International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 2, Issue 12, December 2013
ISSN 2319 - 4847
Perturbation of the cathode Sheath
Characteristics by ALO Dust Particles in
magnetized Plasma
Qusay Adnan Abbas
Dept. of Physics, College of Science, University of Baghdad, Baghdad, Iraq
ABSTRACT
The gas discharge–dust particle interaction for a dc discharge in air with ALO micron-sized particles is investigated. In this
paper, The discharge current and voltage of air plasma in the present of ALO dust particle observed experimentally decrease of
discharge current and increase of voltage discharge. The influence of ALO dust particle on the radial profile of plasma
parameters (namely electron density, electron temperature, ion density, plasma potential and floating potential) are investigated
in air plasma experimentally by using four cylindrical Langmuir probe at pressure 0.6 torr in the presence of magnetic field for
distance 2cm from the cathode surface. This study demonstrates many features in the presence of ALO dust particle which are;
the characteristics of plasma shown uniform distribution along the cathode in the cathode sheath. The plasma potential reduce to
negative values in the presence of ALO dust. This result give evidence to the negatively charge of dust particle in the cathode
sheath. Moreover, when the ALO dust embedded into the discharge all dusty plasma characteristics are reduce in the cathode
sheath region.
Keywords: Dusty Plasma, glow discharge, Langmuir probe, ALO dust, plasma sheath, magnetized plasma.
1 Introduction
Dust particles in a plasma have been studied extensively over the last few decades. Dusty (complex) plasma are a
collection of electrons, positively charged ions, neutral atoms and charged micro-particles called dust [1]-[5]. The term
complex plasma is currently widely used in the scientific literature to designed dusty plasma specially ‘designed” to study
the properties of dust component [6]. There is another definition of dusty plasma which is partially ionized gases
containing small solid particles [7]. The study of interactions between an object and surrounding plasma is a basic
physical problem with many applications ranging from astrophysical topics to operation of gas discharges, technological
plasma applications, and fusion related research . It is especially important in complex (dusty) plasmas–systems
consisting of highly charged micron-size particles in a neutralizing plasma background–since plasma particle interactions
are responsible for a rich variety of phenomena occurring in these systems [8].
Dust in the plasma sheath is a complex subject. This study of dust in the plasma sheath begins with a review of some of
the more pertinent theory. Plasmas have been defined as quasineutral, partially or fully ionized gases that exhibit
collective behavior. The shielding of a plasma from objects is required in order to allow its quasineutral character to exist
for long times over large distances. The transition region between the quasineutral plasma and a boundary is known as
the plasma sheath. In this region, strong fields develop, repelling all but the most energetic electrons and resulting in a
space-charge region consisting almost entirely of ions with directed velocities much higher than their random thermal
velocities [9]-[10].
However, In all dusty plasma experiments, a magnetic field is either absent entirely or else it affects only the ions and
electrons. In either case, the charged dust is treated as an unmagnified component [11]. In this work, a magnetic field is
used to sustain the discharge in a close vicinity of the cathode. Where the magnetic circuit placed behind the cathode
forms above its surface a tunnel of semitoroidal magnetic field. In this closed B field tunnel of B field lines the plasma is
confined and due to a drift of electrons along the tunnel axis in crossed E and B fields the air is very efficiently ionized.
As a results of this magnetic field configuration, the discharge is distributed in a very closed vicinity of the cathode
(where the magnetic field B is strongest).
2 The Dusty Plasma Device
Dusty plasma is produced by suspending micron-sized ALO dust particles in a dc air glow discharge. Figure (1)
shows the diagram of the chamber. The glow discharge is formed between two aluminum electrodes with 8 cm in
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Volume 2, Issue 12, December 2013
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diameter and 2 cm in thickness of both electrodes. The separation displacement is 6.1cm. The electrodes are placed in a
Pyrex tube with 30cm in diameter and 39 cm in length.
A few grams of ALO micron- sized particles (approximately 0.2 gm) are placed in a dust dropper (i.e. duster) which
located in the upper side of the glass tube. The duster is consisted from dc motor which is work by applied 3 dc. voltage
and a circular disc (which used as a container of dust particles). In addition, this device was work by remote control
system. The disc of duster moves two motions which are rotation and vibration motions.
The glow discharge is formed between electrodes when a dc constant voltage of about 2 kV is applied. As a results to
this applied voltage, the electrical breakdown is formed in air at relative pressures, of about ≈ 0.1-1Torr. Figure (2) shows
the photography of the complex plasma at pressure 0.4 torr in the presence of magnetic field.
Teflon
Dust Dropper
Anode
39 cm
6.1 cm
Cathode
Glass
Chamber
Teflon
Figure1 Diagram of chamber.
The magnetic field which used to confine the plasma particles is created by used two coaxial circular permanents
located behind the cathode. This field was measured by using a teslameter model magnetfeldmeβgerat. Figure (3)
illustrated the magnetic field distribution along the cathode surface. It should be remarked from this figure, the magnetic
field distribution has two peaks located at positions -2.3cm and 2.3cm, while it has a minimum value at the center and the
edge regions of the cathode surface.
Figure 2 Photograph of discharge in presence of ALO dust particles at pressure 0.4 torr in
magnetized plasma.
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B (G)
r (cm)
Figure 3 Radial distribution of magnetic field along cathode surface.
3.Langmuir Probe Analysis
In this section ,the methods that used in the calculation of plasma parameters such as electron density, ion density,
electron temperature, plasma potential, and floating potential are review.
3.1 Electron Density
Methods of calculating the electron density described as following: for positively biased of probe, the probe collects all
the electrons and repels all the ions. The electrons current collected is nearly constant. From this current
which is called the electron-saturation current Ies, the electron density can be calculated from the following
relationship[12[-[13]:
n e eA p  2 kT e

I es 
 m
4
e





1l 2
(1 )
where Te is the electron temperature, ne is the electron number density, k is Boltzmann constant , me is the electron mass,
and Ap is the probe tip area.
. 3.2 Electron Temperature
When the probe potential is made less negative, probe collects both ions and electrons. As the potential (probe bias) is
changed further in the positive direction, the ion and electron currents collected just cancel. This current varied
exponentially with probe bias voltage. This current eventually saturates at the plasma space potential value (Vp) due to
space charge limitation in current collection. In the transition region of the I-V curve, the electron current is given
by[12,13]:
Ie 
A p  ne  e  vth
4
 exp
 eV
..
kTe
... (2)
The electron temperature can therefore be calculated directly from the I-V characteristic of the probe. The slope yields
the electron temperature:
Slope
e
....
kTe
..........................(3)
where Te in Ko.
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Slope 
1
....
Te (eV)
..... ..... .......... ( 4 )
3.3 Plasma Potential
The plasma potential (Vp) corresponds to the bias voltage where the plasma and probe are at the same potential. The
plasma potential defines the potential where the electron current changes from the electron repelling current to the
electron saturation current. In the “electron saturation region” electrons experience an attracting potential whereas the
probe delivers a repelling potential to the electrons in the “electron repelling region”. The potential at point of change is
defined as plasma potential and can easily be obtained by looking at the rate of change of the current with respect to the
applied voltage. The maximum of the first derivative
or the zero crossing of the second derivative
of the
probe current with respect to the voltage is the way to find the plasma potential. The floating potential can be calculated
from the following equation, as the bias voltage at which Ii+Ie = 0.
3.4 Ion Density
The ion current passing through an area A in the plasma were determined from the ion currents in the ion saturation
region using the orbital motion limit (OML) probe theory. The advantage of using OML theory is that the ion density can
be determined without the knowledge of the electron temperature. Here it is assumed that the plasma is isotropic, the
electron temperature is much higher than the ion temperature (Te>>Ti) and the probe sheath is thick and non collisional.
Assuming a maxwellian energy distribution in the unperturbed plasma, the following formula for a cylindrical probe is
used to determine the ion current in the OML regime [12,13]:
  eV
I i  A p n i e
 8M
i

12




..... .........( 5)
where Ii, ni and Mi are ion current , ion density, and the ion mass, respectively. By calculate the slope of the linear region
(ion saturation region) of these I2 vs V curves, we can obtain an expression for the ion density.
3.4 Floating Potential
The floating potential (Vf) is defined by Ii=Ie, or Ii(Vf)+Ie(Vf)=0, where Ie is electron current, and Ii is ion current. This means
that, the probe then would draw no net current and would then be at the floating potential. On the other hand, we can calculate
Vf by using the formula[14,15] :
Vf Vp  (

kTe
) Ln  0 .6

e

2  me
M




(6)
It is clear from this equation that the floating potential depends, essentially, only on the electron temperature and the
species of ions involved.
4.Influence of Dust Particle on I,V Characteristic
The change of the current–voltage characteristics of a discharge in the presence of dust particles seems to be an obvious
manifestation of the nonlocal influence of dust particles on gas discharge. This change is the consequence of the increased
loss of plasma electrons and ions in the presence of dust particles and is also the consequence of spatial redistributions of
plasma components in the dc discharge.
Figures (4) and (5) illustrated the influence of ALO dust on the voltage and current of discharge in magnetized plasma,
respectively. It is clear from figure (4) when the ALO dust particle immersed into the glow discharge the discharge
voltage curve show increased while the discharge current decreased. This behavior is consequence of the increased loss of
plasma electrons and ions in the presence of dust particles and is also the consequence of spatial redistributions of plasma
components in the dc. glow discharge. This results mean that the electron losses cause quenching of glow discharge. D
Polyakov et al [16] shown the same results. In addition to that, we noted too, the presence of dust particles in the glow has
no effect of the behavior of both curves of voltage and current when there is no dust presence.
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Vdis (volt)
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P (torr)
Idis (mA)
Figure 4 Schematic discharge voltage as a function of pressure in the presence and
absence of ALO dust particles in magnetized plasma.
P (torr)
Figure 5: Schematic of discharge current as a function of pressure in the presence
and absence of ALO dust particles in magnetized plasma.
5.Langmuir probe measurements of the sheath radial profile:
The effect of ALO dust particles on the radial profile of Plasma parameters, namely electron density, ion density,
electron temperature, and plasma potential in the presence of magnetic field are measured at the cathode sheath of the
plasma chamber with the help of the four single cylindrical Langmuir probe. The theoretical calculation of this
parameters in this region were described in pervious section.
Figure (6) estimated the influence of ALO dust on the radial profile of the electron density at a distance 2 cm from the
cathode surface in magnetized plasma. The data shows the presence of dust reduces the electron density along the
cathode. Since the electron has higher mobility as compared to the ions (due to the much small mass of the electrons), the
ALO dust particles was absorb. Therefore, the electron density is reduced. As well as, the presence of dust causes the
radial profile is uniform.
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ne (cm_3)
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r (cm)
Figure 6: Radial profile of electron density in the presence and absence
of ALO dust particles in magnetized plasma.
The influence of ALO dust particle on the radial profile of electron temperature in cathode sheath in the presence of
magnetized field is shown in figure (7). It is pointed out from the figure the facts that, the energy of electrons reduce in
Te (ev)
the presence of ALO dust. The presence of dust shows the energy of electrons approximately uniform.
r (cm)
Figure 7 Radial profile of electron temperature in the presence and
Absence of ALO dust particles in magnetized plasma.
Figure (8) indicated the influence of ALO dust on the radial profile of plasma potential in the presence of magnetic
field. The profile shows the plasma potential has positive value and non- uniform along cathode. The presence of ALO
dust shows the plasma potential has negative value and the distribution becomes uniform. This behavior give evidence to
the fact that the ALO dust are charged negatively.
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VP (volt)
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r (cm)
Figure 8 Radial profile of plasma potential in the presence and
Absence of ALO dust particles in magnetized plasma.
Figure (9) illustrated the radial profile of ion density in the presence of ALO dust in magnetized plasma. Since the
presence of dust increase the losses of plasma particles in the presence of magnetic field so that the ion density decreased.
In addition to that, when there no dust particles the ion density profile is non-uniform but this profile becomes uniform in
the presence of dust.
Finally, the radial profile of floating potential in the presence of dust particles at the cathode sheath in magnetized
plasma in illustrated in figure (9). The results of this figure shows that, the radial profile of floating potential increases
negatively when the dust particles immersed into glow discharge. This behavior may be attributed to the fact that when
the dust immersed into discharge the dust polarized the plasma particle to stick on their surface. So that the dust was
ni ( cm-3)
charged.
r (cm)
Figure 9 Radial profile of ion density in the presence and absence of
ALO dust particles in magnetized plasma.
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Vf (volt)
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Figure 10 Radial profile of floating potential in the presence and Absence
of ALO dust particles in magnetized plasma.
6.Conclusion:
The variation in the measurable magnetized plasma parameters of a glow discharge in the presence of ALO dust
particles was the subject of this paper. The influence of dust components on the current and voltage discharge at air
pressure range ≈0.4-0.8 torr was described as the first step and then the influence of dust particles on the plasma
characteristics profile along the cathode at pressure 0.6 torr was investigated. The current and voltage of discharge that
present the experimentally observed decrease of discharge current and increase of discharge voltage when the ALO dust
presence.
The influence of ALO dust particle on the experimental radial profile of dusty plasma characteristics in the cathode
sheath demonstrates many features in the presence of magnetic field. In the presence of dust particle , the characteristics
of plasma shown uniform distribution along the cathode in the cathode sheath. The plasma potential reduce to negative
values in the presence of ALO dust. This result give evidence to the negative charge of dust particle in the cathode sheath.
Moreover, when the ALO dust embedded into the discharge all dusty plasma characteristics were reduce.
References
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G. Morfill, S. Khrapak, Yu. Semenov, A. Ivanov, S. Krikalev, A. Kalery, S. Zaletin and Yu. Gigzenko,” Dusty
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Volume 2, Issue 12, December 2013
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Volume 2, Issue 12, December 2013
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[10] B. Pandey and A. Dutta,” The Bohm criterion for a dusty plasma sheath”, Indian Academy of Sciences, 65,1,
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AUTHOR
Qusay Adnan Abbas received the B.S., M.S. and PHD degrees in Physics from Collage of science in
University of Baghdad in 1998, 2001 and 2010, respectively. During 1998-2010, he stayed in plasma
physics Research Laboratory and have many research in in the plasma field suchas magnetron sputtering,
liquid discharge, plasma instability in Q- Machine and Dusty plasma.
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