CVD Diamond Windows for Electron Cyclotron Resonant Heating in Fusion

Electron cyclotron resonant heating (ECRH) in nuclear fusion combines the injection of multi megawatt power with the possibility to focus the millimeter wave beam to small spot sizes of few centimeters. The ECRH allows efficient plasma heating as well as localized current drive for magneto-hydrodynamic stabilization of e.g. neoclassical tearing modes that create local magnetic islands with a fast loss of plasma confinement. One of the key challenges for high power ECRH systems is the provision of suitable confinement windows.
The low loss tangent, high thermal conductivity and outstanding mechanical properties qualify diamond as the state of the art material for high power millimeter wave heating systems in nuclear fusion devices. Artificial diamond disks are grown by CVD in diameters up to 180mm, sufficient for the usual waveguide diameters.
The principles of ECRH systems will be presented with a focus on the permittivity of diamond, which determines the loss tangent and suitable disk thicknesses. Further concepts and status for small and broadband windows for frequencies of 100-240GHz and beam powers up to 2MW will be discussed.