PURPOSE: To obtain ion source whose ion beam intensity is great by constituting the photoionized ion source used for a heavy ion accelerator out of a supply source of electron molecules, an electrode for forming electrostatic inductive resonant conditions in molecules, and an exciting light source for exciting molecules in resonant conditions.

CONSTITUTION: Potassium atoms are ejected from an atom molecule supply source 3 into a vacuum vessel not shown in the figure, and are let pass between a low poten tial electrode 1 for parallel electronstatic field to which proper voltage is applied and a high potential electrode for electrostatic field application. Moreover, the beams from an exciting light source 4, that is, the second higher harmonics, which are generat ed by condensing the fundamentals of the pigment laser for nitrogen laser excitation in DKP crystals, are introduced between the electrodes 1 and 2 so as to produce photoi onization. Next, the produced ions are accelerated immediately in the direction of the electric field, and those are added in an electrostatic lens system 5 and are made into beams. Hereupon, a metallic mesh is spread beforehand in the small hole 6 of the electrode through which ions pass, so as to make the electric field of an ionization area uniform as far as possible. Thus, the low ionization efficiency having a normal photoionizing source to do photoionization in zero magnetic field can be improved sharply.