Dielectric Losses in CVD Diamonds at Frequencies 1 kHz - 200 GHz and Temperatures 70 - 800 K

The low loss materials for output windows still remains to be a weak point in megawatt millimeter waves gyrotrons for controlled plasma heating in fusion reactor. CVD diamond is considered currently as the most promising material for such windows. We determined its dielectric properties in extremely broad frequency and temperature ranges and compared with theory. The combinations of data on dielectric losses in broad frequency and temperature ranges give most information on the loss mechanisms. To measure the loss tangent from 1 kHz up to 200 GHz and temperatures 70 - 800 K different types of resonator technique were used. The diamond wafers were prepared by different producers using a microwave plasma CVD technique. The record breaking low values of loss tangent were observed (tan ~ 4.5.10-6 at T = 300 K, f = 170 GHz). However they are still one order of magnitude higher than the theoretical lower loss limit for diamond. Frequency and temperature dependencies of loss tangent in other materials with diamond-like lattice (Au-doped Si, compensated GaP, InP, GaAs, and Au-doped Si) were measured and analyzed for better understanding of the loss mechanisms. In addition, a semiconducting boron-doped CVD diamond has been also measured as a model material. The dielectric loss mechanisms and application to gyrotron windows are discussed.