Abstract: An electron cyclotron resonance negative ion source comprises an enclosure containing a gas or vapor of a material for forming a plasma, means for injecting into the enclosure a high frequency electromagnetic field forming electrons by ionizing the gas or vapor, means for producing within the enclosure an axially symmetric magnetic field whose amplitude increases along the axis of symmetry, whereby said amplitude, which is at a maximum in the vicinity of and upstream of the negative ion extraction zone, having in the central region of the enclosure a value for which the electron cyclotron resonance condition is satisfied, as well as means for extracting the negative ions formed, brought to a positive potential compared with the enclosure.