MPACVD-diamond windows for high-power and long-pulse millimeter wave transmission

Abstract To satisfy the electrical, thermomechanical and nuclear requirements for continuous wave (CW) millimeter (mm)-wave vacuum windows for electron cyclotron heating in thermonuclear plasma fusion energy research, water-edge-cooled window assemblies using large synthesized diamond disks have been developed in Europe, Japan, Russia and the USA. The diamond employed in these studies has been manufactured predominantly at DeBeers Industrial Diamond Division (UK) Ltd. by microwave plasma assisted chemical vapor deposition (MPACVD) and consists of polycrystalline diamond disks with diameter and thickness up to 119 mm and 2.23 mm, respectively. Due to low mm-wave losses (tan δ=2–3×10 −5 ), outstanding thermal conductivity (≈1800 W/mK), good ultimate bending strength (400 MPa) and radiation insensitivity up to a neutron fluence of 10 20 n/m 2 (10 −5 dpa), water-edge-cooled MPACVD-diamond windows promise to provide a practical technical solution for the transmission of CW millimeter waves at power levels in excess of 1 MW. MPACVD-diamond gyrotron windows with a bakeout temperature of 450°C (Al-braze) have been tested up to 0.55 MW, 10 s at 110 GHz (CPI, Palo Alto, USA) and 0.45 MW, 8 s at 170 GHz (JAERI/FZK, Naka, Japan). A thermally matched ceramic graded seal prototype 75-mm diameter window mounting which is bakeable to 550°C was developed successfully by DeBeers. The present paper reviews the development status and future prospects of this novel application of MPACVD diamond.