An alkali metal vapor arc discharge lamp which operates on DC and employs an amalgam of mercury and an alkali metal such as sodium, has a cathode to anode pressure ratio no greater than 5 and a cathode end temperature at least 50.degree. C. hotter than the anode end temperature to prevent cataphoretic separation of the mercury and alkali metal in the arc discharge during operation of the lamp. The lamp is designed to have a cataphoretic driving parameter (CDP) value of less than 150. The CDP is defined as the product of the arc current in amperes, times the arc gap length in centimeters, divided by square of the inner diameter of the arc tube in centimeters.