Application of electrostatic Langmuir probes for plasmas with
nanostructures produced by an atmospheric pressure anodic arc
A. Shashurin, J. Li, T. Zhuang, M. Keidar
Department of Mechanical and Aerospace Engineering, George Washington University, Washington, DC 20052
I.I. Beilis
School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978, Israel
Abstract: The temporal evolution of a high pressure He arc producing nanotubes
is considered and Langmuir probe technique is applied for plasma parameter
measurements in such discharge. Two modes of arc are observed, namely
cathodic arc that is supported by erosion of cathode material, and anodic arc
supported by ablation of a composite anode (packed with carbon and metallic
catalysts) in which carbon nanotubes are synthesized. Voltage-current
characteristics of single probes are measured and an unusually low ratio of
saturation current on the positively biased probe to that on the negatively biased
(about 1-4) is detected. This effect is explained by increase of saturation current
at the negatively biased probe due to secondary electrons emission from the
probe surface by the long life excited He atom collisions with the probe surface.
Keywords: carbon nanotubes, anode erosion, Langmuir probe, high-pressure arc
1. Introduction
High pressure anodic arcs demonstrated its
efficiency for synthesis of different types of
nanostructures, such as single and multi-wall carbon
nanotubes (CNT), fullerenes and graphene [1,2,3,4].
Extensive interest to arc synthesized nanostructures
facilitates active recent studies of high pressure
anodic arcs [2,5,6,7,8]. Since the synthesis occurs in
arc plasma environment, this promotes application
of standard plasma diagnostics, namely electrostatic
Langmuir probes, to determine the plasma
parameters. The electrostatic Langmuir probes have
not been implied yet for atmospheric arc in the
peripheral areas where nanostructures are produced.
However the plasma state change reflects the arc
evolution during the synthesis. In the present work
we study temporal evolution of high pressure arc
producing nanostructures, and apply the electrostatic
Langmuir probe techniques for plasma parameter
measurements of such arcs.
2. Experimental details
Experiments were conducted in a cylindrical vacuum
chamber (270 mm length and 145 mm diameter).The
He pressure in chamber was controlled in the range
from 0.1 Torrs to 500 Torrs using closed-loop
control system described in details in Ref.[8]. Arc
currents (Iarc) were 30-70 A and corresponding arc
voltages (Uarc) were about 30 V. Arc videos were
recorded by a digital camera (15 frames/s).
Arc electrodes were produced from the carbon, with
the anode being a hollow tube (packed with metal
catalysts – Ni, Y and carbon powder so, that total
anode composition was C:Ni:Y=56:4:1 at.% [8]) and
the cathode being a solid graphite rod. The cathode
and anode had diameters of about 40 mm and 5 mm,
respectively. Size of interelectrode gap (h) was
varied in the range of 3-7 mm.
Three modifications of single electrostatic probes
were used (see Figure 1). First, we utilized two types
of unshielded probes for measurements relatively far
from the arc axis (r ≥1.5 cm). One modification
utilized circular collector oriented perpendicular to
radial plasma flow expanding for the gap as shown
in Figure 1 (a) [circular probe], while another used
large-area prolonged cylinder co-axially surrounding
the electrode axis, and equipped with opening for
recording of arc video [see Figure 1 (b)].
obtained at residual pressure of about 0.1 Torr is
shown in Figure 2. The V-I characteristics
demonstrated the ratio of saturation currents to
positively (Ip) and negatively (In) biased probe of
about |Ip/In|~102, which is in accordance with
conventional collisionless probe theory predicting
this ratio to be about
Second, we utilized single electrostatic probe in
vicinity of the interelectrode gap (distance from the
arc axis r =8 mm, distance above the cathode front
surface z=1-2 mm) equipped with electricallycontrolled shutter able to operate on the millisecond
time scale. Fast shutter was used in order to prevent
probe’s fast deposition by various carbon species
synthesized in the arc, which leads to uncontrollable
growth of collecting area and produce inadequate
readings. The probe consists of collector (a copper
foil strip of 3 mm width was used) installed inside
the ceramic tube (alumina tube with outside
diameter of 6.3 mm was used) with opening of about
5x3 mm2 as shown in Figure 1 (c) [shielded probe].
The cylindrical molybdenum shutter was closely fit
to the outside surface of the ceramic tube and was
able to slide along it, so that collector was exposed
to the plasma solely during the period of time when
shutter was open. The collector surface containing
nanoproducts deposited on it during the exposition
to plasma flux was analyzed using SEM. A 1 kHz
voltage sweeping voltage was applied to the probes
to determine the voltage-current (V-I) characteristics.
3. Results and discussions
The paper considers relatively wide range of gas
pressures (0.1-500 Torr), while the main focus is
made on high pressure range of several hundred Torr
corresponding to conditions of CNT synthesis.
Low-pressure
case.
Voltage-current
(V-I)
characteristic of circular probe [see Figure 1(a)]
and electron mass respectively (about 85 for
Helium) [9]. Visual observation of arcing [see insert
diagram in Figure 2] and post-discharge evaluation
of arc electrodes indicated, that arc was supported by
erosion of the cathode material from the cathode
spots, while anode ablation was not observed. The
electron temperature determined from the slope of
V-I characteristic in semi-log scale yields Te of about
1-2 eV, which lies in the typical range of Te for
cathode jets plasmas [10].
(a)
0.3
I, A
Figure 1. Schematic view of electrostatic probes.
M
, where M and m are ion
m
0.25
Cathode
0.2
0.15
Cathode
spot
0.1
Anode
0.05
U, V
0
-100
-50
-0.05 0
50
100
Figure 2. V-I characteristics of single probe at 0.1 Torr
(h=7 mm, Iarc=60 A, t=100 ms after arc ignition, circular probe:
r=25 mm, z=25 mm, probe diameter=2.6 mm). The ratio |I-/I+|
is close to that predicted by conventional collisionless probe
theory.
Probe V-I characteristics at different background
gas pressures: It was observed that In and Ip
remained approximately constant with background
gas pressure (p) up to some critical pressure and
then, decreased with p. Currents were measured at
50 ms after arc ignition using surrounding probe
shown in Figure 1(b). The critical p for In was about
several Torrs. This dependence of the probe currents
on p is caused by damping of plasma expansion by
background gas atoms that become significant when
distance from arc to the probe becomes comparable
with plasma ion mean free path [11,12]. In addition,
it was observed that behavior of probe V-I
characteristics changed dramatically with increase of
1000
optimal conditions for synthesis of carbon nanotubes
[8].
Cathode
Cathode
Anode
Anode
t=60 ms
t=3 s
100
50
CS mode
Iarc , A
|Ip / In|
100
AA mode
80
40
60
30
40
20
20
10
10
Arc current
Arc voltage
0
0
-1
1
0.1
1
10
100
p, Torr
1000
Figure 3.Ratio of saturation currents as function of background
He pressure (h=7 mm, Iarc=55 A, t=50 ms after arc ignition,
2.6 mm diameter circular probe for p=0.1 Torr and surrounding
probe at r=25 mm for p>0.1 Torr).
It is important to note that the ratio |Ip/In | presented
in Figure 3 corresponds to arc mode when the
discharge was supported by erosion of the cathode
material from the cathode spots (CS mode) [10]. To
this end the V-I characteristics of the probe were
captured immediately after arc ignition (at t= 50 ms),
since on later times the arc was switched to another
operation mode, namely to anodic arc (AA) mode
(for t>few seconds after ignition, p>10-100 Torr)
where the discharge was supported by ablation of the
anode material. This transition from CS mode to AA
mode is illustrated in Figure 4, where arc images in
both modes and temporal evolutions of Iarc and Uarc
are presented. It was observed that the discharge at
CS mode is characterized by relatively high
fluctuations of arc voltage due to unstable behavior
of cathode spots. In the AA mode the dense anode
plume is formed around the hot anode, whose
ablation supported the arcing and led to the more
electrically stable discharge.
Let’s consider both modes of arcing in more details
for the case of 500 Torr He arc as it corresponds to
Uarc , V
background He pressure in comparison with that
obtained at low pressures, namely the ratio |Ip/In|
decreased from about 100 at 0.1 Torr to about ~ 1-4
(spread observed in the different arc runs) for
pressures of about several hundred Torr [see Figure
3].
0
1
2
3
t, s
4
5
6
7
8
Figure 4. Temporal evolutions of Uarc and Iarc, and typical
images of 500 Torr He arc in initial CS and later AA modes
(h=7 mm, Iarc=55 A).
CS mode: This arc mode was supported by the
plasma jets generated at the cathode spots and was
observed during few first seconds after arc initiation,
Typical photograph of the arc in CS mode is shown
in Figure 4 (left image). At the CS mode the anode
was not sufficiently hot to provide the anode
ablation at the rate enough for supporting the
discharge current and thus the anode is a current
collector. The duration of CS stage was readily
controlled by size of interelectrode gap and value of
arc current, namely increase of h and decrease of Iarc
led to prolongation of CS stage. For example, h
increase from 3 to 7 mm resulted in increase of CS
stage duration from ~1 s to ~2-3 s (for Iarc=55 A,
p=500 Torr) and Iarc decrease below 35-40 A
resulted in the fact, that the arc had never switched
to AA mode (h=5 mm, p=500 Torr). Typical probe
V-I characteristic is presented in Figure 5(a). It is
seen that the ratio |Ip/In | was about 4, which is more
than order of magnitude less than ratio predicted by
collisionless probe theory [9].
AA mode: Typical photograph of AA mode is
presented in Figure 4 (right image). It was observed
that the discharge switched to AA mode after first
initial seconds of arcing. The anode in AA mode was
significantly hot and surrounded by the highly
luminous anode plume. The in-situ and post-
experiment evaluation of the arc electrodes indicated
that arc was supported by the anode ablation, while
no cathode erosion occurred. Probe V-I characteristic
is presented in Figure 5(b) for Iarc=55 A, h=3 mm,
t=1 s. It is seen that the ratio |Ip/In| was about 1.5.
I , mA
50
40
Acknowledgements
30
20
10
U, V
0
-120
-80
-40
-10 0
40
80
120
-20
-30
I , mA
-40
2
(b)
pressure dielectric barrier discharge). We
hypothesize that this may be caused by significant
increase of In above the ion saturation current caused
by secondary electron emission from the probe due
to Auger de-excitation of long living He* atoms on
the probe surface [13].
This research was supported by NSF/DOE
Partnership in Plasma Science and Technology (NSF
grant CBET-0853777, DOE grant DE-SC0001169).
We would like to acknowledge PPPL Offsite
Research Program supported by Office of Fusion
Energy Sciences for supporting arc experiments. The
authors would like to thank Dr. Y. Raitses for
valuable discussions.
References
1
U, V
0
-100 -80 -60 -40 -20
0
20
40
60
80 100
-1
-2
Figure 5. V-I characteristics of the arc in (a) CS and (b) AA
modes.
The collector of the shielded probe exposed to
plasma for several tens of ms was analyzed under
the SEM for both CS and AA arc modes (at same
discharge conditions and probe location). It was
observed that large amount of nanostructures
containing entangled bundles of CNTs, catalyst and
soot particles was deposited to the probe surface
regardless polarity of its bias in AA mode, while no
significant deposition of nanostructures on the
collector surface was observed in CS mode. This
indicates that currents supplied to the probe are
mostly governed by the charged plasma particles
while contribution of current delivered with charged
CNTs [7] is negligible.
Thus, V-I curves of single probes measured in both
AA and CS mode in high pressure arcs demonstrated
very low ratio |Ip/In| of about 1-4 (ratio of about 1015 was reported earlier in Ref.[13] for atmospheric
[1] S. Iijima Nature 354, 56 (1991).
[2] S. Farhat and C.D. Scott J. Nanoscience and
Nanotechnology 6, 1189 (2006)
[3]A. P. Moravsky, E. M. Wexler, and R. O. Loufty,
in Carbon Nanotubes: Science and Applications,
edited by M. Meyyappan (CRC, Boca Raton,
FL, 2004).
[4] O. Volotskova, I. Levchenko, A. Shashurin, Y.
Raitses, K. Ostrikov and M. Keidar Nanoscale 2,
2281 (2010).
[5] A. Huczko, H. Lange, Bystrzejewski, Ando Y,
Zhao X, Inoue S, J. Nanoscience and
Nanotechnology 6, 1319 (2006).
[6] Keidar M, Waas A M, Raitses Y and Waldorff E
J. Nanosci. Nanotechnol. 6, 1-6 (2006)
[7] M. Keidar, J. Phys. D: Appl. Phys. 40, 2388
(2007).
[8] A. Shashurin, M. Keidar M, I.I. Beilis J. Appl.
Phys. 104 063311 (2008).
[9] Y. P. Raizer, Gas Discharge Physics (Springer,
Berlin, 1991).
[10] I.I. Beilis in Handbook of Vacuum Arc Science
and Technology edited by (Boxman R L, Martin
P, Sanders D (Ridge Park: Noyes Publishing,
1995).
[11] C.W. Kimblin J. Appl. Phys. 45, 5235 (1974)
[12] I.I Beilis, A. Shashurin, R.L. Boxman IEEE
Trans. Plasma Sci. 35, 973 (2007).
[13] O. Sakai, Y. Kishimoto and K. Tachibana J.
Phys. D: Appl. Phys. 38, 431 (2005).