Evaluation of a newly developed low reflection dummy load for high power millimeter waves

Abstract

A low reflection dummy load (LRD) has been newly developed for gyrotron development and operation. This LRD is designed for 0.5 s of high power (∼1 MW) millimeter wave injection with a low reflection. In order to reduce the reflection, the LRD consists of an inner cylinder, a main chamber, and a conical mirror, which results in the efficient absorption by long pass length between the inner cylinder and the main chamber. The characterization of low reflection has been demonstrated by the water temperature rise of the DC break (DCB) section inside the gyrotron, which works as an indicator of the reflected stray millimeter wave from the dummy load (DL). The water temperature rise of DCB with LRD was decreased by 1%, 15% and 17% at three tested frequencies of 82 GHz, 110 GHz and 138 GHz, respectively, compared with the conventional DLs, and the power absorption of LRD is 4–7% larger than the conventional DLs. A thermal analysis by a finite element method (FEM) with experimentally obtained heat load distribution and cooling time constant showed that the maximum temperature at 0.5 MJ injection is below the melting point of the material but it is not acceptable for the cycle operation considering the allowable stress of the aluminum-alloy used in the present design. FEM analysis suggested that changing its material to CuCrZr enable to use for 0.5 MJ injection. The result of FEM analysis also shows that the CW injection with less than 0.15 MW is acceptable in the present design.