Theme T purpose of this work was to measure the energy losses in the downstream electrode of a highperformance, Huels-type arc heater (the MDC-200) operating over a wide range of pressure and power levels. To define the predominant loss modes, the measurements included axial distributions of both the total electrode heat flux and the radiative heat flux. Development of radiometers capable of sustaining the severe internal environment of the arc heater at pressures of 100 atm and greater became a part of the program after commercial radiometers failed.