Design of a negative helium ion injection system for the 6MV en tandem accelerator at iThemba laboratory for accelerator based sciences

Abstract

Sufficient production of negative helium ions for acceleration of alpha particles is of great interest in tandem electrostatic accelerator facilities. The Tandem accelerator at iThemba LABS makes use of an 860A Cs-Sputtering negative ion source. This particular procedure assisted by sputtering is totally ineffective for atoms of the inert gasses such as helium (He) due to the electronic stability which also prevents them to form chemical compounds. Therefore an ion source is required to produce negative helium ions for various studies and researches. A negative-ion source that operates on a different principle other than sputtering has been designed at iThemba LABS. The design is based on a positive Duoplasmatron ion source coupled to a charge-exchange canal. In this, positive _helium ions produced in the Duoplasmatron ion source are directed to the charge-exchange canal where through atomic collisions with alkali vapour molecules undergo double-electron capture and become negative. Subsequently, they are injected into the EN 6 MV Tandem accelerator for acceleration. The physical processes which occur in the exchange canal have also been studied theoretically in order to enhance the understanding of the source behavior. The computer program TRANSPORT is being used for numerical field calculations which guided the design of the source.