High erosion and re-deposition rates of tungsten in the highly collisional plasmas of Magnum-PSI

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) 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 [1], 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 Ei ≈ 43 eV to Ei ≈ 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 found to be dominant over prompt re-deposition for the exposures investigated. Although the ionization length was long, high re-deposition rates were found, indicating that neutral-ion interactions were important. Thus, entrainment will be a crucial ingredient in the erosion and re-deposition studies of future fusion reactors.