High pressure sodium vapor lamps operated on sonic frequency pulses with short duty cycle in order to raise the color temperature are subject to arc instability near the electrodes and to overheating of the end closures, particularly that at the anode end when unidirectional pulsing is used. Stability and long life is achieved and overheating is prevented by using electrodes of cross-sectional area from 0.3 to 0.4 times the envelope cross section and lengthening the distance from closure to electrode tip so that the arc gap is less than 80% of the gas column length.