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In negative ion accelerators, it is necessary to minimise the stray electrons accelerated at high voltage. Generally, the electrons extracted from the ion source are suppressed by magnetic fields in the ion beam pre-accelerating stage. Different electron suppressors based on this principle have been studied experimentally. It was observed that their suppression efficiency varied considerably with the physical conditions in the accelerating column. This paper points out that secondary electrons (backscattered electrons) are for the most part the cause of the electron leak from the pre-accelerator to the high voltage accelerator stage. A Monte Carlo code, ELSTOP, has been developed to simulate in 3-D the electron behaviour in the accelerating column, by calculating the trajectories in the presence of both electric and magnetic fields and by taking into account the emission of secondary electrons. A comparison with experimental results for different electron suppressors shows good agreement with this model; ELSTOP's trajectories show that the electron leak is correlated with magnetic field topology and accelerating grid potential. ELSTOP was used for the design of an efficient electron suppression grid which will be tested in the near future.

Additional information

Authors: SIMONIN A, CEA, CEN Cadarache, Saint-Paul-lez-Durance (FR)
Bibliographic Reference: Report: EUR-CEA-FC-1502 EN (1993)
Availability: Available from CEA, Département de Recherches sur la Fusion Contrôlée, Saint-Paul-lez-Durance (FR)
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