Design, construction and first tests of a stainless steel load for high power MM-wave radiation

The load is a critical and essential component of a test stand for high-power CW gyrotrons. It both has to absorb and to measure the microwave power in an accurate and reliable way. Up to now cylindrical loads with ceramic coating have been operated at KIT/IHM, which show a good absorption behavior and are capable to absorb up to 1MW of microwave power. Unfortunately this type of load has the drawback that the ceramic layer may degrade and even may flake off due to local overheating. To avoid this problem, a load has been designed and constructed that is purely made of stainless steel. Due to the relatively low absorption rate of this material, we had to enlarge the total absorbing surface of this device by inserting an additional structure made of U-shaped stainless steel pipes. Preliminary measurements with a load without any active cooling have shown that 700kW of microwave radiation with a pulse length of two seconds could be absorbed under atmospheric pressure using a 1MW, 140GHz CW series gyrotron for W7-X as source. During the measurement sequence, the load heated up to temperatures of 100°C at the outer surface and up to 250°C at the inner structure which is a critical value in terms of arcing. In the experiments a water-cooled pre-load showed that the reflected power is approximately 2%, which is considered a very good value. In a second step the load has been equipped with an assembly that allows water-cooling of the inner structure. The outer structure (cylinder, top and bottom sections and a conical scattering mirror) are still not actively cooled. The measurements with this modified setup showed a much better behavior concerning arcs, compared to the results of the former measurement campaign. The tolerated pulse length could be significantly increased up to 30 seconds at a gyrotron output power of 550kW and up to 20 seconds at a power level of 780kW. Calorimetric measurements showed that the internal pipe structure absorbed approximately two thirds of the microwave power. This corresponds well to the ratio of the surface areas under consideration. At the moment we redesign the outer structure of the device to allow active cooling of all parts of the load.