Analysis of transmission characteristics for single and double disk windows

Electron cyclotron resonance heating (ECRH) is one of the most important candidates for plasma heating, non-inductive current drive, plasma start-up, profile control and MHD stabilization in the next generation of fusion plasma machines such as the ITER tokamak. Recent interest of the fusion community for radiation at several frequencies from the same gyrotrons with step tunability stimulates gyrotron development in this direction. Recent results showed the advantage of using frequency tunable gyrotrons for ECRH plasma experiments. It is intended to use step-tunable gyrotrons (105-140 GHz) for the new ASDEX-Upgrade ECRH system. An ECW system being able to operate at two different frequencies, 113.3 and 170 GHz, has been considered within the JET EP program. The geometry of the window unit has been optimized in order to keep the window reflections low within a bandwidth around the chosen frequencies or over a wide-angle range around the Brewster angle.