Operating Characteristics of the Hollow Anode Glow Discharge Ion Source

F.W. Abdelsalam, A.G. Helal , M.M. Abdelrahman and B.A. Soliman

Accelerators & Ion Sources Department, Nuclear Research Center, Atomic Energy Authority P.N.13759, Inchas, Cairo, Egypt.

ABSTRACT

In this work, a new shape of a glow discharge ion source with axial extraction has been designed and constructed. High output ion beam current can be extracted axially in a direction normal to the discharge region without using extraction system. Optimization of the distance between the anode and the cathode has been determined using argon gas. It is found that the optimum gap distance between the anode and the cathode is equal to 3.5 mm, where stable discharge current and maximum output ion beam current can be obtained. The discharge characteristics of the ion source at different operating gas pressures have been measured at the optimum distance between the anode and the cathode. A disk of Teflon insulator has been put between the anode and the cathode. This disk was covering the cathode area and reducing the discharge area on the cathode surface for discharge confinement, therefore, a higher output ion beam current could be obtained. A comparison between experimental and theoretical results is made and a reasonable agreement is obtained.

Key words: Glow discharge ion source, plasma formation, gas pressure and argon gas.

INTRODUCTION

The potential use of ion sources in many fundamental sciences are important, such as atomic physics, plasma physics, plasma chemistry, nuclear physics etc…[1-6]. A conventional ion source is

comprised of a plasma generator and means for ion extraction and acceleration. Usually, the type of

discharge in the plasma generator appears in the name of the ion source, e.g., radio frequency (RF) ion source, electron cyclotron resonance (ECR) ion source, glow discharge (GD) ion source, etc [7-9]. The physical characteristics of these regions depend on the discharge parameters, i.e., geometry of the discharge tube, gas pressure, type of gas, cathode material, applied potential, and the current flowing in the discharge [10]. In most ion sources, the ions are produced by an electrical discharge through a gas or a vapor at pressure in the range of 10-3 mmHg. The discharge creates an ionized medium, plasma, of high electron and ion densities. Ion sources are one of the important components of particle accelerators [11] for studying various nuclear reactions and are essential part of all mass spectrometer [12]. Also they are useful tools in vacuum technology, ion etching, ion sputtering and ion implantation [13]. Ion implantation has been used to modify the mechanical properties of a wide range of metals [14] and alloys using plasma techniques for ion sources [15,16]. In this paper, a glow discharge ion source has been designed and constructed. The discharge characteristics are measured at different pressures for various distances between the anode and cathode using argon gas. From the discharge characteristics curves, we can determine the relation between the discharge voltage versus the gap distance between the anode and cathode for different discharge currents at various pressures. The relation between the gap distance (anode- cathode distance) and the output ion beam current is obtained by measurements at different pressures using argon gas. CONSTRUCTION OF THE ION SOURCE

A schematic diagram of the glow discharge ion source is shown in Fig. (1). It consists of stainless cylindrical anode, its inner radius =10 mm, length =28 mm, and thickness =5 mm, where the cathode is an aluminum disk and its radius =10 mm with 3.5 mm separation from the anode. A Faraday cup is placed at a distance 30 mm from the aluminum cathode. The output ion beam current is extracted from an exit aperture of 3 mm diameter in the aluminum cathode. The working gas is admitted to the ion source through a hose fixed in Perspex flange at up side of the ion source, the ion source is fixed in Perspex container.

Gas inlet

P.S. Anode Anode

Cathode Cathode Faraday Faraday cup cup

Fig.(1) Schematic diagram of a glow Fig.(2) Glow discharge ion source and discharge ion source. its associated electrical circuit.

The operating principle of this ion source is based on the ionization produced by primary electrons colliding with gas molecules due to a potential difference between the anode and the cathode. Therefore high ion beam current can be extracted axially in a direction normal to the discharge region without using extraction system. The anode is made of stainless steel material which is featured by high ionization coefficient and it is hollow to improve the stability of the discharge. The cathode is made of aluminium which has high secondary emission coefficient.

Figure (2) shows a schematic diagram of the glow discharge ion source and its associated electrical circuit. The stainless steel anode is connected to 10 KV power supply which used for initiating the discharge (glow discharge) between the anode and the cathode. A milliamp meter is used to measure the discharge current between the anode and cathode, while the kilo voltmeter is used to measure the discharge voltage between them. The aluminium cathode is connected to earth and the collector (Faraday cup), F.C, is connected to earth through micrometer which used to measure the output ion beam current exit from cathode aperture. A complete vacuum system is used to evacuate the ion source chamber. It consists of stainless steel silicon diffusion pump provided with electrical heater and backed by rotary fore vacuum pump. A liquid nitrogen trap is fixed between the ion source chamber and the silicon diffusion pump in order to prevent the silicon vapour from entering the ion source chamber. The working gas is transmitted to the ion source from a gas cylinder through a needle valve to regulate the rate of gas flow. EXPERIMENTAL RESULTS

Optimization of the Ion Source Properties

In this work, the optimization of the distance between stainless steel anode and aluminium cathode for the glow discharge ion source has been determined using argon gas. Also, the discharge and output ion beam characteristics of the ion source are investigated under different experimental conditions. Measurements of the discharge characteristics have been made using Ar gas with anode diameter equal to 20 mm, cathode diameter equal to 20 mm, and cathode exit aperture equals 3 mm. The relation between the discharge voltage Vd and the discharge current Id at different pressures is shown in Fig.3, 4, 5 and 6, where the distance between the anode and the cathode (d) is equal to 3.5 mm, 5 mm, 7 mm, and 9 mm, respectively. It is seen that the discharge current increases with the discharge voltage according to I α V3/2.

Fig (3): Discharge voltage and discharge current characteristics at a distance of 3.5 mm between the anode and the cathode

Fig (4) : Discharge voltage and discharge current characteristics at a distance of 5 mm between the anode and the cathode

Fig (5): Discharge voltage and discharge current characteristics at a distance of 7 mm between the anode and the cathode

Fig (6): Discharge voltage and discharge current characteristics at a distance of 9 mm between the anode and the cathode

Ion source efficiency

The ion current which can be extracted from a plasma boundary generally exhibits a V3/2 dependence. So the easy way to produce more ion current is to make use of the I α V3/2 law by increasing the voltage. But this is possible only as long as the breakdown rate does not become prohibitive. Complete congruence of the ion beam current Ib and the discharge current Id indicates a linear dependence of the ion current upon the plasma denesity. In order to obtain the efficiency of the ion source, the relations between the discharge current and the ion beam current are given in Figs (7), (8), (9) and (10) at distances of 3.5 mm, 5 mm, 7 mm and 9mm , respectively at the same values of pressures. It is clear that there is a linear dependence. Our experience definitely suggests that there might be no sharp limit referring to the voltage only, but that the minimum gap width also depends on the beam parameters.

Fig (7): The relation between the discharge current and the output ion beam current at a distance of 3.5 mm between the anode and the cathode

Fig (8): The relation between the discharge current and the ion beam current at a distance of 5 mm between the anode and the cathode

Fig (9): The relation between the discharge current and the ion beam current at a distance of 7 mm between the anode and the cathode

Fig (10):The relation between the discharge current and the ion beam current at a distance of 9 mm between the anode and the cathode

Determination of the optimum distance between the anode and the cathode

The relation between the discharge current Id and the ion beam current Ib for the distances 3.5 mm, 5 mm, 7 mm and 9 mm between the anode and the cathode at constant argon gas pressure 3x10-3 mbar is shown in Fig (11). It is seen that high output ion beam currents can be obtained at a distance of 3.5 mm. At distance smaller than 3.5 mm, we can not be able to do our measurements because of instabilities of the discharge.

Fig (11) the relation between the discharge current and the ion beam current for the distances 3.5 mm, 5 mm, 7 mm and 9 mm between the anode and the cathode

Effect of discharge confinement Fig (12) shows the discharge voltage and discharge current characteristics using Ar gas. A disk of Teflon insulator with an inner diameter equals 15 mm and an outer diameter of 20 mm has been put between the anode and the cathode. This disk was covering the cathode area and reducing the discharge area on the cathode surface from a diameter of 20 mm to 15 mm. By varying the discharge voltage, the discharge current and consequently the output ion beam current increases as shown in Fig (13). In the two figures, the measurements have been taken at three values of pressures (4 x10-3, 3.3 x10-3, 2 x10-3 mbar). From the figures it is seen that in this range of pressure we need more high voltage in order to obtain high ion beam currents. Fig (14) shows the variation of the discharge current with the output ion beam current at these values of low pressures (4 x10-3, 3.3 x10-3, 2 x10-3 mbar) for the same insulator inner diameter 15 mm.

Fig (12) Discharge voltage–discharge current characteristics using Ar gas, insulator inner diameter equals 15 mm

Fig (13) Discharge voltage–ion beam current characteristics using Ar gas, insulator inner diameter equals 15 mm

Fig (14) Discharge current–ion beam current characteristics using Ar gas insulator inner diameter equals 15 mm

Results of Ar gas at the inner insulator diameter = 10 mm are shown in Figs (15), (16) and (17). In the three figures, the measurements have been carried out at the pressure range (9 x10-3, 5 x10-3, 3.6x10-3 mbar).

Fig (15) Discharge voltage–discharge current characteristics using Ar gas, insulator inner diameter equals 10 mm

Fig (16) Discharge voltage–ion beam current characteristics using Ar gas, insulator inner diameter equals 10 mm

Fig (17) Discharge current–ion beam current characteristics using Ar gas, insulator inner diameter equals 10 mm

-3 Comparing Fig (7) and Fig (17) for the same pressure (9x10 mbar) it seen that at Id =1.5 mA, the ion beam current is equal to 60 μA, in case of cathode diameter =20 mm without using the insulator Fig (7), while at Id = 1.5 mA, the ion beam current in case of using the insulator of inner diameter = 10 mm, will reach 180 μA (Fig (17)). At a pressure of (3.6x10-3 mbar), by comparing the same two figures the ion beam current in Fig (7) is equal to 180 μA while in Fig (17) it equals

300 μA for the same value of Id = 1.5 mA. So, we conclude that the ion beam current can be greatly increased when inserting the Teflon insulator between the anode and the cathode. It was found that -3 at Id = 2 mA, Vd = 4 KV, one can obtain 400 μA at a pressure of (3.6x10 mbar) as shown in Fig (17), and Fig. (16).So one conclude that high efficiency can be obtained for an insulator inner diameter equal to 10 mm. This value is sufficient for this type of ion source.

Fig. (4.21) The relation between the discharge current and the ion beam current for insulator inner diameters (20 mm, 15 mm and 10 mm) at pressure 3.3 x 10-3 mbar

Comparison between theoretical (the Child – Langmiur equation) and experimental data of Argon gas at optimum operating conditions The ion beam current that can be extracted from an ion source is given by the Child – Langmiur equation for ion current flow under space charge limited conditions [17]:

1 3 ⎛ Z ⎞ 2 V 2 I = 1.72 S ⎜ ⎟ ⎝ A ⎠ d 2 The experimental results are carried out using argon gas in case of pressures 3x10-3 mbar, 5x10-3 mbar, and 9x10-3 mbar ( figs. (18.a), (18.b), and (18.c)), respectively. Theoretical results are obtained using the above equation. From the figures it is seen that a continuous V3/2 curve shows good agreement for beam current values up to 4Kv. The deviation of the beam current at higher voltages can be attributed to losses by charge exchange in the beam line because of pressure rise.

Fig. (18.a) Comparison between theoretical and experimental results at a pressure of 3x10-3 mbar.

Fig. (18.b) Comparison between theoretical and experimental results at a pressure of 5x10-3 mbar.

Fig. (18.c) Comparison between theoretical and experimental results at a pressure of 9x10-3 mbar.

CONCLUSION

An optimization for a glow discharge ion source has been made. This ion source was designed and constructed in the department of Accelerators and Ion Sources, Atomic Energy Authority, Egypt. The different parameters of the ion source such as, the discharge current, discharge voltage, pressure and output ion beam current are measured many times in order to ensure the accuracy. From these measurements, it was found that, the optimum distance between the anode and the cathode is 3.5 mm, where at this distance a stable discharge current could be obtained. Consequently, higher output ion beam current can be measured at this distance. It has been concluded that the ion beam current can be increased greatly when inserting a Teflon insulator μ between the anode and the cathode. An ion beam of 400 A can be obtained for 4 KV – 2 mA at a pressure of 3.6x10-3 mbar. This ion source can be used for etching, sputtering and micro-machining applications. It is featured by a high efficiency with respect to lower gas consumption and long life time.

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