disposed generally downwardly beyond a bottom capping portion 42d 2 of the arc envelope and also advantageously along envelope centerline 42 39 the arc discharge exciting rf coil means 24 has a central plane 46 which is also the plane through the center of the toroidal discharge arc situated below the central preferably substantially circular plane 42 e of the arc envelope wall at its maximum diameter thus there is again a volume 48 above the arc 14 and having at least a portion thereof with a greater radius then the radius of arc discharge 14 in this volume 48 the arc envelope walls are relieved generally away from the flow of hot gases as generally shown by arrows d so that the arc envelope walls 42a and cap portions 42d are at lower and more uniform distributed temperatures for a given power dissipated in arc 14 arc envelope 42 is specifically constructed with two truncated conical wall portions 42a pointing in opposite directions generally upwardly and downwardly to allow the resulting lamp 40 to be mounted with its base in either an upward or downward configuration to allow base up or base down operation of the lamp that is turning arc envelope 42 upside down with respect to at least the excitation coil.