL studies. As of approximately one year ago 13 , the highest reported gain in an ElectricOIL device was 0.2 % cm -1 , with an output power of 28.1 W, and currently the gain and laser power stand at 0.26 % cm -1 and 102 W, respectively. 14 O 2 (a) excitation in an electric discharge is a complicated process. ...
... Simulations using the BLAZE-IV 9,10 discharge model suggested that, in the smaller diameter discharge tubes, the 10 inch electrodes did not provide sufficient residence time for the flow within the discharge to reach an equilibrium of O 2 (a) production. Experiments with a single ¾ inch discharge tube supported this conclusion 13 , and a long, rectangular cross-section quartz tube was fabricated to further exploit this finding for increased O 2 (a) production. Electrodes with lengths of 10, 20, 30, and 42 inches were placed on this tube, and the results from several gas flow rates were measured. ...
Article
Full-text available
Experiments and modeling have led to continued enhancements in the Electric Oxygen-Iodine Laser (ElectricOIL) system. This continuous wave (cw) laser operating on the 1315 nm transition of atomic iodine is pumped by the production of O 2 (a) in a radio-frequency (RF) discharge in an O 2 /He/NO gas mixture. New discharge geometries have led to improvements in O 2 (a) production and efficiency. Studies of electrode gap continue to improve O 2 (a) production at high pressures. Additionally, measurements of species exiting the discharge and the determination of discharge parameters such as E/N continue to expand the understanding of this system. Some of these improvements have already been applied to the laser system, and other advances will be utilized to continue scaling the system to higher laser powers. Over 1 kW of power stored in O 2 (a) has been demonstrated in both the rectangular cross-section and multi-circular tube discharges.
... This paper discusses specifically recent ElectricOIL heat exchanger studies to improve scientific understanding to reduce postdischarge oxygen atom population and bulk flow temperature, without significantly impacting O 2 (a) population; the reduction in temperature enables higher gain and laser power extraction efficiency as discussed below. These studies build on previous ElectricOIL heat exchanger experimental investigations [21][22] , the earlier results encouraging further investigations. Given the additional function of oxygen atom control, referring to such devices as simply heat exchangers may lead to confusion regarding their significance. ...
... The predicted O 2 (a) fraction quenched for both shapes and all sizes was on average 3.5% with a maximum of 4.1% and minimum of 3.2%. This is considerably less than the approximate 10% O 2 (a) fraction quenched using a ducted design similar to STER-6A reported by Zimmerman [21] . The O 2 (a) loss with the diamond was predicted slightly higher than the tubular design, however the difference is small. ...
Article
Full-text available
Experiments[1] with Electric Oxygen-Iodine Laser (ElectricOIL) heat exchanger technology have demonstrated improved control of oxygen atom density and thermal energy, with minimal quenching of O2(a1Δ), and increasing small signal gain from 0.26% cm-1 to 0.30% cm-1. Heat exchanger technological improvements were achieved through both experimental and modeling studies, including estimation of O2(a1Δ) surface quenching coefficients for select ElectricOIL materials downstream of a radio-frequency discharge-driven singlet oxygen generator. Estimation of O2(a1Δ) quenching coefficients is differentiated from previous studies by inclusion of oxygen atoms, historically scrubbed using HgO[2-4] or AgO[5]. High-fidelity, time-dependent and steady-state simulations are presented using the new BLAZE-VI multi-physics simulation suite[6] and compared to data.
... While these simulations were performed for relatively early EOIL experiments, the hardware has since been dramatically improved to attain a gain of 0.30% cm -1 and laser power > 500 W [Benavides, 2012]. Figure 1 shows plasma simulations of the concentric EOIL discharge tubes used in Cav7 [Benavides, 2012], as well as 3D nozzle simulations illustrating the O 2 (a) concentration and the lasing mode of an earlier Cav5 system [Zimmerman, 2009]. EOIL presently shows superlinear scaling with g 0 L and it is anticipated that any future work would result in significant performance enhancement, in part with the guidance of simulations from the sophisticated BLAZE Multiphysics™ plasma-fluids-laser model. ...
... The flows exiting the discharge modules enter independent cross-flow heat-exchanger modules which use Syltherm XLT coolant chilled to -30ºC by LN 2 . Trade studies used to develop heat-exchangers for ElectricOIL are summarized in and [Zimmerman, 2009a]. The design used in Cav7 experiments is reported in [Benavides, 2012]. ...
Article
Full-text available
Continuing experiments with Electric Oxygen-Iodine Laser (EOIL) technology have significantly increased laser power output by increasing the product of gain and gain-length, g0L. Increasing the system size by a factor of 3 resulted in a 5-fold increase in laser output on the 1315-nm transition of atomic iodine. The peak output power observed was 538 W.
Article
Full-text available
Results of experimental and theoretical study of singlet delta oxygen (SDO) production in transverse gas flow RF slab discharge for an electric discharge oxygen-iodine laser are presented. The electric discharge facility operating in both pulse-periodic and CW mode was manufactured: gas flow duct including multi-path cryogenic heat exchanger, dielectric slab channel, and slab electrode system incorporated in the channel for RF discharge ignition. Experiments on SDO production in transverse gas flow RF discharge were carried out. SDO production depending on gas mixture content and pressure, gas flow velocity, and RF discharge power was experimentally studied. It was shown that SDO yield increased with gas pressure decrease, gas flow deceleration and helium dilution of oxygen at the same input power. CW RF discharge was demonstrated to be the most efficient for SDO production as compared to pulse-periodic RF discharge with the same averaged input power. SDO yield was demonstrated to be not less than 10 percent. The model developed was further modified to do simulations of CW and pulse periodic RF discharges. A reasonable agreement between experimental and theoretical data on SDO production in CW and pulse-periodic RF discharges in oxygen is observed.
Article
Herein the authors report on the demonstration of a 95% enhancement in continuous-wave laser power on the 1315 nm transition of atomic iodine via a 50% increase in gain length, flow rates, and discharge power. O2(a 1Δ) is produced by a single radio-frequency-excited electric discharge sustained in an O2–He–NO gas mixture flowing through a rectangular geometry, and I(2P1/2) is then pumped using energy transferred from O2(a 1Δ). A gain of 0.26% cm−1 was obtained and the total laser output power was 54.8 W.
Article
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.
Article
Experiments and modelling have led to continued enhancements in the electric oxygen–iodine laser system. This continuous wave laser operating on the 1315 nm transition of atomic iodine is pumped by the production of O 2 (a) in a radio-frequency discharge in an O 2 /He/NO gas mixture. New discharge geometries have led to improvements in O 2 (a) production and efficiency. Studies of electrode gap continue to improve O 2 (a) production at high pressures, and measurements of species exiting the discharge have expanded the understanding of this system. Some of these improvements have already been applied to the laser system, and other advances will be utilized to continue scaling the system to higher laser powers. Over 1 kW of power stored in O 2 (a) has been demonstrated in both the rectangular cross-section and multi-circular tube discharges.
Article
Full-text available
Experimental investigations of radio-frequency discharges in O 2 /He/NO mixtures in the pressure range of 1-100 Torr and power range of 0.1-2.5 kW have indicated that O 2 (a 1 ∆) production is a strong function of geometry, pressure and diluent ratio. The goal of these investigations was maximization of both the yield and flow rate (power flux) of O 2 (a 1 ∆) in order to produce favorable conditions for application to an electric oxygen-iodine laser (EOIL). As pressure is increased, yield performance is dominated by the influence of geometry and diluent ratio. Numerous measurements of O 2 (a 1 ∆), oxygen atoms, and discharge excited states are made in order to describe the discharge performance dependence on various parameters.
Article
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In the hybrid electric discharge Oxygen-Iodine laser (ElectricOIL), the desired O2(a1Δ) is produced using a low-to-medium pressure electric discharge. The discharge production of atomic oxygen, ozone, and other excited species adds higher levels of complexity to the post-discharge kinetics which are not encountered in a classic purely chemical O2(a1Δ) generation system. Experimental studies over the past six years using electric discharges have demonstrated O2(a) yields greater than 20%, gain, and cw laser power. Several modeling studies have also been performed for ElectricOIL and similar systems. As the development of this type of iodine laser continues, the roles of oxygen atoms and NO/NO2 are found to be very significant in both the discharge region and downstream of the discharge region. A series of O2(1Δ) emission, I* emission, O-atom titrations, gain, and O2(1Δ) yield, NO2* emission, and laser power measurements have been taken to explore the complex phenomena that are being observed. As the overall system is better understood improvements are being made in laser power and efficiency.
Article
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The removal of N2(A 3Σ+u, v′ = 0,1) by O has been studied in a room temperature discharge–flow apparatus by monitoring the temporal decay of the 0,6 and 1,10 bands of the Vegard–Kaplan system. The measured rate constants are (2.8±0.4) and (3.4±0.6)×10−11 cm3 molecule−1 s−1 for v′ = 0 and 1, respectively.
Article
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This work is devoted to the experimental and theoretical study of rf frequency influence on the discharge structure and the O-2(a(1)Delta(g)) yield in a rf discharge in the tubes with HgO coating for removing atomic oxygen to avoid fast O-2(a(1)Delta(g)) quenching. Two series of experiments were carried in two discharge sets up: 1 - tube of large diameter of 14 mm, electrode length of 30 cm; 2 - tube of small diameter of 7 mm, electrode length of 10 cm. In the first series, the increase of rf frequency from 13.56 MHz to 81 MHz resulted in the singlet oxygen (SO) yield Y > 15% at such high oxygen pressure as 10 Torr. 2D self-consistent model was developed to simulate the rf discharges in gas flow in a wide rage of discharge parameters both with and without HgO coating. Results of the simulation agreed very well with the experimental data. It is shown that at the high rf frequency the discharge operates in a mode of the normal current density so that the energy absorbed by electrons from the rf field increases with the frequency. In the second series we scaled up the rf discharge on the pd parameter (p-pressure, d-tube diameter) to increase the oxygen pressure. The pd discharge scaling at the high rf frequency allowed to reach the O-2(a(1)Delta(g)) yield Y > 15% at oxygen pressure up to similar to 17 Torr. The effect of NO admixture on the O-2(a(1)Delta(g)) production has been studied in series 2 at rf frequency of 160 MHz. NO admixture (5 divided by 20%) results in the noticeable increase in the O-2(a(1)Delta(g)) yield in comparison with the discharge tube without HgO coating. It is shown that combination of O-2 + NO mixture with HgO coating of the discharge tube walls provides the most optimal O-2(a(1)Delta(g)) production with the efficiency up to 7-10 % and allows to reach the threshold yield in 15 % at oxygen pressure even above similar to 20 Torr.
Article
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An overview is presented of experimental and theoretical research in the field of physics and engineering of singlet delta oxygen (SDO) production in low-temperature plasma of various electric discharges. Attention is paid mainly to the SDO production with SDO yield adequate for the development of an electric discharge oxygen–iodine laser (DOIL). The review comprises a historical sketch describing the main experimental results on SDO physics in low-temperature plasma obtained since the first detection of SDO in electric discharge in the 1950s and the first attempt to launch a DOIL in the 1970s up to the mid-1980s when several research groups started their activity aimed at DOIL development, stimulated by success in the development of a chemical oxygen–iodine laser (COIL). A detailed analysis of theoretical and experimental research on SDO production in electric discharge from the mid-1980s to the present, when the first DOIL has been launched, is given. Different kinetic models of oxygen low-temperature plasma are compared with the model developed by the authors. The latter comprises electron kinetics based on the accompanying solution of the electron Boltzmann equation, plasma chemistry including reactions of excited molecules and numerous ion–molecular reactions, thermal energy balance and electric circuit equation. The experimental part of the overview is focused on the experimental methods of SDO detection including experiments on the measurements of the Einstein coefficient for SDO transition and experimental procedures of SDO production in self-sustained and non-self-sustained discharges and analysis of different plasma-chemical processes occurring in oxygen low-temperature plasma which brings limitation to the maximum SDO yield and to the lifetime of the SDO in an electric discharge and its afterglow. Quite recently obtained results on gain and output characteristics of DOIL and some projects aimed at the development of high-power DOIL are discussed.
Article
Experimental investigations of radio-frequency discharges in O2/He/NO mixtures in the pressure range of 1-100 Torr and power range of 0.1-1.2 kW have indicated that O2(a1Delta) production is a strong function of geometry, excitation frequency, pressure and diluent ratio. The goal of these investigations was maximization of both the yield and flow rate (power flux) of O2(a1Delta) in order to produce favorable conditions for application to an electric oxygen-iodine laser (EOIL). At lower pressures, improvements in yield are observed when excitation frequency is increased from 13.56 MHz. As pressure is increased, increasing excitation frequency in the baseline configuration becomes detrimental, and yield performance is improved by reducing the discharge gap and increasing the diluent ratio. Numerous measurements of O2(a1Delta), oxygen atoms, and discharge excited states are made in order to describe the discharge performance dependent on various parameters.
Article
An electrically excited O2(a 1Δ) generator packages a bundle of plasma containment tubes into a heat exchanger configuration. Chilled fluorinert circulates rapidly through the structure, and gases (O2, Ar, and He mix at 60–100 Torr) flow transonically through the plasma tubes. An externally sustained, sub-breakdown discharge is ionized by means of applying overvolted [ ∼ 150 Td(1 Td = 10−17 V cm2)] ∼ 10–30 ns pulses at 50 000 pulses/s. The plasma is preionized by UV radiation, iminating from a dielectric barrier discharge plasma. A dc electric field of E/N = 10 Td conducts current longitudinally along the tubes. This process accomplishes: (1) O2(a 1Δ) fractional extraction of 30%, (2) electrical excitation efficiency of 40%, (3) specific power loading up to 150 kJ/m O2, and (4) a minimal plasma temperature rise of <125 °K. The O2(a 1Δ) flow stream carries 2400 W.
Article
Herein the authors report on the demonstration of a 50% enhancement in gain and 38% enhancement in continuous-wave laser power on the 1315 nm transition of atomic iodine through the addition of a secondary discharge to predissociate the molecular iodine in an electric oxygen-iodine laser. In the primary discharge the O2(a 1Δ) is produced by a radio-frequency-excited electric discharge sustained in an O2–He–NO gas mixture, and I(2P1/2) is then pumped using energy transferred from O2(a 1Δ). A gain of 0.10% cm−1 was obtained and the total laser output power was 6.2 W.
Article
Oxygen-iodine lasers that utilize electrical or microwave discharges to produce singlet oxygen are currently being developed. The discharge generators differ from conventional chemical singlet oxygen generators in that they produce significant amounts of atomic oxygen. Post-discharge chemistry includes channels that lead to the formation of ozone. Consequently, removal of I(2P1/2) by O atoms and O3 may impact the efficiency of discharge driven iodine lasers. In the present study we have measured the rate constants for quenching of I(2P1/2) by O(3P) atoms and O3 using pulsed laser photolysis techniques. The rate constant for quenching by O3, 1.8x10-12 cm3 s-1, was found to be a factor of five smaller than the literature value. The rate constant for quenching by O(3P) was 1.2x10-11 cm3 s-1. This was six times larger than a previously reported upper bound, but consistent with estimates obtained by modeling the kinetics of discharge-driven laser systems.