High efficiency trapping of stray electrons in negative ion accelerators

Abstract

A code for the simulation of stray electrons in negative ion accelerators is presented. It performs 3‐D electron trajectory calculations and uses a Monte Carlo method in order to take into account secondary emission from acceleration grids. The code is validated by comparison with various experimental results. It is demonstrated that backscattered electron is the reason for electron leakage in negative ion accelerator columns. The code was used to design the extraction system of the SINGAP accelerator (0.1 A of D− at 1 MeV with current density around 10 mA/cm2). This system has proved to suppress all the extracted electrons. A new design is presented for extracting higher current densities (in the 20 mA/cm2 range), which would conciliate complete electron suppression with high accelerator transparencies (about 56%). This new extraction grid will be tested in the framework of a CEA‐JAERI collaboration on the 400 keV test‐stand at JAERI.