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Ion Beam and Discharge Characteristics of Cold Cathode Ion Source

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Ion Beam and Discharge Characteristics of Cold Cathode Ion Source Indian Journal of Pure & Applied Physics Vol. 58, January 2020, pp. 24-30 Ion beam and discharge characteristics of cold cathode ion source A Attaa,b* , H M Abdel-Hamidb,c, Y H A Fawzyb & M M El-Okrd aPhysics Department, College of Science, Jouf University, Sakaka, P.O. Box 2014, Saudi Arabia bRadiation Physics Department, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority (AEA), Cairo, Egypt cDiagnostic Radiology Department, Applied Medical Sciences Faculty, Jazan University, Jazan, 82911, KSA dPhysics Department, Faculty of Science, Al-Azhar University, Cairo, Egypt Received 10 June 2019; accepted 2 December 2019 In this work, some developments in the acceleration system of cold cathode ion source have been constructed to produce broad beam to be used in different industrial applications. An electrostatic probe with electrical circuit is constructed for study the extracted ion beam distribution. Broad beam 25 mm with ion current in the range of 1 mA is extracted from the constructed extraction system. The obtained optimum distance between the extraction grid and acceleration grid is 3 mm. The characteristics are measured to investigate the ion beam current Ib as a function of different parameters (discharge voltage Vd, gas pressure P, magnetic field intensity B and acceleration voltage Vacc). The magnetic field is collimated and intensifies the plasma that enhances the extracted beam current. The obtained cold cathode ion source can be used in different applications like surface etching, surface modification and deposition due to its long life and compactness. Keywords: Plasma, Cold cathode, Ion beam, Discharge, Acceleration 1 Introduction The cold cathode ion source is composed of two The ion source is the most important part in the mainly parts, the discharge medium and the ion beam system. For this reason, continuous acceleration unit12. The discharge process is produced developments in the design of ion source are needed by the electron collisions with ions along the distance to meets the required applications. Broad beams between the cathode and the anode, where the anode extracted from cold cathode ion source with uniform is surrounding by magnetic field which makes current distributions have an extensive utilization increasing in the electron path with helical motion for in a great variety of research and technology plasma production13,14. The glow discharge constitutes 1,2 applications . an attractive source for plasma-charged particle Cold cathode ion source are used in various systems due to its stability and simplicity. Therefore, applications such as metals deposition, surface 3-7 for long time operation and when reactive gases are cleaning, surface etching and surface modification . used, the hot filaments are usually replaced with cold The cold cathode is characterized by its long life, easy cathode ion sources15,16. The discharge characteristics instrumentation and negligible maintenance due to 8 of the ion source depends on different parameters like filament less operation . The use of extracted ion cathode material, gas pressure, applied voltage and beams for deposition a wide variety of materials has 17 the geometry of the discharge tube . been demonstrated to offer some significant and unique advantages compared to other deposition Magnetic field helps the electrons to confine and methods9,10. Generally, the extracted ion beams from increases the path between the cathode and the anode. cold cathode ion source technique offer a number of The ions produced in the ion source are extracted advantages when compared to the conventional along the axis of the plasma column due to the potential difference between the anode and the plasma techniques such as the control of ion energy, 18 ion current density, and ion distribution width and cathode . For this technique, the addition of magnetic beam direction11. field around the anode discharge chamber increases —————— the lifetime of the ions in the plasma which results in *Corresponding author (E-mail: [email protected]) more successive-single ionization processes, which ATTA et al.: CHARACTERISTICS OF A COLD CATHODE ION SOURCE 25 increases the yields of the plasma density, voltage power supply is used to measure the discharge consequently, a higher extracted ion beam current voltage between them. The stainless steel cathode is could be obtained19. Magnetic confinement of fast connected to earth and the collector (Faraday cup), electrons leads to a creation of plasma density F C, is connected to earth through microammeter gradient normal to the magnetic field.20 which used to measure the output ion beam current The ions beams produced from the cold cathode that exit from the acceleration grid apertures. ion source are extracted along the axis of the plasma A complete vacuum system is used to evacuate the column due to the applied potential difference ion source chamber. The working argon gas is between the cathode and the anode. Different transmitted to the ion source from a gas cylinder geometry of the extraction electrode was studied for through a needle valve to regulate the rate of gas flow. extracts ion beams from the extraction electrode The argon gas is admitted to the ion source through a unit21. The extraction electrode unit of the constructed hose fixed in flange at up side of the ion source. cold cathode ion sources is multi aperture grids for The operating principle of this ion source is based producing broad beams. This development in the on the ionization mechanism produced by primary constructed ion source, applied mainly in surface electrons colliding with gas molecules due to a modifications, sputtering and etching, is capable to potential difference between the anode and the produce low energy ion beams with high-ion current cathode. Therefore, high output ion beam current can densities22. be extracted axially in a direction from the discharge A cold cathode ion source was previously designed region to extraction part. The stainless steel extraction and constructed to produce focused beams23. acceleration electrodes are separated and electrically Therefore, the paper aims to developments of cold insulated from each other by teflon rings. The output cathode ion source with a multi-aperture accelerating ion beam current is increases due to the applied of system working in low pressure to produce collimated acceleration potential. The anode is made of stainless broad beams. The discharge characteristics and ion steel material which is featured by high ionization beam extracted will be characterized and discussed by coefficient and it is cylinder to improve the stability measuring the discharge current and the extracted ion of the discharge. The cathode is made of stainless beam of argon gas under the influence effect of many which has high secondary emission coefficient. parameters such as accelerating voltage Vacc, gas A Faraday cup is placed at a distance 10 mm from pressure P, discharge voltage Vd, magnetic field the stainless acceleration grid. The output ion beam intensity B and extraction- acceleration distance . current is extracted from the acceleration grid. The grid is 313 exit apertures each aperture of 1 mm 2 Experimental Details diameter. Figure 1 shows a schematic diagram of the cold A single cylindrical probe made of tungsten wire cathode ion source and its associated electrical circuit. encapsulated in a glass sleeve was constructed. It is of A milliammeter is used to measure the discharge 0.5 mm in diameter and 1 mm in length. The probe is current between the anode and cathode, while the high made movable in radically and axially position to reach the desired point. The probe is placed in the beam centre at the front of the acceleration grid and at a distance ≈10 mm from the acceleration grid. For measuring the ion beam profile, the electrostatic probe is connected to the ground through a negatively potential ≈ 30 V, and the ion beam is recorded through microammeter under the influence of magnetic field B, and acceleration voltage Vacc. A schematic diagram of the single electrostatic probe and its electrical circuit is shown in Fig. 2. 3 Results and Discussion Figure 3 shows the discharge current Id as a function of the discharge voltage V at different d Fig. 1 – A schematic diagram of broad beam ion source. magnetic fields B with optimum argon gas pressure 26 INDIAN J PURE APPL PHYS, VOL. 58, JANUARY 2020 P=2×10-3 torr. It was found that the increase in probability induced due the more striking electron as magnetic field was accompanied by an increase in the well as the large ionization cross section25. discharge current. The increase is referred to an Figure 5 shows the relation between the discharge increase in magnetic field vector parallel to anode current Id and the gas pressure P with different applied surface, and as a consequence a reduction in the discharge potentials Vd (500, 700 and 900 V) and reverse electron current to the anode occurs. constant magnetic field intensities B 150 G. By The extracted ion beam Ib current increases from adjusting the discharge potential at 500 V, quite 0.65 to 1 mA by increasing the acceleration voltage intensive glow discharge plasma is started between the Vacc from zero to 2000 V at optimum parameters, cathode and the anode. With increasing the discharge argon gas pressure P=2 ×10-3 torr, magnetic field potential to 700 V intensive plasma discharge takes B=150 G and discharge voltage of 900 V as shown in place, and more collisions as well as semi–saturated Fig. 4. This behavior is due to the position and shape plasma is observed by increasing the discharge voltage of the ion beam extracted of the plasma, which are to 900 V. It can be seen that the discharge current dependent on the value of the negative potential relatively increases with increasing the gas pressure. applied to the acceleration electrode.
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