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The dynamic behavior of ac arcs in a supersonic nozzle has been investigated. The close coupling between the arc and its external flow determines the arc characteristics as well as the flow transients within the nozzle in a gas-blast circuit breaker. For affinely related nozzles, the dynamic behavior of the system is found to be fully determined by two nondimensional parameters: the nozzle coefficient N  and the nondimensional frequency Â¿ which is the product of the angular frequency of the current and the time scale of the system. Of these two parameters, Â¿ controls the qualitative behavior of the arc and its external flow. When Â¿ is of the order of 10-2 or less, a quasi-steady period of arcing exists. Arc modeling can then be conveniently divided into a quasi-steady-state (hence dc) phase and a current-zero period.