Development and characterization of a new soft x-ray diagnostic concept for tokamaks

Diagnosing soft x-ray (SXR) emission from tokamaks represents a unique source of information, since it allows the study of several plasma parameters, such as the electron and ion temperature, the investigation of the ionization equilibrium, particle and impurities transport and the study of MHD fluctuations and disruptions. A new SXR diagnostic system called EXODUS (Enhanced X-ray Optimized Detector for Use in multiple Scenarios) is under development with the aim to obtain energy resolved SXR emission profiles from the plasma with a high time resolution (<0.1 ms). The system is based on the Gas Electron Multiplier (GEM) technology coupled with the new data acquisition system especially designed for GEM called GEMINI, which gives the possibility to obtain information about the energy deposited in the detector by the incoming radiation using the so called Time-Over-Threshold technique on each detector channel. The information of the deposited energy allows the study of the SXR emission from the plasma resolved in space, energy and time. There are several advantages in the use of GEM based detector in the harsh environment of a tokamak. First of all, it offers very high rate capabilities (up to 1 MHz/mm2), giving the possibility to obtain sub-ms time resolution together with sub-mm spatial resolution. The high counting rate and the high spatial resolution allow the installation of the detector in a pinhole camera very close to the tokamak vacuum barrier, producing very high statistic measurements. Finally, if properly designed, GEM detectors are intrinsically insensitive to gamma and neutron irradiation, allowing obtaining an excellent signal to noise ratio. In this contribution we describe the design and the production of the GEM based detector with the EXODUS system, together with the characterization of the detector response under quasi-monochromatic x-ray beam obtained using fluorescence of different materials. EXODUS electronics and its architecture allows the measurement of the energy of each detected photon and its time of arrival, resulting in a strong improvement of the energy and time resolution, compared to the old electronics used so far.