Performance of the W7-X ECRH gyrotron diamond output window at 2 MW operation

The current electron cyclotron resonance heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) is in the phase to increase the gyrotron output power from 1 MW to 1.5 MW and, last, up to the target of 2 MW by different steps. The higher heating power shall allow achieving operating regimes with high plasma beta and low collisionality, required by the planned further experimental campaigns. Previous numerical analyses verified the design of the gyrotron diamond output window up to 1.5 MW operation, with temperature dependent material properties. The window must be cooled by water, to assure significant temperature reserve margins. In this work, the investigation of the window performance was further carried on for the target of 2 MW operation. Computational fluid dynamics (CFD) conjugated heat transfer and structural analyses were performed. The sensitivity of the design was checked with respect to different combinations of loss tangent and mm-wave beam radius. The operational limits of the window were also checked. The current design features conical cuffs and a slightly different cooling path with respect to the past. For the first time, the complete window assembly and both the thermal and other operational loads were considered in the analyses. ... mehrThey showed that the window design can work also at 2 MW operation. The maximum loss tangent value of the diamond disk that might be still tolerated is 5.7 × 10$^{–5}$, corresponding to an absorbed power of 2008 W, with a beam radius of 20 mm. Last, going to higher absorbed powers in the disk, maximum temperatures can be significantly reduced by increasing the beam radius within the limits which are however imposed by the window aperture.