Ion Source Optimisation for Proton Beam Quality of the Van De Graaff Accelerator at iThemba LABS for Ion Beam Analysis

M E M Eisa,C A Pineda-Vargas,J L G Delsink,G De Villiers,K A Springhorn,D T Fourie,J L Conradie,P J Celliers

doi:10.4236/wjnst.2013.33014

M E M Eisa, C A Pineda-Vargas + Show 6 more

Open Access

https://doi.org/10.4236/wjnst.2013.33014

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Abstract

The operating principles of the two duoplasmatron ion sources and the PIG source available for the Van de Graaff accelerator have been determined qualitatively, mainly by studying the extensive literature on ion sources. The main characteristics of the three sources have therefore been determined experimentally on an ion source test bench. Important parameters of the sources such as the pressure and temperature in different regions of the sources not measured, because of space limitations. However, in spite of these limitations in quantitative understanding of the sources, sufficient information to continue with the study of the beam transport through the Van de Graaff accelerator and its beamlines could be obtained from these measurements. With regard to beam intensity, lifetime and gas consumption the measurements showed that the hotcathode duoplasmatron is at present by far the most suitable source for the van de Graaff accelerator. The emittance of the ion source was measured with two slits, 90 mm apart, and a Faraday cup in the ion source test bench at an arc voltage of 83 V, an arc current of 1 Amp, a filament voltage of 24 V and an extraction voltage of 5 kV. The measured emittance for 90% of the beam intensity is 48π mm mrad. This figure will be used to calculate the beam transmission through the Van de Graaf accelerator. Different computer programs have been used for these calculations from the ion source through the terminal section, the accelerator and beam line up to the Nuclear Microprobe (NMP) for beam quality.

Highlights

With regard to beam intensity, life-time and gas consumption the measurements showed that the hot-cathode duoplasmatron is at present by far the most suitable source for the van de Graaff accelerator
At iThemba LABS two duoplasmatron ion sources with hot and cold cathodes, respectively, as well as a coldcathode Penning Ionization Gauge (PIG) source are available for the Van de Graaff accelerator
The diagnostic equipment consists of two Faraday cups for the measurement of the beam intensity and adjustable horizontal and vertical slits for selection of portions of the beam, after it has passed through the pole gap of a bending magnet and a drift space

Summary

Introduction

At iThemba LABS two duoplasmatron ion sources with hot and cold cathodes, respectively, as well as a coldcathode Penning Ionization Gauge (PIG) source are available for the Van de Graaff accelerator. The optimum operating conditions, such as the filament current, cathode and anode voltages and the gas flow under which stable beams can be delivered reliably to the users with an acceptable long source lifetime, were determined, since no experimental data were available[1]. Major efforts have been made recently to improve the reliability of this accelerator [2,3] Computer programs such as TRANSPORT program [4,5] which calculates first-, second- and thirdorder effects by matrix multiplication. In order to include space-charge effects in the calculations the program IGUN [7,8,9] which is a two-dimensional ray tracing code that has been developed for simulation of the extraction of positive ions from plasmas. The applicability of the three computer codes for this specific task is reviewed

Measurements and Results

Optimization of the Ion Source by Varying the Gas Flow

Conclusion and Discussion