Kaercher, A.MLZ*

2026

Abstract: his dissertation aims to fill the gap in experimental evidence by first investigatingthe influence of material properties and He plasma parameters on the He-W inter-action broadly and then isolating a parameter variations directly linked to the Heinteraction mechanisms of interest.For this, recrystallised polycrystalline tungsten (polycrystalline tungsten (poly-W)) samples, undamaged and irradiation-damaged to varying doses, were first usedin several exposure series screening the influence of He flux, fluence, energy, andpre-existing defects. To optimise the surface preparation for single-crystalline tung-sten (single-crystalline tungsten (sc-W)), open-volume defect concentrations in thenear-surface region of the samples were characterised by Doppler-broadening spec-troscopy of the positron annihilation line (Doppler-broadening spectroscopy of thepositron-electron annihilation line (DBS)). In subsequent exposures, sc-W was ex-posed alongside poly-W reference samples to isolate the effects of He clusteringmechanisms from pre-existing lattice defects and impurities. The He retention in allsamples was quantified using elastic recoil detection analysis (ERDA). For selectedsamples, high-resolution depth profiles of the He concentration were obtained bysuccessive surface removal via anodic oxidation between ERDA measurements.Both undamaged and pre-damaged poly-W samples were exposed at a sub-strate temperature of 300 K to He plasma with fluences ranging from 1×1019 to1×1023 He/m2, fluxes from 1×1017 to 5×1019 He/m2s, and energies of 50 , 100 , and200 eV. Several undamaged single-crystalline tungsten (sc-W) samples were ex-posed to He plasma with He energies of 50 and 100 eV, fluxes between 1×1017and 5×1019 He/m2s, and fluences of 1×1021 and 1×1022 He/m2. Two additionalelectron-beam irradiated (e-beam)-irradiated sc-W samples were exposed at low flux(1×1017 He/m2s) and 50 eV to a fluence of 9×1021 He/m2.At medium He fluxes (∼1×1018 He/m2s) and 100 eV, the total He retention inboth pre-damaged and undamaged poly-W samples increased linearly with fluenceuntil the He uptake flattened before reaching saturation values around 1×1019 He/m2.Pre-damaged samples reached saturation at two orders of magnitude lower fluencethan the undamaged samples. The saturation behaviour also depended on the Heflux: at high fluxes (>1×1019 He/m2s), undamaged samples saturated earlier andthe saturation value was higher independent of damage dose or intrinsic defects. Atlow flux (1×1017 He/m2s), in contrast, none of the poly-W samples saturated up tothe He fluence of 1×1022 He/m2.With increasing He flux, He retention rose significantly, particularly in undam-aged samples. Undamaged sc-W samples exposed to 50 eV exhibited the strongestdependence on He flux with a threshold behaviour: at low to medium fluxes(≤1×1018 He/m2s), no measurable amounts of He were retained, whereas a sharpincrease to levels comparable to pre-damaged poly-W occurred at fluxes of1×1019 He/m2s. The He retention in undamaged poly-W at low He flux in turn ex-hibited threshold behaviour when the He energy was increased from 50 eV to 100 eV.For sc-W at low flux, the change in He energy from 50 to 100 eV likewise had a de-cisive but less substantial effect on He retention: the measured He retention was notdetectable at 50 eV and was small but significant at 100 eV.He concentration depth profiles showed the highest concentrations and the ma-jority of all retained He within the first few nm below surface. Surface-near Heconcentration increased with the increase in defect concentrations, He flux, He flu-ence, and He energy. High damage doses inhibited measurable He accumulationbeyond 34 nm in depth. Increasing fluence mainly raised the near-surface He con-centration but not the penetration depth at medium flux and 100 eV. Beyond 30 nm,undamaged samples showed flat He concentrations of 0.02 to 0.03 at.%, suggestinga baseline trap level. The depth profile of an undamaged sc-W sample after high-flux exposure showed retention confined to the implantation zone, while the poly-Wprofile exhibited a markedly deeper distribution.The systematic exposure variations yielded clear experimental evidence for Heself-trapping in sc-W: the threshold behaviour of He retention in sc-W betweenthe He fluxes of 1×1018 He/m2s and 1×1019 He/m2s at a low He energy of 50 eV.This behaviour is attributed to concentration-dependent He–He interactions thatform growing clusters in interstitial sites until they become large enough to displaceW atoms and become immobilised in the resulting vacancies. The near-identicalretention values of all exposed samples at high flux, independent of the initial defectconcentration, demonstrate that self-trapping dominates He retention under theseconditions.Different dominant retention mechanisms were identified for the respective fluxregimes: at low flux, impurity trapping and vacancy occupation dominate. At me-dium flux, interactions of He with impurities increase retention in poly-W throughtemporary binding while He–vacancy clusters trapped in pre-existing defects growvia trap mutation until a He saturation is reached. At high flux, high He concentra-tions form in the implantation volume of all W substrates, leading to self-trapping as the dominant mechanism. Subsequent trap mutation contributes to the developmentof a He-saturated zone.Impurity-aided He trapping is postulated as the explanation for enhanced Heretention in poly-W relative to sc-W at fluxes below 1×1019 He/m2s. Depending onHe flux and energy, the underlying processes are temporary immobilisation of He atimpurity defects and the kinetic displacement of impurities from occupied sites. Incontrast to the limitation of He trapping in sc-W to the implantation zone, the lowHe concentrations deeper in poly-W are attributed to a permanent immobilisationof He clusters at impurity defects.

Keyword(s): Engineering, Industrial Materials and Processing (1st) ; Nuclear Physics (2nd)

1. NEPOMUC (NEPOMUC)

1. No specific instrument