Development of Hydrogen Retention Model Based on Plasma-Tungsten Interaction Analysis

The hydrogen retention model using hydrogen particle balance equation was developed to explain temporal and spatial variation of hydrogen plasma properties (n_e, T_e) near the plasma-facing material boundary. The balance contains the outflux of hydrogen ions from plasma to wall and the influx of hydrogen neutrals from wall to plasma. Especially, the model considered the plasma-facing area where hydrogen retention occurs during operation. Tungsten was selected as wall material because it is a representative metal boundary as well as a promising fusion plasma-facing material. The influx equations from wall to plasma is functions of desorption energy set (Edes), which represents the various hydrogen retention reactions. Hydrogen retention reactions in tungsten can occur as hydrogen solution, hydrogen oversaturation-induced vacancy trapping, implanted impurity-induced chemical trapping, physical damage-induced defect cluster trapping. The types of retention reactions are dependent on various plasma-wall interaction (PWI) conditions. The model was firstly constructed with the assumption that plasma properties can be changed by neutral gas influx from wall material because it changes boundary condition between plasma and wall. The influx is recycling flux, which is dependent on volume retention reactions of hydrogen in wall material because retention reactions decide amount of recycling flux and period of recycling. Thus, the purpose of hydrogen retention model is to expect the variation of plasma as functions of hydrogen retention reactions in volume of wall material. The volume retention reaction rate is governed by desorption energies (E_des) of specific set of retention reactions because different set of retention reactions are formed by different PWI conditions. Experiments to determine the hydrogen desorption energy (E_des) was in tungsten was performed with various PWI conditions as the deuterium plasma exposure onto tungsten, the deuterium plasma exposure onto carbon-implanted tungsten, the deuterium plasma exposure onto defect-formed tungsten, the gas-admixed (P_{He or Ar} ~10-20%) deuterium plasma exposure onto tungsten, the deuterium plasma exposure onto recrystallized tungsten. Plasma was consistently exposed onto tungsten with electron cyclotron resonance (ECR) plasma system. The deuterium was used as hydrogen isotope because it has higher measurement reliability of thermal desorption spectroscopy (TDS) than hydrogen. The set of desorption energies of specific retention reactions were obtained by using TDS. Because accurate measurement of desorption energy is the precondition for present work, the reliability of TDS was confirmed by international TDS round robin experiment (TDS-RRE). Hydrogen retention reactions in tungsten were figured out with corresponding desorption energies; the hydrogen solution (E_des,0: 0.75-0.95 eV), the hydrogen oversaturation-induced vacancy trapping (E_des,1: 1.84 eV), the implanted carbon impurity-induced chemical trapping (E_des,2: 2.33 eV), and the physical damage-induced defect cluster trapping (E_des,3: 2.39 eV). In terms of variation effect in fusion-relevant condition, both He ash and Ar puffing gas effects were indirectly understood by using admixing condition. However, both gases did not change the desorption energy (ΔE_des,i=0) compared to deuterium plasma case due to no formation of extrinsic trapping site. The effect of tungsten recrystallizationwas also analysed that can reduce hydrogen retention amount (ΔN_wall=30-50%) due to reduced fabrication-defects without the change of desorption energy (ΔE_des,i=0). For the consideration of ion incident energy, the dimensions of volume retentions were extended from subsurface (nm ~ μm) to bulk (μm ~ mm) depending on the implanted plasma ions (100 eV/D₂⁺), the impurity ions (300 eV/C⁴⁺), and the high energyions(2.8MeV/W²⁺).By using experimentally-obtained desorption energy data, the hydrogen retention model was constructed with long-term volume retention reactions and corresponding desorption energies. Based on the model with experimentally-obtained desorption energy data, validation to expect temporal plasma variation with wall recovery time was performed. The wall recovery time explains settling time of plasma property as functions of retention reactions and corresponding desorption energies. The validation experiment showed that the longer wall recovery time (0 ~ 14,400 sec) for the hydrogen retention conditions with higher desorption energies (0.75 ~ 2.39 eV). Thus, long-term volume retentions dominate the settling time of plasma property as a rate determining step. The spatial variation of plasma was observed with distance from wall to plasma, where the variation of plasma density occur by hydrogen recycling, is comparable to mean free path (MFP) between neutral particles (D₂). The variation of plasma by volume retention reactions cannot be explained or expected by using conventional particle balance equation, which considers wall as fixed boundary. In this dissertation, the extended particle balance was proposed with newly developed hydrogen retention model by considering volume retention reactions with dependence on plasma-wall interaction conditions. Therefore, the plasma-wall interaction must be considered to analyse wall as a transient boundary condition of plasma system