Seinälämpötilan vaikutus kaasudynamiikkaan JET:in subdivertorissa

Tokamak-type fusion reactors are a possible source of abundant clean energy, but among other issues, there is still incomplete understanding of the vacuum pumping systems. To control the plasma density and, in future fusion power plants, to remove helium ash, neutral particles are removed by a cryopump through a structure called a subdivertor. The pumping needs to be well understood, efficient, and as controlled as possible. In this thesis, the impact of the wall temperature on the pumping and the conditions in the subdivertor is investigated. The study is conducted with a Boltzmann-equation Monte Carlo solver called EIRENE. To verify the simulations, the conductance calculated by EIRENE in a simplified simulation is compared to analytical solutions with ideal gas assumptions. The simulations are conducted in a toroidally symmetric approximation of the subdivertor geometry in JET with uniform wall temperatures, with the exception of a cool pump surface. The simplified geometry for EIRENE validation is a 2 m×1 m×9 m box with the source on the bottom, the pump at the top, and a thin aperture in the middle. Both uniform and non-uniform wall temperatures are tested. The results demonstrate that when the wall temperature is increased from 373 K to 1160 K, the pressure and the pressure gradient near the louvres increase by a factor of approximately 1.7. The difference between the wall and cryopump temperatures also affected the pressure and caused non-isothermal conditions. The EIRENE simulations in the simplified geometry produce conductance estimates that differ from the analytical results by a constant factor of approximately 1.35 for small apertures. With large apertures, the values exceed the analytical solution by up to a factor of 2. The gas temperature, and consequently the wall temperatures, are an important factor when comparing pressure values. Also, pressure gauges in experimental reactors should be coupled with temperature sensors as temperature greatly affects the measured values. EIRENE provides values on conductance in restricted geometries and conditions within a factor of 1.35, but further studies are required to understand the constant factor in relation to analytical solutions