Effect of negative oxygen ions on the characterist

Authors: Davoud Dorranian Mahsa Alizadeh

Publish Date: 2014/04/03

Volume: 8, Issue: 2, Pages: 122-

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Abstract

Langmuir probe measurements are performed in cylindrical dc glow discharge plasma Plasma was generated in an evacuated glass tube with circular plane disk electrodes Measurements were carried out at different points along the axis of tube for different working pressures of pure argon and mixture of argon–oxygen gasses to obtain the plasma density and temperature as well as plasma and floating potentials Variation of discharge potential as a function of discharge pressure for both plasmas is observed It is shown that electron temperature plasma potential and floating potential in constant current mode and constant pressure are increased from cathode to anode on the axial points of the discharge tube while electron density is decreased To sustain the discharge process after adding oxygen to plasma higher voltage is required since electrons are more energetic at lower densityClassical low pressure electrical discharge has been the topic of several researches in the field of plasma physics for many years Recently development of plasma technology in industry has motivated plasma physicist to focus their researches on different phenomena occurred in the electrical discharge tube at a wide variety of discharge conditions Thin film deposition surface etching of materials and plasma sterilization are some important samples of application of plasma in industryThe glow discharge is the best known type of nonthermal discharge A nonthermal glow discharge occurs at low pressures An electric discharge can be viewed when two electrodes are inserted to a glass tube and connected to a power supply The tube can be filled with various gasses or evacuated As the applied voltage across the two electrodes increases the current suddenly increases sharply at a certain voltage required for sufficiently intensive electron avalanches If the pressure is low on the order of a few torrs and the external circuit has a large resistance to prohibit a large current a glow discharge develops This is a lowcurrent highvoltage discharge widely used to generate nonthermal plasma 1In the case of cold dc plasma with Maxwellian distribution Langmuir probe LP is a very useful and trustable convenient device for measuring the local plasma parameter Langmuir probe is simply a wire immersed in the plasma to determine the electron temperature electron density and plasma potentialIn this work the effect of pressure of the discharge tube on the characteristics of argon and argon–oxygen plasmas is studied experimentally Measurements were carried out in pure argon and Ar/Ar + O2 = 80  plasmas when the plasma current was kept constant Oxygen is an electronegative gas The electronegative gas in the discharge has a strong effect on the electron dynamics behavior because of the attachment of electrons to the electronegative radicals thus becoming negative ions The presence of negative ions complicates the transport properties of the discharge There has been considerable scientific and technological interest in electronegative plasmas 2 3 4 and there have been various methods in the determination of negative ion density 2 3 4 5 6 Electrostatic probes 2 3 4 5 6 7 8 laser photodetachment LPD 2 3 4 5 6 7 8 9 and laser Thomson scattering LTS 2 3 4 5 6 7 8 9 10 are frequently used diagnostic tools for detecting negative ions in plasma Among them electrostatic probe technique is simple and inexpensive and provides the spatial resolution of plasma parametersTo determine the plasma parameters a movable single LP is designed and constructed The Langmuir probe is made of a narrow quartz pipe with cylindrical tungsten tip The length of the probe tip is vertically 10 mm and its radius is 03 mm The probe can be moved longitudinally along the tube axis z axis The probe was connected through an electrical circuit to a dc power supply ranging from −50 to 50 V The probe current as a function of the probe voltage was measured by a convenient digital ampere meter between the probe tip and the ground as shown in Fig 1

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