Miniature high pressure arc lamps containing a substantial pressure of xenon, in addition to metal halide and mercury, can provide instant light at turn-on and are suitable for automotive headlamps. The xenon aggravates convection which causes arc-bowing and overheating of the envelope above the arc. By operating the lamps on unidirectional current upon which a frequency-modulated high frequency ripple has been imposed, acoustic resonance is used to straighten out the arc. The use of unidirectional current permits a reduction in cost and size of the ballast control circuits operated from auto storage battery. Frequency modulation of the ripple broadens the band allowing acoustic straightening of the arc. Additional control of the arc plus reduction of cataphoresis may be achieved through a horizontal magnetic field at the arc, transverse to current flow.