PURPOSE: To obtain an ignitor with a life not affected by plasma which is mounted easily, by mounting on the outside of an electric discharge chamber an ignitor comprising a laser generator for producing laser beam which transmits through quartz glass, γ radiation and β- radiation, a γ radiation source and a β- radiation source.

CONSTITUTION: Xe gas introduced into a gas discharge chamber 8 from a gas introduction system is made to collide with laser beam discharged from an ignitor laser generator 10 to form initial ionization plasma. Electron in the initial ionization plasma formed is accelerated in a radial direction in a high frequency field induced by an induction coil 6 and collides with the Xe gas to increase ionization plasma. After such a situation has continued for a few milliseconds, the amount of plasma which disappears at a wall and the like is balanced with the amount of plasma which is formed to make an equilibrium condition. Xe+ ion is extractedly accelerated by electrodes 2-4 and then it is discharged to the outside of an ion thruster. Therefore, an ignitor with a life not affected by plasma can be mounted easily.