Shuyu Dai

Modelling of local carbon deposition on a rough test limiter exposed to the edge plasma of TEXTOR

A Monte-Carlo code called SURO was developed to study the influence of surface roughness on the impurity deposition characteristics in fusion experiments. SURO uses the test particle approach to describe the impact of background plasma and the deposition of impurity particles on a sinusoidal surface. The local impact angle and dynamic change of surface roughness as well as surface concentrations of different species due to erosion and deposition are taken into account. Coupled with the three-dimensional Monte-Carlo code ERO, SURO was used to study the impact of surface roughness on 13C deposition in 13CH4 injection experiments in TEXTOR. The simulations showed that the amount of net deposited 13C species increased with surface roughness. Parameter studies with varying 12C and 13C fluxes were performed to gain insights into impurity deposition characteristics on the rough surface. Calculations of the exposure time needed for surface smoothing for TEXTOR and ITER were also carried out for different scenarios.

Modelling of surface roughness effects on impurity erosion and deposition in TEXTOR with a code package SURO/ERO/SDPIC

The roughness-induced uneven erosion–deposition behaviour is widely observed on plasma-wetted surfaces in tokamaks. The three-dimensional (3D) angular distribution of background plasma and impurities is expected to have an impact on the local erosion–deposition characteristic on rough surfaces. The investigations of 13C deposition on rough surfaces in TEXTOR experiments have been re-visited by 3D treatment of surface morphology to evaluate the effect of 3D angular distribution and its connection with surface topography by the code package SURO/ERO/SDPIC. The simulation results show that the erosion/deposition patterns and evolution of surface topography are strongly affected by the azimuthal direction of incident flux. A reduced aspect ratio of rough surface leads to an increase in 13C deposition due to the enhanced trapping ability at surface recessions. The shadowing effect of rough surface has been revealed based on the relationship between 3D incident direction and surface topography properties. The more realistic surface structures used by 3D SURO can well reproduce the experimental results of the increase in the 13C deposition efficiency by a factor of 3–5 on a rough surface compared with a smooth one. The influence of sheath electric field on the local impact angle and resulting 13C deposition has been studied, which indicates that the difference in 13C deposition caused by sheath electric field can be alleviated by the use of more realistic surface structures. The difference in 13C deposition on smooth graphite and tungsten substrates has been specified by consideration of effects of kinetic reflection, enhanced physical sputtering and nucleation.

A general model for chemical erosion of carbon materials due to low-energy H+ impact

Modeling the chemical erosion of carbon materials due to low-energy H+ impact is of paramount importance for the prediction of the behavior of carbon-based plasma-facing components in nuclear fusion devices. In this paper a simple general model describing both energy and temperature dependence of carbon-based chemical erosion is presented. Enlightened by Hopf’s model {Hopf et al., [J. Appl. Phys. 94, 2373 (2003)}, the chemical erosion is separated into the contributions from three mechanisms: thermal chemical erosion, energetic chemical sputtering, and ion-enhanced chemical erosion. Using input from the Monte Carlo code TRIDYN, this model is able to reproduce experimental data well.

Kinetic modelling of material erosion and impurity transport in edge localized modes in EAST

The kinetic modelling of material erosion and impurity transport on divertor plates in edge localized modes (ELMs) in tokamaks has been performed using the SDPIC and ITCD codes. The kinetic information on impinging particles and background plasma such as incident angle and energy, electric potential and field during ELMs can be well treated by the SDPIC code; this is generally difficult to obtain in experiments and fluid simulations due to diagnostic and technical limitations. The heat flux modelled with the SDPIC code can well reproduce the experimental result in EAST. The major power delivered by ions results in a strong increase of the erosion rate for both carbon and tungsten substrates during ELMs. The variation of the incident angle of impinging particles can be interpreted by the change of the potential drop induced by fast electrons during ELMs. The time difference in the peak values of erosion rate between the carbon and tungsten substrates is determined by the particle flux and sputtering yield. The heat loads for present ELMs in EAST cannot cause the ablation and melting of surface material. The eroded tungsten species show different transport properties from the eroded carbon species due to the small mean free path of W atoms and consequent prompt redeposition. A W self-sputtering avalanche is not observed during ELMs, ascribed to the small kinetic energy carried by the redeposited W species.