[en] Several different beam dumps using parallel groove structures were made for use in absorbing electron cyclotron radiation emitted from fusion plasmas. The materials used were Pyrex, Macor, alumina, and graphite. Broadband absorption measurements indicate more than 99% power absorption for dumps made of Pyrex and Macor, while absorption by alumina and graphite dumps are noticeably less. Waves polarized with E parallel to the grooves undergo higher absorption compared to those polarized with E perpendicular to the grooves. Three different mechanisms are identified as the source of reflected power. They are: (1) front surface reflection; (2) back surface reflection; and (3) diffraction grating effect. All the dumps exhibit greater reflection at lower frequencies