Design of a Vacuum Pumping System for the Closed Helical Divertor for Steady State Operation in LHD

Abstract

A vacuum pumping system is installed in a Closed Helical Divertor (CHD) in the Large Helical Device (LHD) at the National Institute for Fusion Science for active control of the peripheral plasma density and impurity suppression in the core plasma. In the CHD configuration, the distance between the pumping system and the divertor plates (heat and particle source) is very short (only ∼0.1 m). One of the major issues in designing the pumping system is the reduction of heat load by radiation and thermal conduction due to the neutral particles being released from the heated divertor plates while keeping a high pumping efficiency. Here the heat load and the pumping efficiency are analyzed using a neutral particle transport simulation and a finite element method based software for multi-physics analysis. We propose a new design for a pumping system with an expanded area of the inlet of the water-cooled blinds and a bottom slit beneath the pumping system. This increases the pumping efficiency by approximately 60% over that of our previous design. It also predicts that the increase in heat load on the pumping system for the new design would be reasonably suppressed by a buffer plate with high emissivity on the surface of the vacuum vessel on the inboard side of the torus.