Recent results of ECH diamond window development

Resent results of a diamond window development for an electron cyclotron heating (ECH) system are reported. High power millimeter wave transmission experiment of a neutron irradiated diamond window shows that loss tangent doubles and thermal conductivity decreases to approximately half. It is, however, verified that the irradiated window is applicable to a fusion reactor like the International Thermonuclear Experimental Reactor (TIER). In terms of reliability on the window structure, a diamond window with a copper (Cu) coated edge is developed. Thickness of the Cu layer is 0.5 mm. High power mm-wave transmission experiment of the Cu-coated window shows that the edge cooling is effective for an 1 MW transmission even if the edge is covered by the Cu layer.