Wall Probe as a Gas Analytical Detector
V. I. Demidov1, S. F. Adams2, I. Kaganovich3, M. E. Koepke1, A. A. Kudryavtsev4, J. M. Williamson5
1
West Virginia University, Morgantown, WV 26506, USA
2
Air Force Research Laboratory, WPAFB, OH 45433, USA
3
Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA
4
Sankt-Petersburg State University, Sankt-Petersburg 198504, Russia
5
UES, Inc, Beavercreek, OH 45432, USA
Abstract: A novel approach to the development of gas analytical detectors is
reported. It is based on measurements of peaks from plasma-chemical reactions
in the energetic part of the electron energy distribution function (EEDF) with
wall probes. EEDF measurements were conducted in the near-cathode plasma of
a cold cathode, short DC discharge. DC discharge measurements are technically
simpler since temporal resolution is not required and have dramatically better
sensitivity with respect to previous work in the afterglow. The working area of
the wall is also much greater than standard cylindrical probes resulting in a
dramatic increase in probe sensitivity. This research can potentially be exploited
for the development of a micro-analytical gas sensor operational up to
atmospheric pressure.
Keywords: gas analysis, wall probe, plasma
1. Introduction
Atomic and molecular processes can modify and
shape the form of the electron energy
distribution function (EEDF) in a plasma. It
means that measurements of the EEDF allow, in
principle, analyzing those processes and
measuring densities of participating particles.
This in turn can be used for the development of
gas analytical detectors.
However, the
formation of the EEDF depends on many
processes which may be difficult to separate in
the measured EEDF. Therefore, for practical
implementation of the method, for developing
gas analytical sensors, it is necessary to produce
a plasma with a well structured EEDF where
different plasma-chemical processes can be
easily separated.
Generally, plasma-chemical process separation
is simpler to obtain in plasmas with very low
electron temperatures Te (say, less than a few
tenth of an eV). In this case, thermal electrons
do not affect the energy interval between 1 and
25 eV, which is convenient for probe
measurements. One type of low temperature
plasma is an afterglow plasma. The method for
analyzing fine structures in the high energy part
of the EEDF with a cylindrical Langmuir probe
has been described and is known as plasma
electron spectroscopy (PLES) [1, 2]. The PLES
method has been further developed in a number
of papers (see Ref. [3] and references therein).
More details of the PLES method are presented
in the next section.
In the present work, the PLES method was
extended by application with a dc discharge
plasma near a cold cathode (where Te is also
close to room temperature) and a wall probe. In
a dc discharge, the EEDF measurements are
technically simpler and have significantly better
sensitivity than in an afterglow plasma since
temporal resolution is not required. Application
of a wall probe results in additional sensitivity
and allows development of micro-scale sensors
for atmospheric pressures since a wall probe
eliminates the necessity of an inserted
cylindrical Langmuir probe into the plasma.
Details of the wall probe method are discussed
in Sec. 3. Section 4 describes the experimental
approach to the measurements and Sec. 5
demonstrates some results of fine structure
measurements in EEDFs.
Measurements of the high energy portion of the
EEDF can, therefore, give analytical
information about the gas mixture. In this
paper, we report a novel approach to probe
measurements in the vicinity of a cold cathode
2. Plasma electron spectroscopy
In afterglow plasmas, there are some volumetric
processes which generate groups of energetic
electrons. These processes include: a) Penning
ionization,
A∗ + B(∗) → A + B+ + ef
or AB+ + ef,
(1)
b) collisions of the second kind between excited
atoms and slow electrons,
A∗
+ e → A + ef,
(2)
and c) electron detachment in electronegative
gases,
A + B− → AB + ef,
(3)
where A and B are ground states of the same or
different atoms, AB is a molecule, A∗ and B∗ are
atoms in excited (metastable) state (B(∗) is a
ground or excited state), A+ and AB+ atomic and
molecular ions, respectively, B− is a negative
ion, and ef is an energetic electron.
Under the condition of non-locality of the EEDF
[4], energetic electrons, arising in plasmachemical reactions (1) – (3), do not lose their
energies in the plasma volume giving rise to
characteristic maxima in the EEDF.
An
example of the measured EEDF by a cylindrical
Langmuir probe in a helium afrerglow plasma is
shown in Fig. 1.
These measurements
determined the density of metastable atoms in
the plasma [5].
Figure 1. Measured EEDF in the afterglow of a powermodulated rf ICP helium plasma. The red curve shows the cold
(bulk) electrons and the blue curve is multiplied by 500 to show
the high-energy peaks in the EEDF. The arrows mark electrons
produced primarily by reactions (1) and (2).
leading to the development of gas analytical
sensors (dc PLES detectors). For this purpose, a
short (without positive column) dc discharge
with cold cathode and a wall probe have been
used. The basics of the wall probe is explained
in the next section.
3. Wall probe method
Typically, a wall probe is an electrically isolated
segment of the plasma volume wall, serving to
either replace or cover the otherwise continuous
plasma volume wall and collects the current
from the plasma for different probe potentials.
The wall probe does not require a probe holder
and thus reduces the disturbance of the plasma.
The area of the wall probe is significantly larger
than the more typical cylindrical Langmuir
probe resulting in a dramatic increase in the
probe sensitivity. The distortion of the EEDF
measurements by the ion current is significantly
reduced due to the much greater probe radius of
curvature than the near-wall sheath thickness.
Consequently, the ion current only weakly
depends on the probe voltage for negative
potentials.
The wall probe can be a convenient instrument
for the measurement of micro-discharge
plasmas where ordinary cylindrical probes are
difficult to apply due to obvious construction
limitations. The EEDF is measured by applying
negative potentials to the probe and using the
appropriate probe theory [2]. If the probe
dimension (radius) R is smaller than the electron
mean free-path-length λe (see, Fig. 2),
collisionless theory is used. In this case the
EEDF is proportional to the second derivative of
electron probe current Ie with respect to the
probe potential V.
Figure 2. Collisionless theory wall probe case.
If R is greater than λe (see, Fig. 3), the electrons
have diffusive movement to the probe and the
EEDF is proportional to the first derivative, with
respect to the probe potential, of electron probe
current, dIe/dV.
a function of full (kinetic and potential) energy
[2,4]. If the electron energy relaxation length is
larger than the plasma volume size, L, (this is
the nonlocal EEDF in the volume), the EEDF is
the same at any point of the plasma, as a
function of full electron energy and therefore
can be measured by the wall probe for the entire
volume. For elastic collisions of electrons, in
noble gases, the condition is typically met for p
×L < 10 Torr×cm, where p is the gas pressure.
Thus, for atmospheric pressure noble gases, L is
of the order 100 μm.
The probe disturbs the plasma for the distance
of the probe dimension R. If the measurements
are conducted with a small wall probe of
dimension R much smaller than L, the plasma
distortion should be negligible.
In the case of large wall probes, where the probe
dimension is comparable to the plasma volume
size, (R ∼ L), the application is more
questionable. However, using large wall probes
may be important for applications where high
sensitivity is essential, but exact knowledge of
the undisturbed plasma EEDF is not very
important and/or the results of the
measurements can be correctly interpreted.
Under these circumstances, development of gas
analytical sensors based on the dc-PLES
approach is possible.
4. Experimental
Figure 3. Druyvesteyn method wall probe case.
The probe measures the EEDF within the
distance of electron energy relaxation length λε
from the probe. Within λε, the EEDF is nonlocal
and does not depend on the spatial coordinate as
Experiments in a short discharge (near-cathode
plasma) with cold cathode were performed to
demonstrate the practical usage of a large wall
probe. A picture of the experimental device is
shown in Fig. 4. The discharge occurs between a
plane, disk-shaped, 2.5 cm diameter molybdenum
cathode (right) and anode (left). The plasma channel
is bounded by a thin cylindrical stainless steel wall
with a slit (shown between cathode and anode). The
distance between the cathode and anode was 1.2 cm.
In this configuration the wall was used as the large
probe. Experiments were performed with helium,
neon, argon, and in mixtures of oxygen and argon or
helium and argon. The total gas pressure ranged
from 0.2 to 15 Torr, and the discharge current varied
from 0.2 to 10 mA.
under the discharge conditions used for Fig. 5 is less
than 0.1 cm. Thus it is expected that diffusive probe
theory is the appropriate theory. However, as seen
in Fig. 5, for reason that are not clear at the present
time, the second derivative, d2Iw/dV2, best resembles
the near cathode plasma EEDFs measured and
modeled by other authors.
Figure 4. Wall probe used in Druyvesteyn case.
5. Results of experiments
Typical results of measurements in pure helium of
probe current, its first and second derivatives,
dIw/dV, d2Iw/dV2, respectively, with respect to the
probe voltage are shown in Fig. 5. Two peaks at
wall voltages of approximately -15 and -20 eV, are
clearly seen in d2Iw/dV2.
Figure 6. High energy portion of d2Iw/dV2 in a Ne (3 Torr), Ar
(0.5 Torr) and O2 20%)/ Ar(80% (0.5 Torr) dc discharge.
Discharge currents were 10, 2, and 3 mA, respectively. Maxima
at 16 eV and 11.5 eV are due to collisions of Ne and Ar
metastables with slow electrons (reaction 2). Maximum at ∼4
eV is due to electron detachment from oxygen (reaction 3).
Fig. 6 shows the maxima in d2Iw/dV2 for neon,
argon, and oxygen (20%)/argon (80%) mixture.
More results on the method are provided in [6].
This work was supported by the DOE OFES
(Contract No. DE-SC0001939), GK 14.740.11.0893
and AFOSR.
References
[1] V.I. Demidov and N.B. Kolokolov, Sov. Phys. J.
30, 97, 1987.
Figure 5. Probe current and its first, dIw/dV, and second
derivatives, d2Iw/dV2, with respect to wall probe potential in a
helium (4 Torr) dc discharge. Discharge current is 5mA.
Maximum at about -15 V is connected to Penning ionization of
two metastable He atoms (reaction 1). Maximum at about -20 V
is connected to deactivation of He metastables by slow electrons
(reaction 2).
The first peak corresponds to electrons arising from
reaction (1) and the second peak corresponds to
electrons arising from reaction (2) for He metastable
atoms. The mean free path length for electrons
[2] V.I. Demidov, S.V. Ratynskaia and K. Rypdal,
Rev. Sci. Instrum. 73, 3409, 2002.
[3] N.B. Kolokolov and A.B. Blagoev, PhysicsUspekhi 36, 55, 1993.
[4] L.D. Tsendin, Plasma Source Sci. Technol. 4,
200, 1995.
[5] V.I. Demidov and C.A. DeJoseph, Jr., Rev Sci.
Instrum. 77, 116104, 2006.
[6] V.I. Demidov et al. CPP 50, 808, 2010.