Physics drivers for the plasma-facing component design of the COMPASS-U tokamak

Abstract The main drivers for the metallic plasma-facing component (PFC) design of the COMPASS Upgrade (COMPASS-U) tokamak ( B T ⩽ 5 T, I P ⩽ 2 MA, R geo = 0.9 m, a geo = 0.27 m, t flattop ⩽ 3 s) derived from physics considerations are presented in this paper. Requirements on the plasma-wall gaps are given based on previous experimental observations in other devices. Gaps at the inner/outer midplane are set to minimum values 20 and 40 mm, respectively, to allow all magnetic equilibria in the lower and upper single-null configurations to fit in the PFC outline, to allow an efficient flow in the scrape-off layer towards the divertor and to minimize the impurity flux from the wall. The closed tungsten divertor was designed with a similar shape to ITER’s divertor since COMPASS-U has a similar aspect ratio and uses an ITER-like equilibrium for its baseline scenario. Steady-state thermal loads are calculated at the outer strike-point, where they are expected to be the highest, in attached conditions using a 0D energy balance (⩾150 MW m −2 ) and in detached conditions using BIT1 kinetic calculations (∼70 MW m −2 ). Transient thermal loads range from 1.2 MJ m −2 for ELMs (maximum parallel peak energy fluence at targets) to 0.6–1.2 GW m −2 in ∼0.5 ms for high-energy vertical displacement events (maximum perpendicular heat flux density). Electromagnetic loads are calculated during disruptions using an in-house parametric model, accounting for current quench (CQ) and halo current contributions and validated by simulations using the CarMa0NL code, which is able to describe the nonlinear evolution of axisymmetric plasmas in the presence of 3D volumetric structures. The CQ rate is expected to be at least δI P / δt = 3 MA ms −1 , and the halo current magnitude is set to 75% of the pre-disruptive maximum plasma current, according to the ITPA disruption database, yielding I halo = 1.5 MA. Maximum total force acts on the bottom divertor baffle with a value of ∼60 kN and the corresponding total torque is ∼4 kN m.