Design of soft-X-ray tomographic system in WEST using GEM detectors

Abstract

In metallic tokamaks, the interplay between particle transport and MagnetoHydroDynamic (MHD) activity might lead to impurities accumulation and finally to disruption. Studying such phenomena is thus essential if stationary discharges are to be achieved. Measuring the soft X-ray (SXR) radiation ([0.1 keV; 20 keV]) of magnetic fusion plasmas is a standard way of accessing valuable information on particle transport and MHD. Generally, like at Tore Supra (TS), the analysis is performed with a 2D tomographic system composed of several cameras equipped with silicon barrier diodes (SBD). On WEST the installation of an upper divertor masks many of the actual TS vertical diodes so that no proper tomography is possible. This paper presents the design of a new SXR diagnostic for the WEST project developed in collaboration with IPPLM (Poland) and the Warsaw University of Technology, based on a triple gas electron multiplier (GEM) detector. Preliminary simulations performed to size and position the detector and its electronics inside the vertical thimble are also presented, in particular estimation of magnetic field and temperature variation affecting GEM spatial resolution and signal quality. As a conclusion, perspectives about tomographic capabilities of the new system for studying impurity transport are given

Parallel computing in soft X‐rays plasma diagnostic systems for thermal fusion reactors—feasibility studies for GPUs

“…The objective of the plasma impurities diagnostic is to convert photons originating from plasma metallic content into a sampled electric signal and to digitally process it to obtain information on the time, energy, and location of the radiation. In a dedicated signal converter and amplifier called the gaseous electron multiplier (GEM), photons are converted as electron clouds , observed as a collection of electric impulses . The occurrences of the impulses , called events , are aggregated as valid data for further processing.…”

“…The diagnostic system with such functionality is currently deployed in the Tungsten Environment in Steady‐State Tokamak (WEST) reactor in Cadarache . It is dedicated to low‐latency measurements of soft X‐rays to investigate impurities in plasma.…”

“…It is dedicated to low‐latency measurements of soft X‐rays to investigate impurities in plasma. The device provides output signals allowing tomographic reconstruction of spatial radiation distribution . It acquires data at high rates and reduces their throughput at runtime.…”

“…In the back‐end, the signals from the detector must be associated with the original X‐ray photons. The GEM‐specific amplification converts each photon to an electron cloud . An occurrence of an electron cloud is subsequently detected as impulses .…”

Tomographic capabilities of the new GEM based SXR diagnostic of WEST

“…Unfortunately, most of the former SXR LoS of Tore Supra are lost due to the WEST upper divertor. Thus the design of the SXR diagnostic has been refurbished [5,6] in order to be adapted to the WEST configuration, see Fig. 1.…”