A plasma lamp for an electrodeless plasma lamp having a waveguide body. The non-radiative thermal losses from the bulb are controlled to prevent the bulb from melting while providing a high level of radiation from the bulb. The thermal conductivity of the waveguide, configuration of the heat sink and size and placement of the bulb may be selected to provide a brightness of more than 80 lumens per watt and a total brightness of more than 12,000 lumens at a power level of 150 watts.