Investigation of Hot Cathode and Hollow Anode of Argon Glow Discharge Plasma

Hot cathode and hollow anode argon DC glow discharge plasma at different pressures of (0.04, 0.05, 0.06, 0.07, 0.09, 0.2, 0.4, and 0.8 mbar) has been investigated. The experiments were carried out under the influence of pressure and filament cathode current on voltage – current characteristics of glow discharge and its breakdown voltage. Plasma parameters have been measured and obtained using single probe method at fixed discharge current (Id=1.88 mA) and hollow anode diameter. A computer MATLAB program is performed for this purpose. It was shown that the discharge voltage – current characteristics curve has a positive resistance and represents an abnormal glow region at pressure (0.04 and 0.06) mbar for different filament current. The breakdown voltage increases as the filament current is increased. In different pressure, electron temperature shows different behavior with increasing filament current. Electron density varies nearly inversely with the filament current, but it is increase due to increase of pressure from (1 to 3 mbar), then tends to decrease for the higher pressure. There are two groups of electrons according to the two peaks of (EEDFs), and the peaks amplitude decrease, with the increases of both filament current and gas pressure.


Introduction
The detail of a gas discharge by a hot filament cathode has been studied by many researchers, Ehlers and Leung (1979) had investigated the electron emission from filament cathodes in gas discharge, and shown magnetic field produced by the filament heating current cause inhomogeneous emission of electron concentration of discharge current at a localized position of a filament.Hot cathode discharge plasma with negative ions is widely used in fusion researches and development and in modern plasma processing technologies (Cercek & Gyergyek, 2004).Stable operation of a cylindrical hall thruster has been achieved by Granstedt, Raitses, and Fisch (2008), using a hot wire cathode, which functions as a controllable electron emission source.Miyamoto, Imakita, Kasuya, Shimamoto, and Wada (2009) directly heated high temperature cathodes of refractory metals such as tungsten run electric current of more than several tens of amperes.The electric current makes magnetic field around the cathode wire and the magnetic field cause inhomogeneous emission of electrons from the cathode.Very low electron temperatures have been obtained by Handly and Robertson (2009), in a hot-filament discharge device by having a set of conditions designed to minimize heating of the confined electrons.Zhe, Zhigang, Yikang, and Xiaozhang (2010) generated a sheet plasma is by a mesh anode and a single hot-filament cathode with a DC power supply, and its characteristics are experimentally investigated.Flaxer (2011) produced electros through thermionic emission by heating a wire filament, accelerating the electrons by high voltage, and ionizing the analyzed molecules.Yasserian, Borkhari, and Dorranian (2012) investigate the characteristics of a DC electrical breakdown accompanied with a hot tungsten filament.The device includes two flat electrodes and a moveable filament which is located behind the cathode.The left-hand of the Paschen curve is obtained for different of the currents of the filament as well as various locations in the presence of argon and nitrogen gases.Chen and Jiang (2013) show that the difference of hot cathode operation mode has a great influence on the arc discharge of high current ion source.Borkhari and Yasserian (2013) studied the influence of a hot filament on the electrical breakdown characteristics for different ratios of argon and nitrogen gases for a wide range of pressure.
On the other hand, hollow anode discharge can be more widely applied; it can be used, in ion-electron sources and spectroscopy, when the hollow anode discharge is established very bright plasma is formed in the hollow anode.Matsuura and Wagatsuma (2013) employed Glow discharge optical emission spectrometry (GD-OES) for the direct analysis of solid samples, because sample atoms are directly introduced into the plasma through cathode sputtering.A Grimm-style glow discharge excitation source, whose hollow anode had an inner diameter of 8 mm, was employed as the excitation source.Mujawar (2013) deal with the study of oxygen negative ion formation in the anodic glow plasma.The anodic glow is characterized by a double layer having a steep gradient in T e and electron density.The anodic glow is created via D.C discharge between a hollow tubular-anode in conjunction with parallel plate cathodes.Tutyk, Ovcharuk, Gasik, and Maslenikov (2013) made an experimental researches of features of plasma-beam discharge (PBD) generation mode in the gas discharge electron gun operation with cold cathode and the hollow anode on the basis of the high-voltage glow discharge in a range of helium pressure P = 10 ÷ 130Pa (Abdelsalam, Abdelrahman, Soliman, Basal, & Authority, 2013).Study of optimal ion optic system for extraction of low current ion beam from plasma ion source based on glow discharge.The study based on experimental investigations and computer simulations results.Shevchenko, Tarala, Shevchenko, and Titarenko (2014) investigate an influence of the deposition conditions on the characteristics of amorphous carbon films obtained with the help of the ion source based on reflective discharge with hollow cathode.Jiang et al. (2014) present experimental results of a novel electron gun with a vacuum-arc-plasma cathode used in microwave tube devices.The plasma cathode is based on a vacuum-arc discharge with a novel hollow anode and a Pierce-type electrostatic focusing system.
The present work aims to investigate experimentally the effect of filament current on voltage -current characteristic curve of hot cathode and hollow anode argon discharge as well as its breakdown voltage V b .Furthermore, study this effect on some plasma parameters, such as electron temperature and its density.As well as plasma potential and electron energy distribution function under different working argon gas pressure.

Experimental Setup
A picture of the cylindrical discharge chamber is shown in Figure 1.A cylindrical Pyrex glass tube of 5 cm diameter and 25 cm length was used.Both sides of tube are opened in order to move the electrodes through them to obtain good homogenous discharge and also to change the distance between the electrodes.Two cylindrical plastic rubbers were used to close both sides of the chamber to prevent the leakage of gas as well as to insert the electrodes through them.The distance between two electrodes was fixed to 112 mm.Tube was providing with two pipe connections, one for rotary vacuum pump and the other for single probe and gas inlet which is a capillary copper tube of 1.7 mm outer radius.Both single probe and gas inlet are mounted in the another plastic rubber as shown in Figure 1.Argon gas of high purity 99.99 was used in our experiments and Mechanical rotary pump was used [Trivac E2, Pr. Nr. 140000, Lybold], to evacuate the system to an ultimate pressure (7.5×10 -3 ) torr, this is in turn connected to discharge chamber through a piping tube, thermocouple vacuum gauge is used for measuring pressure and needle valve to control the argon gas flow.The experiments were carried out under pressure ranged from 1.5×10 -2 to 7.5×10 -1 torr.Figure 2 represents a schematic diagram of the discharge system.A hollow cylindrical anode of 4cm inter diameter, 3.5 cm of hollow depth and 0.5 cm thickness is made from copper metal as illustrated in Figure 3.The purpose of using this type of anode to get more stable and denser plasma at the axial of the discharge.The external area of hollow anode has been isolated by an insulator (Teflon) in order to get the discharge just inside the hole of the anode as shown Figure 3.The end of Hollow anode is connected to the external electric circuit through cylindrical plastic rubber to prevent leakage.Cathode filament was manufactured from (tungsten) of 1.5 cm spiral diameterand 5 cm length as shown in Figure 3. Filament was heated using a DC power supply [3B power supply U21060].It is a variable power supply of range of (0-24) V and maximum current 20 A. In the present work the filament was heated by passing the current ranged from (0-1.5).Beyond this range the filament began to glow the system.
Single probe was constructed from tungsten wire of radius (0.1 mm) and the length of 6 mm which is exposed to discharge.This in turn, inserted to the chamber through a glass tube and the latter is entered to the positive column through the cylindrical plastic rubber.The probe was fixed within a capillary tube of glass and shielded by a plastic insulator to prevent their connection.Before each experimental measurement, Probe was cleaned by using a fine sand paper, acetone, deionizer water.Discharge voltage has been supplied to the electrodes system by a DC power supply [3B power supply U21060], of range (0-6000) V and the maximum output current (2 mA).A digital multi meter [Mastech M9803R True RMS MULTIMETER] was used to measure the discharge current ( ).The non-linear protective resistor (lamp) was used to limit the discharge current and avoid the streamer to pass through the chamber to make the spark breakdown (Lochte-Holtegreven, 1968).This type of non-linear resistor was chosen because it provides a relatively high power of 25watt out cooling process (Salim, 2007).The probe voltage was obtained from (220 V-50 HZ), using variac and an isolated transformer (100 V-50 Hz) to bias the probe about 100 Volt as shown in Figure 4. Also a direct resistance (10 KΩ) is used to convert the voltage to real value of current through MATLAB program.The capacitor [C50 SAMER 29539] was used to reduce the phase difference in characteristic of probe.The x-y recorder (oscilloscope) traces the characteristic of the probes, and the voltage on the probes is recorded on the x-axis, while the current through the probes is recorded on the y-axis, as the voltage drop across the resistance of value (10 KΩ).The present work includes two different parts.The first part concerns with the influence of the filament current on the voltage -current characteristics of discharge and its breakdown voltage under different gas pressure without using single probe.For this purpose, we used the circuit shown in Figure 4 without the probe circuit.The second part include the plasma parameters measurements such as Electron temperature, electron density and plasma potential as well as (EEDFs) at various filament current using circuit shown in Figure 4.The experiment were started for two cases, firstly to evacuate the system to the required pressure under the flow of argon gas to the chamber continuously.Plasma was created by applying high voltage from DC power supply sufficient for occurring breakdown of gas and obtaining an abnormal glow discharge ( positive resistance) with and without the filament current ranged from (0-1.5)A for the first part to obtain the discharge characteristics and the Paschen's curve under different filament current and gas pressure.This procedure was repeated for the second part using single probe and an oscilloscope to obtain single probe characteristics.For more details see Hajy (2014).

Experimental Results and Discussion
Voltage-current characteristics of the discharge were obtained using voltmeter and ammeter.Figure 5 represents an abnormal glow discharge characteristics at pressure of (0.04 and 0.06) mbar for different filament currents.It is clear that an increase of the discharge voltage was accompanied by an increase of the discharge current (positive resistance).Also figure shows that increasing filament current causes shifting the characteristics towards higher discharge voltage.This latter behavior means that increasing emitted electrons from the filament enhance the negative space charge near the anode and consequently to that increasing discharge voltage as well as its resistance (Jomachi & Klobes, 2005).
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Figure 1 .
Figure 1.Picture of cylindrical discharge chamber

Figure 3 .
Figure 3. Cylindrical hollow anode and Cathode filament with rubber holders