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
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.
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.
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]
Circular, Parallel Plate
Rectangular, Parallel Plate
Circular, Parallel Plate
Figure 1. Sketches of experimental flow setups.
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.
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
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
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×YO2(a)×NA×1.57 ×10−19 J
where NA is Avogadro’s number,
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.
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
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.
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,
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
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®.
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
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.
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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