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Description
Heavy metallic first walls are attractive for fusion devices, but the impact of high-Z impurities in the plasma remains a concern. Recent design studies carried out for the STEP tokamak (planned for completion by 2040) have used a sequential approach with multiple simulation codes to model the radiative impact of tungsten (W) eroded by fast ions. This contribution presents the results of W transport and core radiation modelling.
The work focuses on two different design candidates for STEP modelled using the JETTO transport code as the primary modelling tool [1]. The simulations include the effects of the fusion product He and the seeded impurities Xe and Ar, which in total result in more than 300 MW of impurity radiation. The guiding-centre following ASCOT code has been applied to calculate the flux of fast ions on the first wall [2], and the erosion rate of W (~1e18 s$^{-1}$) is obtained by interpolating the erosion yields calculated using the SDTrimSP code. Finally, this total rate is used as a separatrix boundary source in the JETTO simulations to estimate the radiative impact of W.
The simulations show that in both designs, W radiation stays below 10 MW if the escape velocity of W is assumed to be above 0.5 m/s. Smaller escape velocities at the separatrix boundary increase the W radiation but are considered unlikely based on more extensive edge simulations carried out for the other impurities. The simulations suggest a difference in W screening between the two design candidates: in the larger configuration (R$_{\mathrm{geo}}=4.2~$m), W core density reduces towards the plasma centre, whereas in the smaller configuration (R$_{\mathrm{geo}}=3.6~$m), no such reduction is seen. These observations are discussed together with possible future steps to improve the predictions.
This work has been funded by STEP, a major technology and infrastructure programme led by UK Industrial Fusion Solutions Ltd (UKIFS), which aims to deliver the UK’s prototype fusion powerplant and a path to the commercial viability of fusion.
[1] E. Tholerus et al, Nucl. Fusion 64 (2024) 106030
[2] A. Snicker et al, Proceedings of the 2025 IAEA FEC Conference