Limiter observations during W7-X first plasmas

G.A Wurden,S Marsen,M Jakubowski,F Pisano,B Cannas,R König,H Niemann,S Brezinsek,F Effenberg,C Biedermann,O Schmitz,J Fellinger,J.H Harris,H.P Laqua,L Stephey,E.A Unterberg,S Bozhenkov

doi:10.1088/1741-4326/aa6609

Abstract

During the first operational phase (referred to as OP1.1) of the new Wendelstein 7-X (W7-X) stellarator, five poloidal graphite limiters were mounted on the inboard side of the vacuum vessel, one in each of the five toroidal modules which form the W7-X vacuum vessel. Each limiter consisted of nine specially shaped graphite tiles, designed to conform to the last closed field line geometry in the bean-shaped section of the standard OP1.1 magnetic field configuration (Sunn Pedersen et al 2015 Nucl. Fusion 55 126001). We observed the limiters with multiple infrared and visible camera systems, as well as filtered photomultipliers. Power loads are calculated from infrared (IR) temperature measurements using THEODOR, and heating patterns (dual stripes) compare well with field line mapping and EMC3-EIRENE predictions. While the poloidal symmetry of the heat loads was excellent, the toroidal heating pattern showed up to a factor of 2× variation, with peak heat loads on Limiter 1. The total power intercepted by the limiters was up to ~60% of the input ECRH heating power. Calorimetry using bulk tile heating (measured via post-shot IR thermography) on Limiter 3 showed a difference between short high power discharges, and longer lower power ones, with regards to the fraction of energy deposited on the limiters. Finally, fast heating transients, with frequency >1 kHz were detected, and their visibility was enhanced by the presence of surface coatings which developed on the limiters by the end of the campaign.

Highlights

During the first operational phase (OP1.1) of the new Wendelstein 7-X (W7-X) stellarator, specially shaped poloidal graphite limiters, each consisting of nine discrete tiles, served as the main plasma facing
Due to the helical symmetry with five modules in W7-X, there will eventually be on the order of 10–20 camera systems (2 or more IR cameras viewing in each module) to cover the entire vessel for long pulse operation, all with real-time analysis and large data streams
This paper focuses on high resolution infrared observations which enabled heat flux and power load measurements on the limiters

Summary

Introduction

Because the machine had a large surface area of exposed copper tiles, and no plasma exposed water-cooled components (yet), it was important to know that most of the power was either going to the 5 graphite limiters, or that it was being radiated away to the entire vessel. Infrared camera systems are used in magnetic fusion experiments, world-wide, to measure heat loads on vessel components [3,4,5,6]. Due to the helical symmetry with five modules in W7-X, there will eventually be on the order of 10–20 camera systems (2 or more IR cameras viewing in each module) to cover the entire vessel for long pulse operation, all with real-time analysis and large data streams. For OP1.1, we started with a more basic diagnostic set

Methods

Findings

Discussion

Conclusion