Production and Diagnosis of low
Density of Plasma
MINOR RESEARCH PROJECT
SUBMITTED TO
UNIVERSITY GRANTS COMMISION
(WRO)
V.R.KHADSE
Department of physics,
Moolji jaitha College,
Jalgaon
1.
Technical write up of the project
I. Experimental System :
As proposed, the objective of this project was to setup a laboratory plasma system
and to carry out diagnostic measurement using electrical probes. The plasma system
fabricated during the present work consists of following sub systems
1. Discharge tube assembly .
2. Rotary vacuum system.
3. Electrical double probe system for diagnostic purpose.
1. Discharge tube assembly :
The schematic diagram of complete discharge tube assembly is shown in Figure 1 a.
Actual photograph of the plasma system along with power supplies and the measuring
instruments is shown in figure 1b. The plasma system consists of a T type discharge tube 30
cm in length and 4 cm in diameter made up of corning glass to which the discharge
electrodes (anode and cathode ) are connected through the side ports with B20 joints. Three
B14 joints are , placed 6 cm apart and are used for inserting Langmuir probe and a double
probe for diagnostic measurement. The anode and cathode were made-up of commercial
aluminium of the size 25 mm dia. , and 4 mm thick. The electrode edges and surfaces were
smoothly rounded to avoid sparking. The lower part of the discharge tube is connected to a
rotary –diffusion vacuum system.
Double Probe
Langmuir Probe
B14 joint
Cathode
Anode
To Vacuum System
Figure 1a : schematic diagram of complete discharge tube assembly
To
Figure 1 b : Plasma system with power supplies and the measuring instruments
2. Rotary vacuum system.
The vacuum system consists of a two stage rotary pump of 100 litres/min (Hind
Hivac , model no. HHV -----). The discharge tube was directly mounted on the rotary pump
flange provided with a ‘O’ ring and was fixed by nut and bolts. This arrangement provided
an air tight environment in the tube and the desired pressure ( from 1 torr to .oo1 torr ) could
be obtained by adjusting the needle valve fitted to the vacuum system. A thermocouple
Pirani gauge was used to measure the vacuum in system.
3. Electrical double probe system
The double probes were made from tungsten wire 0.5 mm in diameter.
Spacing
between two probes was 2 mm and the length exposed to plasma of each probe is 3 mm (
see figure 2a). Double probes were inserted through B14 cones on the discharge tube. Figure
2b shows the circuit diagram for the double probe measurements. A continuously variable
DC voltage is applied between the two probes and the corresponding probe current was
measured on a DC micro ammeter. The probe voltage was obtained from a 100 V regulated,
stabilized
power supply and was measured on a voltmeter. The Pohl’s commutator
switch changes the polarity of voltage.
Figure 2a : Double probe
V
Pohl’s commutator
µA
double probes
plasma
Figure 2b : The Double Probe Assembly
II. Methodology:
2.1 Experimental procedure for generating plasma :
The discharge tube was evacuated to a pressure of 0.01 torr by using rotary pump and
dry air or desired gas was introduce through the needle valve till a stable vacuum condition
between 0.1 torr to 0.5 torr is obtained. Continuously variable high voltage ( 0 to 1600V )
supply is connected between anode and cathode. The discharge voltage and the discharge
current are monitored. The voltage is gradually increased till the discharge is produced in the
tube. The discharge voltage is suitably adjusted so that fairly uniform plasma extends the tube
and the double probe lies n the positive column of the discharge. Figure 3a shows the nature
of discharge at the operating conditions of pressure = 0.5torr voltage = 800 V and current
I = 50 mA . The violet coloured cathode glow and pink positive column
are clearly
observed . Out of these regions the positive column is treated as plasma. The discharge
voltage was varied from 650 V to 1000 V and the corresponding discharge current is
measured.
A transition towards an arc plasma is observed above 1000V and the current
becomes uncontrollable. Therefore in all the experiments the discharge was operated in the
range 650V to 850V.
Figure 3b shows the discharge characteristics. It is observed that
the discharge current increases linearly with the discharge voltage indicating that the system
is operating in discharge the normal glow region.
2.12 Plasma generation with different cathode geometries
When the cathode shape is changed , the type of plasma generated is also changed.
We have attempted an hallow cathode and a brush cathode geometry as they create a high
density
plasma which finds applications in
sputtering, thin film deposition and
nanotechnology . Figure 4 shows the cathodes fabricated with these geometries. Figure 5
shows the hallow cathode discharge with corresponding discharge characteristics.
45
40
35
30
25
20
15
10
5
0
Discharge Current mA
Figure 3a : Discharge at 0.5 torr, 800 V, 40 mA.
Discharge Voltage volts
600
650
700
750
800
850
Figure 3b :The discharge characteristics.
Figure 4. Hallow Cathode and Brush Cathode Geometries
Figure 5a. The Hallow Cathode Discharge
140
Discharge Current mA
120
100
80
60
40
20
Discharge Voltage volts
0
250
350
450
550
Figure 5b. Hallow Cathode Discharge Charactristics
650
2.2 Experimental procedure for probe measurements :
The double probes , inserted in plasma , are interconnected as shown in Fig.2b. The
differential probe voltage Vp is varied from 0 - 40 V and the associated probe current Ip is measured
. A Pohl’s commutator, (DPDT switch ) used to change the probe polarity and thus the negative
half of the probe characteristics is obtained. Typical double probe characteristics obtained under
different plasma conditions are shown in the following figures. ( Figure 6a ,6b,6c,6d)
10
Vd = 730 V
I d = 20 mA
3
1
8
Probe current µ A
Vd = 650 V
I d = 10 mA
Probe current µ A
5
6
4
2
0
-35
-15
-1
5
25
Probe voltage V
-40
-30
-20
-10
-2
0
10
20
30
40
Probe voltage V
-4
-3
-6
-8
-5
-10
Figure 6a
Figure 6b
Vd = 780 V
I d = 30 mA
12
-35 -25 -15
Vd = 820 V
I d = 40 mA
Probe current µ A
Probe current µ A
12
8
4
0
-5
-4
-8
-12
5
15 25 35
Probe voltage V
-35
-25
-15
8
4
0
-5
-4
-8
-12
5
15
25
Probe voltage V
35
Figure 4 : Experimentally observed double probe characteristics.
2.21 Analysis of double probe charactristics :
Johnson and Malter [1] were first one to described the floating double probe technique for the
measurements of plasma properties. They have extended the Langmuir single probe technique and
developed equivalent resistance method to obtain the plasma electron temperature . Their theory is
applicable if the probe characteristics is symmetric as shown in figure 5. The electron temperature
(Te ) can be determined from the Vd – Id characteristics using the given expression .
Te = 11 ,600 (G-G 2) Ro ΣIp
where Ro = [ dVd/dId ] vd=O
Ro is denoted as equivalent
resistance
.
Figure 5. Symmetric Double Probe Characteristics { Johnson and Malter [1] }
However under most of the experimental conditions the probe characteristics may not
be symmetric. An example of the asymmetric double probe characteristics is presented in
figure 6 . Yamanto and Okuda [2] , Dote [3] have modified the
equivalent resistance
method for an asymmetric probe characteristics of figure 6. Further modifications have been
made by, Phadke et. al [4], Liberman [5] and Kaneda [6] . The ultimate expression for the
electron temperature Te and the electron density is given as [3] :
Figure 6 : Asymmetric Double Probe Charactristics { Dote [ 3 ] }
2.22 Results and Discussions :
1.
The effect of cathode geometries can be understood by comparing the discharge
characteristics of the plane cathode and the hollow cathode ( Fig. 3b and Fig.5b) . We see
that relatively high values of currents voltages (120 mA compared to40 mA) are available
with the hollow cathode discharge than the plane cathode geometry. Also the discharge
operates at lower voltages (550 V compared to 800 V) making the handling easier. In the case
of
the hollow cathode discharge high sputtering rates are observed due to the ionic
bombardment on the cathode material . Therefore these type of discharges find applications
in surface coatings , generation of nano particles etc.
2. Experimentally observed probe characteristics shown in figure 4 were analysed using
Dotes [3] method.
From the curves of figure 4 the parameters (dVd/dId)
0
, ∑ Ipo, and
slope S were determined and the electron temperature Te , the electron density ne of the
discharge plasma were calculated using was determined. Table I summarizes these results.
Obs.
No.
Discharge
Voltage
Volts
Discharge
Current
mA
Electron
Temperature Te
0
K
Electron
Density
Ne cm-3
1
650
10
3.7682t x 104
6.24 x 109
2
730
20
3.7104 x 104
1.07 x 1010
3
780
30
3.7565 x 104
2.41 x 1010
4
820
40
2.9515 x 104
6.10 x 1010
We see that the electron density increases with discharge current with a corresponding
decrease in the electron temperature. In other words, the rise in the number density of
electrons in the discharge is associated with a fall in their mean kinetic energy. This is
attributed to the fact that the mean free path of the electrons becomes shorter and the
increased electron neutral collision frequency results in reduction in the electron temperature
of the discharge plasma.
****
3. References :
1. E. O. Johnson and L. Malter, Phys. Rev. (U.S.A) 80 (1950) 58.
2. K. Yamanto and T. Okuda, J. Phys. Soc. Japan 11 (1956) 57.
3. T. Dote, Japan J. Appl. Phys. (Japan) 2 (1968) 964.
4. Phadke et. al, Indian Journal of Pure and Applied Phys. 26 Nov. (1988) 678.
5. M. A. Liberman and A. J. Lichtenberg, Principles of Plasma Discharges and Material
Processing (Wiley, New York 1994) p 165.
6. T. Kaneda, Japan J. Appl. Phys., Vol. 16 (1977) No.10.
****
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