22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
OH and H number density distributions in near-surface nanosecond pulse
discharges at a liquid / vapor interface
Z. Yin2, C. Winters1, V. Petrishchev1, W.R. Lempert1 and I.V. Adamovich1
1
2
Department of Mechanical and Aerospace Engineering, The Ohio State University,43210 Columbus, OH, U.S.A.
Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Institut für Verbrennungstechnik, 70569 Stuttgart, Germany
Abstract: Laser Induced Fluorescence (LIF) and Two-Photon Absorption LIF (TALIF)
line imaging are used for in situ measurements of two-dimensional spatial distributions of
absolute OH and H atom number density distributions in near-surface repetitive
nanosecond pulse discharge plasma over distilled water surface in argon buffer.
Keywords: liquid-vapor plasma, nanosecond pulse discharge, laser induced fluorescence
1. Introduction
Kinetics of nonequilibrium plasma chemical reactions
in electric discharges at liquid-vapor interfaces is of
significant fundamental interest [1], and is critically
important for development of applications such as plasma
chemical reforming of liquid hydrocarbons and
oxygenates [2], generation of reactive nitrogen and
oxygen species, removal of organic compounds from
water and aqueous solutions, and plasma activation of
water for sterilization and wound treatment [3]. In spite
of numerous experimental and modeling studies [1-4],
kinetics of liquid-vapor plasma chemical reactions
remains poorly understood, primarily due to the lack of
adequate diagnostic and modeling tools.
One of the most significant challenges encountered in
quantitative studies of liquid / vapor phase plasma
chemistry is sustaining the plasma at controlled,
reproducible, optical access conditions, making possible
in situ optical diagnostics. In the present work, diffuse,
highly reproducible plasmas are sustained near a plane
liquid-vapor interface, using a discharge generated by
high-voltage, nanosecond duration pulses [5]. This
simple, “canonical” geometry provides optical access for
laser diagnostics, and lends itself to quantitative, spatially
and time-resolved studies of plasma dynamics and
non-equilibrium plasma chemistry.
The objectives of the present work are to obtain in situ
measurements of absolute OH and H number density
distributions in nanosecond pulse discharges generated
near the liquid water – water vapor interface in simple
geometry, using single-photon OH Laser Induced
Florescence (LIF) and H atoms Two-Photon Absorption
LIF (TALIF). For this, we are using a repetitively pulsed,
nanosecond pulse duration, double dielectric barrier
discharge sustained near a plane liquid-vapor interface.
2. Experimental
A schematic of a liquid-vapor discharge cell is shown in
Fig. 1. The cell is made of a rectangular cross section
quartz channel (10 mm width x 22 mm height) 10 cm
long, fused at both ends to 25 mm outside diameter quartz
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tube sections, with quartz tubes used for gas flow inlet
and exit and CaF 2 windows glued to the ends of the
channel at Brewster’s angle. Between the experiments,
the inlet and exit tubes were used to partially fill the cell
with a liquid (distilled water), to the depth of 6 mm. The
flow inlet and exit were connected to gas delivery and
vacuum lines. Argon buffer flow (flow rate 0.1 SLM)
was maintained in the channel, at the conditions when the
water vapor pressure in the cell is close to saturated vapor
pressure, ~15 - 20 Torr at room temperature (18 – 22 °C),
and at total pressure in the cell of 30 Torr. The buffer
flow rate has to be sufficiently low to prevent excessive
evaporative cooling of the test cell, as well as water vapor
condensation on the outside walls of the cell. The
windows provided optical access to laser beams used for
LIF and TALIF species number density measurements in
the plasma sustained in the cell. The test cell was placed
on a translation stage, such that its vertical position (i.e.,
the distance between the laser beam and the liquid
surface) could be adjusted, to collect the fluorescence
signal at different heights above the liquid surface.
Fig. 1. Schematic of experimental setup.
1
The electrodes, made of adhesive copper foil 10 mm
wide and 20 mm long, were attached to the bottom of the
cell 30 mm apart from each other, as shown in Fig. 1, to
sustain nanosecond pulse, double dielectric barrier
discharge inside the cell. Kapton dielectric tape was
placed between the electrodes to prevent breakdown
outside the cell. The electrodes were powered by FID
GmbH FPG 60-100MC4 pulse generator (peak voltage up
to 30 kV, pulse duration 5 ns, repetition rate up to
100 kHz), to sustain a near-surface dielectric barrier
discharge inside the test section. The positive polarity
(+8 kV) and the negative polarity (‒8 kV) terminals of the
FID pulser were connected to the electrodes, as shown in
Fig. 1. Note that peak voltage between the electrodes
may be higher than the difference between the incident
pulse peak voltages (up to a factor of two), due to incident
voltage pulse reflection. In the present work, the pulser
was operated in repetitive burst mode, at pulse repetition
rate of 1 kHz, 20 pulses in the burst, and burst repetition
rate of 10 Hz. Custom made, high bandwidth voltage and
current probes [6] were used to measure applied voltage
pulse and current pulse waveforms.
Fig. 2 plots discharge pulse voltage and current
waveforms, as well as the coupled energy waveform.
Pulse peak voltage is about 30 kV and voltage pulse
duration is about 10 ns FWHM. Multiple peaks in
voltage, current, and coupled energy waveforms are
caused by multiple pulse reflections off the electrodes and
pulse generator. The energy coupled by each discharge
pulse at these conditions is approximately 3.7 mJ.
Fig. 2. Voltage, current, and coupled energy traces.
the fundamental to be frequency-mixed again in another
type-I BBO crystal to produce a frequency-tripled output
of 205 nm. A Pellin-Broca prism separates the three
beams, and an identical prism (not shown in Fig. 1) is set
symmetrically, to compensate for alignment changes of
308 nm and 205 nm beams. The two beams are aligned to
overlap as they passed through the test cell, and they are
softly focused with an f = 1 m lens approximately in the
center of the test cell. The two beams were tuned to
excite R2(2.5) transition at 307.792 nm in the OH
A2Σ+ ←X2Π (v′ = 0, v″ = 0) band and the H 3d 2D 3/2,5/2
1s 2S 1/2 transition (two photons at 205.144 nm),
respectively. Band pass filters are placed in front of the
ICCD camera lens to isolate OH and H fluorescence
signal from plasma emission. Changing the position of
the laser beam relative to the liquid surface, by moving
the cell mounted on the translation stage, a series of OH
LIF and H TALIF line images are obtained and collected.
The fluorescence signal in the line images is analysed to
obtain absolute one-dimensional H and OH number
density distributions along the laser beam, as discussed
below. Two-dimensional [H] and [OH] distributions are
reconstructed from several line images, taken at different
heights above the liquid surface, h. For this, species
concentrations as functions of h are inferred based on
spline-fit of concentrations measured at several heights
above the liquid surface (6 values ranging from
h = 300 µm to 14 mm for [H] and 13 values ranging from
h = 300 µm to 12 mm for [OH]). The use of approach is
justified since pulse-to-pulse reproducibility of the data is
very good.
Fluorescence yield efficiency was determined assuming
H 2 O vapour in the cell to be the only quenching species.
In the present work, we assume room temperature and a
total pressure of 30 Torr, with water vapor pressure of
20 Torr (close to saturated vapor pressure). Relative
fluorescence intensity was put on an absolute scale using
signal-comparison-based in situ calibration techniques.
For OH LIF, Rayleigh scattering signal at 308 nm was
collected with N 2 as the buffer gas in the same test cell, a
technique demonstrated in our previous work [8]. For H
TALIF, Kr was used as a reference gas [9]. In all LIF
measurements, the laser pulse energy was kept low, to
avoid signal saturation, photoionization, and photolytic
effect, reported for H TALIF by Goldsmith [10] and
Gasnot [11]. The camera gate was set wide enough to
capture the entire duration of fluorescence decay after the
laser pulse.
The schematic of OH LIF / H TALIF laser diagnostics,
shown in Fig. 1, is similar to the one used in our previous
work [7], where it is discussed in greater detail. Briefly, a
Nd:YAG (Continuum Model Powerlite 8010) pumped
tunable dye laser (Laser Analytical Systems, Model LDL
20505) output at 615 nm is frequency-doubled in a type-I
BBO crystal. The frequency doubled output at 308 nm is
separated from the fundamental, sent through a wave
plate to adjust its polarization, and then recombined with
3. Results and discussion
A typical ICCD image of the plasma is shown in Fig. 2.
In this image, the camera gate is 100 ns. The bottom of
the image corresponds to the approximate location of
distilled water surface, and the top of the image is 13 mm
above the surface. As can be seen on the ICCD image,
the plasma is formed 1 - 2 mm above the liquid surface
between electrodes, and is near the surface above the
electrodes, the locations of which are indicated below the
2
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image in Fig. 2. Fig. 2 also shows OH LIF and H TALIF
line images (13 for OH and 7 for H). The laser beam was
brought to within 300 µm from the liquid surface.
Fig. 4 shows a two-dimensional distribution of absolute
H number density, inferred from H TALIF line images.
Peak [H] value is ≈ 2∙1016 cm-3, such that H atom mole
fraction in the mixture is about 2%, much higher
compared to OH mole fraction. Note that OH and H in
the plasma are likely to be generated at the same rate,
primarily due to H 2 O dissociation by electron impact and
by reactive quenching of metastable electronically excited
Ar* atoms, H 2 O + e- → H + OH + e-, and H 2 O + Ar* →
H + OH + Ar. The significant difference between OH
and H number densities is expected, since the estimated
characteristic time for the dominant OH decay reaction
OH + OH → H 2 O + O, τ OH ~ (k[OH] t=0 )-1 ~ (1.5∙10-12
cm3/s ∙ 2∙1016 cm-3 )-1 ~ 30 µs, is much shorter compared
to that of H atom decay by recombination: H + H + M →
H 2 + M, τ H ~ (k[H] t=0 ∙[M])-1 ~ (9∙10-33 cm6/s ∙ 2∙1016 cm-3
∙ 1018 cm-3)-1 ~ 5 ms. As can be seen from comparison of
Figs. 3 and 4, H atoms diffuse further away from the
liquid surface / plasma region. Note that peak H atom
number density is achieved above the negative electrode,
unlike peak OH number density, where the peak is located
closer to the positive electrode (compare Figs. 3 and 4).
H number density decay near the top wall of the test
section is also detected.
Fig. 2. Plasma emission and LIF / TALIF line images of
H and OH.
Fig. 3 shows a two-dimensional distribution of absolute
OH number density inferred from OH LIF line images.
Comparing this plot and the ICCD image of plasma
emission in Fig. 2, it can be seen that OH distribution
basically follows plasma emission intensity. Peak [OH]
value measured at these conditions is ≈ 3∙1014 cm-3, such
that the estimated OH mole fraction in the gas/vapor
phase is about 3∙10-4. The highest OH number density
region is located near the positive electrode (compare
Fig. 2 and Fig. 3). Near the top wall of the quartz test
cell, [OH] decreases to near detection limit. At the low
flow rate used in the present experiments (estimated flow
residence time of ~ 1 s, much longer compared to burst
duration, 20 ms), both the flow and diffusion do not seem
to affect the location of peak OH number density region.
µ
Fig. 3. Two-dimensional distribution of absolute OH
number density, inferred from OH LIF line images.
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Fig. 4. Two-dimensional distribution of absolute H
number density, inferred from H TALIF line images.
4. Summary
Laser Induced Fluorescence and Two-Photon
Absorption LIF line imaging are used for in situ
measurements of two-dimensional spatial distributions of
absolute OH and H atom number density distributions in
near-surface repetitive nanosecond pulse discharge
plasma over distilled water surface in argon buffer at a
pressure 30 Torr. The measurement results after a burst
of 20 discharge pulses at a pulse repetition rate of 1 kHz
demonstrated
that
peak
H
number
density,
[H] = 2∙1016 cm-3, is much higher compared to peak
OH number density, [OH] = 3∙1014 cm-3. Highest OH
number density was measured in the region where plasma
emission peaks, while H atoms convect downstream with
the flow and diffuse further away from the surface
compared to OH. The results demonstrate experimental
capability of in situ radical species number density
distribution measurements in liquid-vapor interface
plasmas, in simple canonical geometry that lends itself for
3
validation of kinetic models.
5. Acknowledgments
This work has been supported by an NSF grant
“Kinetics of Non-Equilibrium Fast Ionization Wave
Plasmas in Gas Phase and Gas–Liquid Interface” and by
US DOE Plasma Science Center “Predictive Control of
Plasma Kinetics: Multi-Phase and Bounded Systems”.
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