Recent characterization of hybrid radiofrequency/arc discharge for generation
of singlet oxygen
Josef Schmiedberger1, Jan Gregor2, Vít Jirásek1, Miroslav Čenský1, Karel Rohlena1, and Anna Vodičková3
1
Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
Institute of Plasma Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
3
Faculty of Biomedical Engineering, Czech Technical University in Prague, Kladno, Czech Republic
2
Abstract: A novel high-pressure hybrid radiofrequency/direct current (RF/DC) plasmatron
for generation of singlet oxygen is presented. The apparatus is based on a fast mixing of
hybrid Ar+He plasma jet of DC electric arc sustained by an RF discharge with an injected
neutral O2+He+NO gas stream. Recently intended applications of the device include
pumping of discharge oxygen-iodine laser and plasma medicine.
Keywords: singlet oxygen, plasma jet, discharge oxygen-iodine laser, plasma medicine.
1. Introduction
Our research on a hybrid radiofrequency/direct current
(RF/DC) plasmatron had been motivated originally for
pumping of a discharge oxygen-iodine laser (denoted in
the current literature as DOIL, EOIL or ElectricOIL).
There are 4 types of discharge singlet oxygen generator
(DSOG), which were used for DOIL experiments – an RF
discharge [1], a pulse sustained DC discharge [2], a MW
cavity discharge [3], and MW swirl plasma jet [4]. Our
concept of DSOG is based on a proposal [5] of a fast
mixing of Ar hybrid RF/DC plasma jet with a neutral O 2
stream. It means an energy transfer from the Ar hybrid
plasma jet to a neutral oxygen stream, which has a
potential to compensate for disadvantages of purely RF
plasma jets of electronegative oxygen – the lack of
electrons and the low working pressure [5]. A more
detailed survey of DSOG related papers can be found
elsewhere [6, 7].
We have also proposed and started application tests of
our DSOG in a low-pressure biomedicine application [8].
Cancer cells of mouse skin (primary melanoma) are being
exposed to gaseous singlet oxygen flow for cancer
research purposes.
Regardless of the laser pumping of DOIL and the
biomedical application, the hybrid RC/DC plasmatron
itself is an attractive as a new and promising type of
potentially high-power high-pressure flowing nonequilibrium plasma source.
2. Experimental Scheme of RF/DC Plasma Jet Device
The scheme of our DSOG, denoted as DSOG-6, is
shown in Fig. 1. The hybrid RF/DC plasma torch is
produced in an Al nozzle with a cylindrical hole, using a
gas mixture Ar+He. The diameter/length of the nozzle is
5/10 mm. The neutral oxygen stream in the gas mixture
O2+He+NO is injected radially at the nozzle exit plane
perpendicular to the axis of the plasma jet. The injection
slit, surrounding the nozzle exit, has a diameter/width of
5.3/0.5 mm. The stability of the plasma torch is achieved
by (1) a short axial distance of 1.5 mm between the
Fig. 1 Experimental scheme of DSOG-6.
cathode tip and the anode inlet hole, (2) a stabilizing
circuitry in the DC part (serial resistance and inductance
and parallel capacities), (3) an enhanced vortex flow
upstream the anode nozzle, and (4) an optimized gas
mixture of the plasma torch Ar:He = ~10:5 mmol/s. The
vacuum pump velocity is 3000 m3/h. Besides the water
cooling of all the electrodes and of the metal parts in the
discharge chamber, DSOG-6 has an additional aluminium
water cooler of its effluent.
The combination of RF and DC technology requires a
careful filtering of RF noise in the DC circuitry and an
effective shielding of DSOG-6 to prevent an
electromagnetic leakage. The discharge chamber, the RF
filters and their electric connections have a shielding,
which is connected to the ground of the RF power source.
The impedance matching consists of an adjustable serial
inductance and an adjustable parallel capacity. The RF
filters are 2-stage LC filters (LC-LC), suppressing the RF
signal to the level of -55 dB in the frequency range of 6.525 MHz and yet allowing a transmission of long term DC
current of up to 100 A without any additional cooling.
The RF discharge is used for ignition and sustaining the
DC arc discharge by RF pre-ionization. When the RF
power is switched off, the DC arc discharge disappears.
This mode of performance could be described as an RF
sustained DC arc discharge. The visible length of plasma
jet is only ~5 mm due to the presence of electronegative
oxygen.
in the I2 dissociation processes. An excessive portion of
atomic oxygen has to be, however, suppressed by the wall
recombination and by a NO/NO2 titration prior to
reaching the laser region. In the case of the biomedical
application, the extinction point of O titration must be
safely far upstream from the biological samples to
exclude atomic oxygen completely.
3. Basic Features of RF/DC Plasma Jet DSOG
The arc plasma jets have a much higher density of
electrons (by several orders of magnitude) than the RF
plasma jets and they can be easily operated at a higher
pressure, which is desirable for both the efficient
supersonic expansion cooling in DOIL and for biomedical
applications. The arc plasma jet mixing with the neutral
oxygen stream can lead to considerably higher power
densities in much smaller plasma volumes than the
conventional non-self sustained discharges.
The electrode coupled RF discharge is applied to the arc
plasma jet at the plasma torch region. The RF discharge
feeds in some additional energy preferentially to electrons
in order to keep them unattached (oxygen is an
electronegative gas forming negative ions) at a low
reduced electric field E/N~10 -16 V cm2 (the limit of nonself sustained discharge existence region in the oxygen
mixtures), while ions and neutrals are cooled down by a
neutral cold oxygen stream and a non-equilibrium plasma
is created. The arc plasma jet technology is a much
cheaper source of plasmas with high electron densities
than the electron beams or the electron avalanches,
especially at scaling up to high power operation.
The hollow anode coupled RF discharge influences
significantly also the arc discharge itself, especially in the
close vicinity of the anode inner surface. The plasma
anode formed inside the nozzle by the RF part enables the
diffusive mode of DC arc root. We observe a one order of
magnitude increase of the anode voltage drop caused by
the diffusive mode. It switches the arc root from a hotspot mode to a diffusive mode, decreasing thus the current
density of plasmatron anode by several orders of
magnitude. That is advantageous with the use of
aluminium anode, which has a lower melting point and a
significantly lower rate of O2(1Δ) quenching compared to
a standard copper anode routinely used in the gas
plasmatron.
A special attention is aimed at the problem of high
temperature, which accompanies the use of the arc
discharge. A subsequent cooling of the gas mixture is
done at and downstream the mixing point. The fast
mixing of neutral oxygen stream with the plasma jet chills
it dramatically and there are also further additional
counter-measures. A combination of higher molar flow
rates, cooling by the quenching finger together with
chamber walls, and supersonic expansion cooling is used.
One of the consequences of arc discharge regime is an
excess of atomic oxygen, which belongs to quenchers of
O2(1Δ). A limited optimum concentration of O has a
positive influence on the kinetics of DOIL due to its role
4. DC Current-Voltage Characteristics
The experimental determination of basic characteristics
in hybrid plasma jet of DSOG-6 includes detailed
measurements of current-voltage (A-V) characteristics in
the DC part in dependence on the working conditions and
the RF share of fed-in power, especially under the
diffusion mode conditions (see Figs. 2 and 3). The
description of the discharge in the hybrid RF/DC torch is
divided into two parts – the DC part channel (thermal
plasma close to local thermodynamic equilibrium (LTE))
and the space of RF excited plasma (highly nonequilibrium plasma) [9]. The ignition of discharge in the
DC channel is done by RF excitation of plasma. The DC
voltage is gradually increasing in the interval 0.1-0.6 A.
The dynamic resistance in the DC part is positive with
main charge carriers being electrons generated by the RF
discharge. This part of A-V characteristics may be
ascribed to the abnormal glow discharge. The RF plasma
spreads from the RF chamber to the DC channel upstream
the working gas. Since the DC source is technically a
source of constant current, the arc start is accompanied by
changes of voltage course in the DC channel. After
reaching a certain value of current (~0.6 A) an additional
thermionic emission of electrons from the cathode occurs.
It significantly enhances the conductivity of plasma
column in the channel, resulting in a sudden decrease of
voltage in the DC part. A visible constricted arc column
(adjacent to the cathode tip) occurs, which, however,
optically disappears in the space of RF excited plasma,
which behaves itself as a diffusion plasma anode. Further
increase of current brings another increase of voltage in
Fig. 2 A-V characteristics at standard flow rates.
working gas (higher currents are achievable and thus also
a higher power from the DC source is delivered into the
hybrid plasma). The region with the positive dynamic
resistance of the arc part is larger at lower RF powers.
There are more expected effects of thermic discharge in
combination with the RF discharge contrary to pure RF
plasma. The significantly higher density of electrons in
the DC arc column (generated by thermionic ionization)
expands into the RF space as a result of electric field
between the cathode and the RF space. The RF field
operation accelerates also termionically emitted electrons
in the RF part, whereby the originally equilibrium plasma
from the DC part changes in the RF part into the nonequilibrium plasma. The degree of non-equilibrium in the
hybrid plasma obviously depends on the optimum ratio of
DC power and RF power, which will be the subject of
further research of this hybrid torch.
Fig. 3 A-V characteristics at increased flow rates.
the DC part (the dynamic resistance of plasma in the
hybrid arc continues to be positive). Reaching a certain
current density the arc column tends to continue also in
the space of RF excited plasma and it tends also to create
a diffusive arc root (i.e. without any hot spot typical for
the classic DC arc root). This region is characterized by a
voltage instability at power feeding from a DC source
with a current limitation (from that point the dynamic
resistance in the DC region of plasma channel changes to
negative, which is typical for a freely burning electric arc)
and with respect to danger of chamber damage in the RF
part of torch by an insufficient heat removal from the
place of arc root localization a further current increase is
excluded.
The determination of A-V characteristics under
different working regimes was done at various values of
working gas flow rate (the typical mixture of Ar:He=10:5
mmol/s or higher by 20 %) in the DC channel with the RF
power as the main variable parameter in the range 100500 W. Consequently, the DC current used was in the
range 0.1-1.8 A and the DC voltage was in the range 40230 V. The total power (the sum of RF and DC powers)
was in the range 110-760 W. The power ratio of DC and
RF powers was in the range 0.1-3.6. The comparison of
A-V characteristics courses allow us to make the
following conclusions. There is the typical value of
current ~0.6 A nearly independent on the flow rate of
working gas and on the RF power, when the thermionic
emission from the cathode occurs. The dynamic resistance
of conductive column in the DC channel is positive not
only at the ionization by the RF excitation, but also in the
hybrid plasma (after the thermionic emission from the
cathode). The voltage in the DC part (at comparable
values of RF power) is higher at higher flow rates of
working gas (it demonstrates more intensive cooling of
the arc column, leading to a decrease of its conductive
cross section and thus to an increase of the total electric
resistance of the arc). The region with the positive
dynamic resistance of arc is larger at higher flow rates of
5. Singlet Oxygen Generation
Calculations of O2(1Δ) content in a thermal plasma with
an enhanced electron temperature show that the nonequilibrium plasma (Te ~3 eV, Ti,n ~2500 K) is promising
for a high yield of O2(1Δ) [9]. According to our simplified
model, the value of ~42 % seems to be feasible.
The generation of O2(1Δ) is achieved by a laterally
symmetric injection of neutral mixture O2:He:NO =
1.5:2.44:0.22 mmol/s into the hybrid RF/DC plasma jet of
Ar:He = 9.84:5 mmol/s. The RF forward/reflected power
is 500/5 W. The DC voltage/current is 120 V / 1 A. The
temperature at the discharge chamber is 90ºC and at the
outlet of DSOG-6 is 35ºC. The yield of O2(1Δ) is ~5 % at
the nozzle upstream/downstream pressure of 22/1.4 kPa.
The estimate of singlet oxygen yield is done by means of
the change in the absorption peak of atomic iodine at the
laser transition of DOIL at 1315 nm. This experimental
procedure is described in detail in reference [9]. The
specific energy is 41 J/mmol. Recent experiments (now in
progress) reached a power level of ~1 kW and are heading
to a level of ~6 kW, where a specific energy of 400
J/mmol (the optimum for singlet oxygen generation)
should be reached.
6. Biomedical Tests
Under the conditions mentioned in the paragraph 5, the
hybrid plasmatron DSOG-6 was tested in preliminary
biomedical tests – an exposure of cancer cells of mouse
skin (primary melanoma). Contrary to the photodynamic
therapy of cancer cells, which utilizes singlet oxygen
produced in situ, it is possible to influence the cells
significantly also by gaseous singlet oxygen generated
outside the tissues in the plasma of electric discharge and
to transport it to surface accessible places of tissues. It is
perspective especially for skin tumors, some skin diseases
and wound healing (i.e. plasma medicine). One of the
consequences of arc discharge regime is an excess of
atomic oxygen, which belongs to quenchers of singlet
oxygen and it could also damage the biological samples.
Atomic oxygen has to be suppressed by the wall
recombination and by a NO/NO2 titration prior to
reaching the exposure region. In the case of the
biomedical application, the extinction point of O titration
must be safely far upstream from the biological samples
to exclude atomic oxygen completely. A short exposure
of cells to singlet oxygen (few minutes) enhances their
ability to survive a chemotherapy treatment by cisplatin.
A longer exposure (8 minutes and more) kills them
completely. Expected changes of DNA of the surviving
cells as well as the explanation of their longer survival are
still under investigation.
7. Cold Flow Modeling of Plasma Jet
A cold flow upstream and inside the hollow aluminum
electrode was simulated by the CFD-ACE+ software. A
mixture of Ar:He=9.5:5 mmol/s was split into four
tangential inlets. The chamber was cut at the point of
oxygen admixing, because the modeling of the injection
of mixture O2:He:NO into a high-speed hot gas is a
difficult task. An aerodynamic choking by an O 2:He:NO
stream was for this purpose replaced by a geometric
choking instead. The diameter of the outer opening was
set as to match the pressure upstream the plasma jet to the
experiment (14 kPa). A vortex caused by the four
tangential inlets is shown in Fig. 4. In order to assess the
influence of the vortex on the transport of heat (and mass)
from the region near the cathode tip, in another simulation
the cathode end was kept hot at 1200 K (visual estimation
from its color brightness), while the anode was cooled
down to 290 K (cooling water temperature). Exactly the
same case was then still simulated with a non-tangential
inlet (perpendicularly to the jet axis) with no vortex. The
resulting temperature field is shown in Fig. 5 for both the
tangential and the non-tangential case. The difference
between the both cases is fairly small and is localized near
the axis, where the vortex-case is warmer. It was further
found that the temperature field does not have azimuthal
inhomogeneities. A similar conclusion was drawn also for
the mass transfer. It is therefore possible to simulate the
plasma jet in an axisymmetric approximation.
8. Conclusions
The recent characterization of hybrid RF/DC discharge
includes both the experimental and modeling data. The
latest experimental results – the detailed A-V
characteristics bring the deeper understanding of
transition from the hybrid abnormal glow discharge to the
hybrid arc discharge with the diffusive arc root. The
transition occurs at the DC current of ~0.6 A. The hybrid
abnormal glow discharge has the same course of A-V
characteristics as usual DC abnormal glow discharges do.
However, the hybrid arc discharge has different course of
A-V characteristics compared with classic DC arc
discharges. This difference is caused mainly by the higher
voltage drop at the plasma anode (i.e. diffusive arc root).
Acknowledgments
This work has been supported by the Czech Science
Foundation under the grant project P102/12/0723.
Fig. 4 Streamlines colored by velocity magnitude in the
region upstream O2 admixing.
Fig. 5 Temperature near the cathode tip and outlet
for vortex and non-vortex case.
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