Power load limits of the WENDELSTEIN 7-X target elements—comparison of experimental results and design values for power loads up to the critical heat flux

The power load limits of the WENDELSTEIN7-X divertor target elements were experimentally evaluated with heat loads considerably exceeding the expected operating conditions. The water-cooled elements are designed for steady-state heat flux of 10 MW m(-2) and to remove a power load up to 100 kW. The elements must allow a limited operation time at 12 MW m(-2) steady-state and should not fail for short pulses of up to 15 MW m(-2) for cooling conditions in the subcooled nucleate boiling regime. In the framework of the qualification phase, pre-series target elements were loaded up to 24 MW m(-2) without loss of CFC tiles. A critical heat flux at the target of 31 MW m(-2) was achieved. The paper discusses the results of the tests performed at the high heat flux test facility GLADIS. The experimental results compared to transient nonlinear fine element method (FEM) calculations confirm a high thermal safety margin of the target design sufficient for plasma operation in W7-X.