Two designs of electron suppressor have been tested in a volume negative ion source operating dc with a large extraction aperture diameter (16 mm). Both suppressors employ magnetic fields to divert the electrons near the extraction aperture onto collector electrodes. However, while one suppressor uses a fixed field produced by permanent magnets, the other employs a variable field produced by a solenoidal coil. These suppressors allow low values (3-4: 1) of extracted electron to ion currents to be obtained. The results are compared with a diffusion model and a hydrodynamic model of the transport of the electrons across the magnetic field. They show that the hydrodynamic model gives a good description of the suppression and transport of the electrons.
