ion‐production mechanism in an indirectly heated Penning source has been studied at NIRS. Taking the number of primary electron passages between two cathodes into account, we calculated the effective electron current density, Ji ; it is the order of kA/cm2 for Ar, where the index ‘‘i’’ is the charge state. In a pulsed arc condition of 2.6 A and 1300 V with a neutral atom density of 4×1013/cm3(cw), the calculated ionization times for Ar5+→Ar6+ and Ar6+→Ar7+ are 18 and 28 μs if their production process is step‐by‐step. These values agree well with the experimental value of 20 μs. Ji is proportional to the arc voltage and inversely proportional to the neutral‐atom density N. The yield of multiply charged ions is in proportion to 1/Ni−2. This expression agrees well with the experimental results, which suggest that multiply charged ions are produced from Ar2+ through step‐by‐step ionization by primary electrons.

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