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DEVELOPMENT OF TABLETOP 50 KV ION ACCELERATOR Raj Kumar, Rajeev Ahuja, C. P. Safvan, Amit Roy, IUAC, New Delhi- 110 067 Abstract A low energy accelerator is designed, fabricated, assembled & tested at IUAC with the concept of having a compact table top type machine. The accelerator is specially developed for students & faculty at the undergraduate & postgraduate level. Students will get a chance to leverage their existing theoretical knowledge in physics to explore ion source, bending/analyzing magnet, analyzed - ion, molecular & neutral beams at different energies, vacuum systems, beam transport, energy selection, building an accelerator etc. Faculty & students can conduct experiments using alpha, proton & hydrogen beams up to 50 kV without any radiation hazard. These would include, but not be restricted to positive ion implantation, ion induced fluorescence studies, particle detector experiments, ion scattering experiments, production of neutral atomic species in beam form etc. The accelerator consists of a cold cathode penning ion generator capable of generating gaseous beams upto 350 µA with acceleration voltage of 50 kV. An electrostatic quadrupole triplet, permanent magnet based analyzing magnet, beam scanner etc have been designed & developed in-house for the accelerator. The accelerator components are controlled & monitored through a programmable logic controller. INTRODUCTION Development of low energy accelerator was started in IUAC with a concept of having stand alone tabletop type machine requiring minimum components & utilities. Cold cathode penning ion generator has been designed & fabricated in-house by IUAC which can deliver gaseous beams like, Alpha, Proton, hydrogen, argon, nitrogen etc up to 350 µA. The source is assembled inside nylon cylinder to provide isolation of 50 kV. The source consists of a small permanent magnet, stainless steel cylindrical anode, feed thru's, mild steel cathode body with face plate, extractor etc. This has been tested for maximum current of 400 µA. Stability test at 350 µA has been carried out for 24 hrs. An electrostatic quadrupole triplet has been designed and fabricated for use in above accelerator. It has poles of 100, 185, 100 mm long with 56 mm radius. The quadrupole has a total length of 421 mm with a 60 mm aperture. It has been designed to use a maximum of 3 kV for focusing of 50 keV beam at 450 mm distance. An analyzing magnet has been assembled by using pallets of permanent magnets to get a stable field of up to 2110 Gauss as required for accelerator. The poles of magnet have been fabricated by using 25 mm thick mild steel plates. Field shorting plates mounted on a common plate having vertical movement arrangement using a common hand wheel has been used to vary the magnetic field for beam energy and mass selection. Field variation of 900 to 2110 Gauss has been achieved. Figure 1: IUAC's 50 kV Ion Accelerator All parts of the accelerator have been assembled on a common stand with vacuum system having a 400 lpm turbo molecular pump, rotary pump, vacuum gauges controllers etc. Penning ion generator in a grounded extractor mode has been directly mounted onto the inlet flange of the quadrupole triplet. A small experimental chamber has been installed with an isolation valve after the magnet. It has a faraday cup, suppressor, beam profile viewer, target mounting ladder etc. The overall physical size of the accelerator is 1600x1400x1500 mm and is shown in Figure 1. The accelerator has been satisfactorily tested for beam energy range and currents as tabulated below. Beam H+ H2+ He+ He2+ Beam energy range and currents obtained Energy (keV) Field (Gauss) Current (µA) 23.4 - 46.8 891 - 1230 4.45 14 - 46.8 1007 - 1758 52 14 - 33.7 1398 - 2110 43.5 37.5 - 91.8 1130 - 1748 0.16 SUB SYSTEMS OF THE ACCELERATOR Penning Ion Generator (PIG) Ion Source A cold plasma based PIG ion source[1] is designed, developed and used with IUAC's 50 kV ion accelerator. The ion source is capable of delivering up to 350 μA of stable current. In a penning ion generator the electrons oscillate between two cathode electrodes inside cylindrical anode. The plasma production is improved by providing an axial magnetic field using a permanent magnet having a surface field density of around 3.5 kG. The ion source is assembled in a nylon housing and connected to a 50 kV power supply. The required anode voltage for the source is derived from the main 50 kV supply. A copper tube of 1/8 inch diameter is tightened on the cathode body with an adapter and connected to a precision needle valve mounted on the high voltage divider plate to supply the source gas to the source. The complete assembled view of PIG ion source is shown in Figure 2. stable field having variation from 900 to 2110 Gauss has been achieved. Figure 4: Bending Magnet Assembly Figure 2: Ion Source Assembly Electrostatic Quadrupole Triplet An electrostatic quadrupole triplet has been designed and fabricated for use in the above accelerator. It has a stainless steel vacuum jacket of 250 mm diameter with nine 2.75” ConFlat ports for power connections and an 8” ConFlat port for mounting the turbo molecular pump. The quadrupole elements are 100, 185 and 100 mm long with a radius of 56 mm providing 60 mm aperture for the beam. It is designed to focus a 50 keV beam at 450 mm away from its exit port by applying up to 3 kV on the poles. Glassman make 3 kV power supply modules are used to power the quadrupole lens. These power supply modules are controlled & monitored by a programmable logic controller (PLC) through analogue input/output modules. The assembled view of the internal components of the quadrupole is shown in Figure 3. Beam Scanner Beam scanner for the accelerator has been developed to get uniform implantation throughout the target surface. Glassman power supply modules of 3 kV have been used for the scanner. By applying voltages across a set of parallel plates, the beam is made to scan the surface of the target, thereby eliminating any non uniformity in the dosage received in the target The power supplies are controlled through PLC analogue outputs to get triangular voltage on the scanner plates. Discharge resistors are used across scanner plates to get symmetric triangular voltages. The beam scanner assembly is shown in Figure 5. Figure 5:Electrostatic Beam Scanner Vacuum System Figure 3: Quadrupole Triplet Inner Assembly Permanent Magnet Based Bending Magnet A bending magnet has been designed, fabricated & assembled by using pallets of permanent magnet of size 25x50x12 mm having surface flux density of 3.5 kG. The pallets are stacked between two 25 mm thick rectangular MS plates acting as poles. Orientation of all the permanent magnet pallets & stacks have been kept the same. Four vertically moving field shorting plates have been mounted onto a third MS plate for varying the field. This third MS plate moves the field shorting vertical plates between the magnet poles to vary the field that is seen by the ion beam. A common hand wheel is used for movement of the third plate holding the vertical shorting elements to provide field continuous variability. The magnet has beam pipe of cross section 60x75 mm with a bending radius of 200 mm. Homogeneous & One turbo molecular pump of 400 litres per minute is mounted on the quadrupole triplet vacuum jacket with backing of rotary pump to provide vacuum to the accelerator. Vacuum in the range of 2.0x10-7 bar is achieved by this pump. ACCELERATOR PERFORMANCE CURVES The accelerator has been tested for various ion beams, energies and currents. The current is varied by varying the gas flow into the PIG source using a needle valve. The beam energy is varied by varying the high voltage power supply and simultaneously adjusting the magnetic field to select the desired ion species. Proton beam is extracted by injecting hydrogen gas into the ion source. Figures 6 and 7 show the accelerator output as a function of pressure at a constant energy and as a function of beam energy at a constant pressure respectively. Figure 6: Current vs Pressure at a constant energy Figure 10: Beam current vs time in hrs. PROPOSED USES OF THIS ACCELERATOR Figure 7: Current vs beam energy at a constant pressure Helium beam is extracted by injecting helium gas into the ion source. Fig. 8 & 9 shows accelerator output as a function of pressure at a constant energy and as a function of beam energy at a constant pressure respectively for He+ ions. Figure 8: Current vs Pressure at a constant energy This ion accelerator has been specially designed & developed for educational institutions. UG & PG students and faculty will get a chance to leverage their existing theoretical knowledge in Physics to explore: Ion source Bending magnet Analyzed-ion beams, Molecular beams & Neutral beams at different energies Vacuum systems Beam transport Energy selection Beam Scanner Building an Accelerator etc..... Students & faculty can conduct experiments using alpha, proton and hydrogen beams up to 50 kV without any radiation hazard for following. Positive ion implantation Ion induced fluorescence studies Particle detector experiments Ion scattering experiments Production of neutral atomic species in beam form etc …. ACKNOWLEDGMENT We acknowledge with thanks the support provided by D Kanjilal in the completion of this project. REFERENCES [1] Joshua L. Rovey, Brandon P. Ruzic, and Thomas J. Houlahan, Rev. of Sci. Instr. 78 (2007) 106101. Figure 9: Current vs beam energy at a constant pressure ACCELERATOR STABILITY TESTS Tests for beam current stability are carried out for longer durations. Fig.10 shows stability test for 14 hrs. It is observed that the current stability depends mainly on the vacuum stability. It is clear from Figure 10 that the vacuum stabilizes after 3-4 hrs from the start of the machine and current also stabilizes with the vacuum and remains very stable for longer times.