The performance of a plasma generator which used arc heating was studied with helium and hydrogen as the working gases. An optimum electrode gap and chamber diameter were determined for a stagnation pressure between 1 and 1.5 atmospheres by computing the efficiency of the unit for various values of these parameters. Two methods of increasing the specific impulse of the test gas at sonic velocity were studied using optimum chamber conditions. First, the energy input to the gas was increased to approximately 1.5 X 105 cal/gram for hydrogen; and a maximum specific impulse of 2300 grams force over grams mass/sec was obtained. Further increases in the specific impulse resulted from removal of boundary layer gas at the throat. A theory, assuming separate regimes of flow at the throat, was developed to explain this increase.