22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
High current density discharges in water - a discussion of mechanisms
D.C. Schram
Eindhoven University Technology, Department of Physics, Eindhoven, The Netherlands
Abstract: A new picture of discharges is developed on basis of experimental information.
With rising current a very dense plasma column is created which in the final stage is
Coulomb collision dominated. The plasma is in the thermal regime, very dense and weakly
non ideal. Molecule formation takes place mostly at the surface and in the water below.
Keywords: plasmas in water; thermal plasmas; molecule formation
1. Introduction
In this contribution the physics of discharges in water
with high current density will be discussed and
consequences of the physical picture in terms of
dissociation and molecule formation mechanisms. This
discussion is based on earlier measurements by Marinov
et al. on a discharge in low conductivity water [1].
They show that first a single discharge is formed at the
beginning of the high V pulse. From their measurements
it is shown that pressure increases to up to GPa level and
that electron density reaches values of 1026 m-3. In these
high energy density thermal plasmas high ionization
degrees and very high electron densities are reached after
a short ionization period.
2. Time development
At the beginning of the current rise, the conductivity is
electron neutral collisions controlled and starts thus at a
low value. The power density becomes very fast high as
the voltage is high because of the high impedance
associated with the high resistivity. In this initial period
the electron temperature is high to ensure fast ionization.
Later, with increasing n e and thus n e /n 0 , the resistivity
and thus impedance decrease; still the product of current
density and field remains high (can reach
1017 - 1018 W/m3). The electron density rises fast and
n e /n 0 reaches the % 0 - % level at which Coulomb
collisions start to dominate. Then the thermal plasma
domain is reached with a constant conductivity only
dependent on the electron temperature which has dropped
to around 1 eV. During this process the channel
lengthens and increases of diameter (µm’s). As a
consequence of the good Coulomb conductivity the
potential drops even before the current approaches its
maximum value.
As then the potential is only for a small part over the
channel, most of the potential is over the water circuit and
the discharge branches in many branches, observed if the
conductivity of water is low. The discharge current is
then divided over many branches, and in each of them
lower densities will result and probably the Coulomb
thermal plasma regime will not be reached. In higher
conductivity water the situation may be different: then the
current will rise to higher values and the channel will
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lengthen and widen: the full voltage then remains over the
channel, driving larger currents. It is found that then a
bubble appears, rather than branching [2].
3. Plasma structure drawing
The plasma structure is a central channel with high
ionization and an ionized gas envelope with the recycling
diffusion flux and interaction with the water surface
(Fig 1). At first the neutral density will be in the order of
the water density > 1028 m-3; the electron density is still
low and the electron temperature high. The ionization
degree rises fast and soon the heavy particle temperature
will increase in the center, because of good coupling
between electrons and heavy particles. As radially there
is no pressure gradient, in the center the density is lower
(1027 m-3) because of the high temperature, with
T 0 ∼ T e ∼ 1 eV. In the envelope the neutral density is
thus higher, in the 1028 m-3 range, at the lower
temperature there. The center and the envelope are both
atomic: molecules generated at the water surface will very
fast dissociate by thermal collisions and by molecular
assisted processes as charge transfer and dissociative
recombination. The result is a very efficient dissociation
of molecules, also efficient with respect to energy use.
The products resulting from the last process may be
exited as OH* resulting from H 2 O+ + e  OH* + H.
which thus can serve as indicator for molecules which are
dissociated by charge transfer and dissociative
recombination.
4. Efficiency for dissociation and radical production
The energy balance of the fully developed plasma
channel has as main terms, Joule dissipation as energy
input and ionization as main energy loss. Electron heat
conduction is less important provided T e remains smaller
than 2 eV: it limits in essence the electron temperature.
Radiation is relatively less important as the high electron
density ensures that all excitations lead to ionization.
Only at very high densities continuum radiation may be a
significant energy loss. We note that continuum emission
can be used for measurement of electron density.
We note in passing that contrary to general belief a very
large benefit of thermal plasmas is that only a fraction of
the gas (in the plasma center) needs to be heated to have
1
Fig. 2. Electron temperature of argon plasma as function
of n e /p1/2 for various non-equilibrium constants and
pressure between 104 and 106 Pa.
Fig. 1. Sketch of plasma and gas envelope in water,
showing the ionized central plasma, the envelope and
water, with parameters.
all the benefits of high electron density. It is just not true
that all the gas has to be heated to 1 eV: it is only a small
part and the inelastic energy input is the most important
energy loss, rather than heating. Thermal plasmas are
capable to dissociate molecules with very high energy
efficiency.
5. Thermal plasma and approximate modeling
The dense plasmas at high ionization ratio have some
very simple characteristics which makes also simple
estimates possible. The conductivity and electron heat
conduction are in the Coulomb regime mainly T e
determined, and can be estimated by use of approximate
modeling. This approach makes use of the fact that
plasma properties do not depend strongly on nonequilibrium: over population of neutral ground state with
respect to Saha and differences between T e and T 0 . The
electron temperature (∼1 eV) can be eliminated and all
quantities as electron heat conductivity can be expressed
as function of n e /p1/2 (Fig. 2). Hence plasma properties
can be estimated if electron density and pressure are
known. It is thus essential to get experimental evidence
for electron density and for the pressure. Details can be
found in Burm et al. [3].
It should be remarked that at these high electron
densities the plasma enters the (weak) non Debye regime:
then the number of particles in the Debye sphere is
smaller than 1, screening changes with as consequence
changes in conductivity and in line radiation
characteristics.
6. Processes at the surface
In a very short time (sub-ns) a large number of radicals
are produced and the water surface is soon covered with
radicals. Molecules are formed which in the gas phase
will be dissociated as described above. Also radicals as
OH start to enter the water. H radicals may diffuse faster
into the water, which makes the situation favourable for
OH radicals absorbed at the surface. Hence the formation
2
of H 2 O 2 can be expected and final population may be
more determined by loss processes. H 2 and O 2 are
expected also and measuring these gases may also help in
estimating the formation processes (Fig. 3).
Fig. 3. Sketch (left half view) with atomic H and O
plasma, atomic gas envelope and water with fluxes.
7. Conclusions
The plasmas appearing in discharges in water are after a
short time dense thermal plasmas with high pressure and
electron densities. In the plasma column molecules will
be readily dissociated and thus an atomic plasma with
ionization ratio above 1% is produced, of which the
properties can be estimated by approximate modeling.
These dense plasmas with its atomic envelope give rise to
large fluxes of atomic radicals. This gives rise to
molecule formation in water and in envelope, where
recombination processes may give rise to line emission as
OH*. Also radicals (and in particular H radicals) may
diffuse fast in the surrounding water and form H 2
molecules further away. This situation: fast radical
production in a thermal plasma and quenching of the
plasma may be favorable for H 2 O 2 formation. Maybe
thermal plasmas are not so bad after all.
8. Acknowledgement.
The author acknowledges gratefully discussions with
Antoine Rousseau and with several other colleagues of
the plasmas in liquids community.
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9. References
[1] I. Marinov, S. Stariskoskaia and A. Rousseau.
J. Phys. D: Appl. Phys., 47, 224017 (2014)
[2] P. Bruggeman, et al. Plasma Sources Sci. Technol.,
18, 025017 (2009)
[3] Burm, et al. Plasma Chem. Plasma Process., 12,
413-435 (argon I) (2002)
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