Erosion of hydrocarbon films at room temperature due to argon ions and thermal atomic hydrogen is investigated in a particle-beam experiment. Physical sputtering by the ions is observed at energies ⩾200 eV and reaches a yield of 0.5 at an ion energy of 800 eV. The measured yields are in agreement with TRIM.SP computer simulations, and a threshold energy of ≃58 eV is derived for physical sputtering. Erosion by simultaneous fluxes of argon ions and thermal hydrogen atoms is observed at all energies investigated down to 20 eV and reaches a yield of about 3 at an ion energy of 800 eV and a hydrogen-atom-to-argon-ion-flux ratio of 400. It is proposed that the significant decrease of the threshold energy as well as the increase of the absolute yields is due to the process of chemical sputtering: Within a collision cascade caused by the incident ions, bonds are broken and instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the common range of ions and hydrogen atoms. Finally, the molecules diffuse to the surface and desorb. The threshold energy of chemical sputtering is on the order of typical carbon–carbon bond energies in organic compounds of several eV. Based on this mechanism a model for the energy dependence of the chemical sputtering yield is presented, which leads to good agreement with the data.

Wendelstein 7-X is the first comprehensively optimized stellarator aiming at good confinement with plasma parameters relevant to a future stellarator power plant. Plasma operation started in 2015 using a limiter configuration. After installing an uncooled magnetic island divertor, extending the energy limit from 4 to 80 MJ, operation continued in 2017. For this phase, the electron cyclotron resonance heating (ECRH) capability was extended to 7 MW, and hydrogen pellet injection was implemented. The enhancements resulted in the highest triple product (6.5 × 1019 keV m−3 s) achieved in a stellarator until now. Plasma conditions [Te(0) ≈ Ti(0) ≈ 3.8 keV, τE > 200 ms] already were in the stellarator reactor-relevant ion-root plasma transport regime. Stable operation above the 2nd harmonic ECRH X-mode cutoff was demonstrated, which is instrumental for achieving high plasma densities in Wendelstein 7-X. Further important developments include the confirmation of low intrinsic error fields, the observation of current-drive induced instabilities, and first fast ion heating and confinement experiments. The efficacy of the magnetic island divertor was instrumental in achieving high performance in Wendelstein 7-X. Symmetrization of the heat loads between the ten divertor modules could be achieved by external resonant magnetic fields. Full divertor power detachment facilitated the extension of high power plasmas significantly beyond the energy limit of 80 MJ.

The surface loss probabilities of hydrocarbon radicals on the surface of amorphous hydrogenated carbon (C:H) films are investigated by depositing films inside a cavity with walls made from silicon substrates. This cavity is exposed to a discharge using different hydrocarbon source gases, and particles from the plasma can enter the cavity through a slit. The surface loss probability β is determined by analysis of the deposition profile inside the cavity. This surface loss probability corresponds to the sum of the probabilities of effective sticking on the surface and of formation of a non-reactive volatile product via surface reactions. By comparing the deposition profiles measured in CH4, C2H2 and C2H4 discharges one obtains for C2H radicals β = 0.90 ± 0.05 and for C2Hx>2 radicals β = 0.35 ± 0.1, whereas the surface reaction probability for CH3 is below 10 −2 , as known from the literature. The growth rate of C:H films is, therefore, very sensitive to any contribution of C2Hx species in the impinging flux from a hydrocarbon discharge. The very same growth precursors can be formed in a divertor plasma and should therefore dominate the formation of re-deposited layers. A scenario for the occurrence of these re-deposited films in fusion experiments on the basis of typical divertor plasma and surface parameters is being discussed. Strategies are proposed for prevention of these re-deposited layers and for their removal.

The evolution of carbon/boron deposition and the deuterium inventory were determined during the transition from a carbon dominated to a full tungsten ASDEX Upgrade. In the carbon dominated machine about 17 g of carbon were deposited at the inner divertor and in remote areas during one standard discharge campaign. Main carbon sources were the ICRH antennae protection limiters in the main chamber. After coating these limiters with tungsten the carbon deposition decreased to 35 g. The remaining carbon originated mainly from erosion at the outer divertor strike point. Transition to a full tungsten machine resulted in a further decrease of the carbon deposition to about 1 g. 1.31.7 g deuterium was trapped in codeposited carbon/boron layers in the divertor and in remote areas during the carbon dominated campaigns. The deuterium inventory decreased to 0.140.22 g in the full tungsten machine.

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