Design of the torus interface for the neutral beam injectors of Wendelstein 7-X

Abstract

A neutral beam injection (NBI) heating system for the stellarator project Wendelstein 7-X (W7-X) is currently under construction at the Max Planck Institute for Plasma Physics (IPP) Greifswald, Germany. The NBI at W7-X is based on the similar system in operation on the tokamak ASDEX Upgrade at IPP in Garching, Germany, and makes use of radio-frequency-driven positive ion sources. Two NBI injectors are going to be installed in W7-X, each being capable of hosting four ion sources. Initially each injector is being equipped with two ion sources of 1.8 (2.5)MW beam power each in H (D). Accelerated positive ions are going to be neutralized and transported inside an NBI-box, which also hosts a residual ion dump, titanium sublimation pumps and a calorimeter. The beams are focused at a point 6.5m downstream of the ion sources and are injected into the plasma vessel of W7-X through two ports. The NBI boxes and their NBI ports are connected by NBI–torus interfaces, which are made of several components that minimize beam losses while protecting the port walls and bellows from energetic re-ionized particles and beam intersection. This paper describes the design of the NBI–torus interface, including the shape optimization of a beam scraper and the integration of beam diagnostics.