References(14) Cited-By(1) This paper describes breakdown mechanism of the metal halide lamp containing Sodium, Scandium and Thorium iodides in DC operation. Current growth, voltage collapse and the formative time lag of a discharge were measured. The time lag was compared with the calculated drift time of both electrons and ions in order to presume the dominant secondary process. It was found that a Townsend-type breakdown with γi process occurs in the starting gap and that two types of breakdown, i.e. (1) prebreakdown→arc and (2) prebreakdown→glow→arc occur in the main gap. Which type will occur depends on the burning period, the impressed voltage and its polarity. Townsend mechanism with γi process and/or γp process as the dominant secondary process has an important role for the breakdown. On the comparison with the results in case of the mercury lamp, the appearance of the prebreakdown stage and the glow-to-arc step discharge are principally considered to be due to the suppression of electron multiplication by the electron attachment and/or quenching in impurity gases.
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