The W7-X ECRH gyrotron diamond output window: Oil and water cooled window performance at 1.5 MW operation

The 140 GHz 1 MW gyrotron (Thales TH1507) for the electron cyclotron resonance heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) is being upgraded to 1.5 MW continuous wave operation to increase the total heating power for achieving operating regimes with high plasma beta and low collisionality. The design of the new 1.5 MW gyrotron (Thales TH1507U) has been significantly improved to the new power target. It features a chemical vapor deposition (CVD) diamond output window cooled by the silicon oil Dow Corning 200(R) 5cSt, in direct contact with the circumference of the diamond disk. ... mehrThe oil has been selected to prevent any corrosion of the brazing material between diamond disk and copper cuffs. Previous analyses verified the suitability of the window design for both, operation at 1 MW and at 1.5 MW if assuming constant properties for diamond and copper. In this work, the window is investigated by computational fluid dynamics conjugated heat transfer and structural analyses with temperature dependent material properties to assess its performance if cooled by water and oil at different absorbed powers in the disk. Temperature dependent properties are assumed as, at those power levels, the disk achieves temperature ranges at which, the reduction of the diamond thermal conductivity becomes remarkable. Otherwise, the maximum achieved temperatures are deeply underestimated. Both water- and oil-cooled window cases work up to about 1.2 kW absorbed power in the disk, with temperatures and stresses lower than the maximum allowed limits. However, it is of fundamental importance to assure high temperature reserve margins in the window to counteract any potential degrading factor of the disk quality from microwave transmission perspective. Water-cooling represents the only solution as it provides significant margins, even at higher values of absorbed power. In the worst-case scenario of 1.5 kW absorption, the maximum temperature in diamond is 173 °C while the oil-cooling option would lead to temperatures well above the 250 °C design safe limit.