Deuterium Inventory in Tore Supra (DITS): 2nd post-mortem analysis campaign and fuel retention in the gaps

Abstract

A dedicated study on fuel retention has been launched in Tore Supra, which includes a D wall-loading campaign and the dismantling of the main limiter (Deuterium Inventory in Tore Supra, DITS project). This paper presents new results from a second post-mortem analysis campaign on 40 tiles with special emphasis on the D retention in the gaps. SIMS analysis reveals that only 1/3 of the thickness of deposits in the plasma shadowed zones are due to the DITS wall-loading campaign. As pre-DITS deposits contain less D than DITS deposits, the contribution of DITS to the D inventory is about 30–50%. The new estimate for the total amount of D retained in the Tore Supra limiter is 1.7 × 10²⁴ atoms, close to the previous estimate, with the gap surfaces contributing about 33%. NRA measurements show a stepped decrease of D along the gap with strong asymmetries between different gap orientations.

Introduction

High performance plasma discharges in modern magnetic confinement devices lead to the development of plasma facing components with castellated surfaces in order to reduce mechanical stress induced by heat loads or transient magnetic fields. However, the enlarged surface of these castellated plasma facing components raised concerns about the possibility of increased fuel retention. In order to assess the fuel retention in gaps and on castellated structures, several dedicated studies were performed (e.g. [1], [2], [3]). In most of these studies, the individual discharges lasted only a few seconds and no active control of the sample temperature during the discharges was attempted. This gap can be breached in Tore Supra as it can achieve long plasma durations combined with castellated, actively cooled plasma facing components (PFCs). The Deuterium Inventory in Tore Supra (DITS) project was launched to study fuel retention in true steady state from a plasma wall interaction point of view.

The DITS project consists of three phases: a dedicated campaign of repetitive discharges to load the walls with deuterium, the extraction of representative parts of the PFCs (namely one sector of the toroidal pump limiter (TPL), and tiles from different inboard and outboard protection PFCs), and an extensive analysis program in collaboration with European partners. Prior to the DITS loading campaign, ¹³C and ¹¹B marker layers were deposited inside the vacuum vessel via glow discharges, using a mixture of He and ¹³CH₄ or ¹¹B₂D₆, respectively. For the actual D wall-loading campaign, more than 160 long (>1 min), identical discharges were performed for a total of ∼5 h of plasma operation [4]. Gas balance measurements showed a constant, non-saturating D-retention rate of 1.7 × 10²⁰ D atoms s⁻¹ resulting in a total increment of the D vessel inventory of ∼3.1 × 10²⁴ D atoms [4].

An extensive post-mortem analysis program on the extracted PFC samples has been established to estimate the retained D. A first analysis campaign on 10 TPL tiles gave a first overview of the D content in the TPL and tested the technical procedures (e.g. sample cutting and handling, relevant analysis techniques) [5], [6]. A second analysis campaign, with 40 additional tiles has been started to address specific questions regarding retention and to broaden the statistical basis for the D quantification. Particularly, special care was taken for the detection of the ¹³C and ¹¹B marker layers, in order to identify the part of the D inventory specifically due to the DITS loading campaign and to allow a more accurate comparison between the amount of retained fuel deduced from post-mortem analysis and from particle balance measurements. This article presents first results from this second post-mortem analysis campaign with special emphasis on the contribution of the gap surfaces to the D retention.