Abstract
The central column of the COMPASS Upgrade tokamak, where the plasma discharges will be started and shut down, is covered by a continuous first wall composed of 8 bulk tungsten guard limiters with recessed Inconel tiles in between. In diverted configurations, the plasma will be limited for a short duration (similar to 0.2 s-0.4 s) during the plasma current ramp-up and ramp-down phases but high heat fluxes due to short power decay lengths are foreseen.
An adapted shaping is therefore needed to spread the heat loads over a maximum area. A novel method taking into account the different magnetic equilibria and the incident angle of the field lines was used to design a roof shape able to ensure a constant heat flux on the toroidal direction, accounting for the lambda(gnear-SOL) and lambda(gfar-SOL) Due to the diversity of plasma scenarios considered on COMPASS-U, the optimized shape of the inner wall tiles strongly differs from one scenario to another, thus a compromise has to be found among all foreseen scenarios.
The tile shaping also needs to take into account possible short wave misalignments. Calculations with the 3D magnetic field line tracing code PFCFlux validates the design, confirming the constant toroidal profile of deposited heat flux for the COMPASS-U workhorse scenarios, or slightly decreasing or increasing heat flux profiles (from the center to the sides of the tile) for scenarios with smaller or higher lambda(gnear-SOL) respectively, within the expected large range (1.7 mm<lambda(gnear-SOL)<5 mm), but still providing acceptable loads.
Estimated heat fluxes are in the range of 5 MW/m(2) to 35 MW/m(2) on the 8 guard limiters, receiving 70%-80% of the power deposited to the inner wall. In case of shortwave misalignments up to 0.5 mm in the radial direction, calculations show that leading edges are still protected, confirming the robustness of the design.