We have observed three modes (I, II and III) of the current-voltage characteristics and luminous patterns in argon dc glow discharges with two plane-parallel cathodes. Mode-I: The discharge current was low and week luminous region was located in the midplane parallel to the cathodes. Mode-II: The luminous region separated into two parts parallel to the cathodes at certain discharge current. The width of each glow region extended with the current and then they finally coalesced. Mode-III: With further increment of the current the intensity of the glow region became brighter, then the features of the discharge showed those of hollow cathode discharge. Using the cathode fall model proposed by Fujii et. al. (IEEE, Quantum Electronics, QE-15 (1979) 35), we have examined the mechanism of the mode transition. Mode-I: Ion current mainly governed the discharge current. Mode-II: The secondary electron current emitted by photons was 50percent to 80percent of the discharge current. Mode-III (hollow cathode discharge): The discharge current mainly consisted of the secondary electron current emitted by photons. Hence, we have inferred that it is the primary factor of the transition from modes-I to II that the secondary electrons emitted from the cathodes by photons exceeded those by ions. In mode-III, the secondary electrons by photons were dominant.
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