Stable operation of a corona excited supersonic expansion for the production of cold radicals or ions requires control of the geometry, chemistry, and electrical parameters. The nozzle must taper rapidly on the high-pressure side to a throat that opens up into a free expansion on the vacuum side. Optimum radical and ion production is obtained with only a few percent of precursor in an inert carrier gas. Capacitive loading must be kept to a minimum to prevent oscillation. It is shown that the finite response time of the plasma synthesizes an inductance that serves to decouple the negative resistance of the plasma from the rest of the circuit. Practically, oscillation is prevented if the circuit RC time constant is shorter than that which the plasma can follow.