Thermal Balance Test of the STIX DEM for ESA Solar Orbiter Mission

ESA Solar Orbiter mission is going to work on elliptical orbit around the Sun with perihelion of 0.28AU and aphelion of 0.952AU. Its main goal is to determine how Sun creates and controls the heliosphere. The Solar Orbiter spacecraft consists of ten different instruments one of which is the STIX instrument – Spectrometer/Telescope for Imaging X-rays. The STIX instrument is divided into three subsystems: (i) feedthrough with two X-ray windows, (ii) the grids with aspect system and (iii) detector and electronic module (DEM). The subsystems operate in two different thermal environments. The grids and DEM are located inside the spacecraft, while the feedthrough is surrounded by the heatshield and one of the windows is directly expose to the Sun. The spacecraft interior temperature is kept at 50°C and -20°C in hot and cold operational modes respectively, while the CdTe detectors located inside the DEM are kept around -20°C by a cold element provided by the spacecraft. Main objective of the STIX DEM STM 2.0 thermal balance tests was to verify the design of electronics unit with respect to the heat flow budget and resulted temperatures. The most critical issue was the insufficient MLI performance, which was the main reason of too high heat flux through the cold interface. The paper presents the modifications done to DEM STM, new MLI design, influence of the integration process (in particular MLI assembly), thermal testing phase in vacuum chamber with new heat flux sensor and two constant temperature interfaces ensuring appropriate environment, which lasted around 2 months. The tests were successful and resulted in valuable integration process conclusions and provided necessary data to correlate thermal model in ESATAN software.