Gerald Pintsuk

Prompt re-deposition strongly reduces the migration of eroded material in current fusion devices, thus lowering core dilution and extending the lifetime of the plasma-facing components. However, under the high-density ( n e > 2 × 10 20 m⁻³), low-temperature ( T e < 5 eV) divertor plasma conditions anticipated for future fusion devices like ITER, Coulomb collisions become highly efficient. These collisions not only prevent the ions from completing their Larmor orbits but also strongly couple the sputtered impurities to the plasma, leading to impurity entrainment. Entrainment drags impurities with the plasma flow to the surface, leading to enhanced impurity impact energies as discussed in the companion paper (Cornelissen M.J.H. et al 2025 Nucl. Fusion 65 026009), and the re-deposition of sputtered particles. This entrained re-deposition was studied with the linear plasma generator Magnum-PSI, whose linear geometry allowed for a clear distinction between entrained and prompt re-deposition. Five W targets were exposed to high-density ( n e ≈ 5 × 10 20 m⁻³), low-temperature ( T e ≈ 1.2 eV) argon (Ar) plasmas. The deposition pattern of tungsten (W) on molybdenum (Mo) witness plates was determined using ion beam analysis and electron scanning microscopy. As the ion-impact energy of Ar⁺ increased from E i ≈ 43 eV to E i ≈ 94 eV, the local re-deposition rate decreased from R ≈ 93% to R ≈ 68%. Entrained re-deposition became evident from an up-downstream asymmetry in the re-deposition profile and was dominant over prompt re-deposition for the exposures investigated. Although the ionization length was long, high re-deposition rates were found, revealing that neutral-ion interactions were important. Thus, entrainment will be a crucial ingredient in the erosion and re-deposition studies of future fusion reactors.

The Joint European Torus (JET) fusion reactor was upgraded to the metallic wall configuration in 2011 which consisted of bulk beryllium (Be) tiles in the main chamber and bulk tungsten (W) and W-coated CFC tiles in the divertor (Matthews G.F. et al 2011 Phys. Scr. T148 014001). During each campaign, a series of wall damages were observed; on the upper dump plates (UDP) positioned to the top part of the vessel walls and on the inner wall—mainly affecting the inner wall guard limiters (IWGL). In both cases, it was concluded that the causes of these damages were unmitigated plasma disruptions. In the case of JET with the metallic wall configuration, most of these plasma disruptions were intentionally provoked. The overall objective was to study the behaviour of these phenomena, in order to assess their impact on the wall, improve understanding of morphological material changes, and—based on that—to develop, implement and test mitigation techniques for their prospective use on ITER. The current results bring additional information on the effects of the unmitigated plasma disruptions on the UDPs and are a significant extension of work presented in (Jepu et al 2019 Nucl. Fusion 59 086009) where the scale of the damage after three operational campaigns on the Be top limiters of JET was highlighted. In addition, new data is presented on the damaging effect that the high energetic runaway electrons had on the Be IWGL in JET.