The magnetic filter is essential in all current negative ion sources for neutral beam injectors. The negative ions produced are easily destroyed by collisions with fast electrons (>2 eV). While the fast electrons are necessary to produce the plasma (e, H+, H2+, H3+, H) through ionizing collisions with the gas. To resolve this conflict, the magnetic filter is applied to form a negative ion extraction region in front of the plasma grid, where the electron temperature is cooled down through the filter field. Combined with the confined magnetic field, there are several filter field configurations. Because the plasma generation has a strong relation with the primary electrons in the filament-arc negative ion source. Hence, the influence of the magnetic filter field on the plasma has been studied through the distribution of primary electrons in this paper.