Investigations on CO$_2$ plasma jets with the ICOTOM radiometers of the EXOMARS descent module.
Résumé
Introduction: The back heat shield of the Exo-mars descent module Schiaparelli, that entered the Mars atmosphere on October 19, 2016, was equipped with three COMARS+ packs located on a radial line. Each COMARS+ pack contains a heat flux sensor, a pressure gauge and two CNES infrared radiometers called ICOTOM. Each ICOTOM radiometer col-lects light from the back entry plasma in a 17.5° half-angle cone. Each pair is composed of a radiometer operating in the range 4.17 – 5 µm (B1) and a radiom-eter operating in the range 2.6 – 3.36 µm (B2). Some of the radiometers designed for the Exomars missions were not used for the mission on board but in laborato-ry wind tunnels in order to help to characterize the emission of CO2 plasmas and to better understand the Schiaparelli measurements during the entry phase.
Calibration: Calibration experiments were carried out on non-flying ICOTOM in order to use them in plasma wind tunnels and to study their measuring be-haviour with their own temperature. Those measure-ments were crosschecked with manufacturer's calibra-tion. Manufacturer's data were also post-processed in order to obtain direct flux density information from the ICOTOM electrical signal collected from the ground-based plasmas and to make ready the recep-tion of the in-flight measurements.
The experiments carried out with a blackbody-like electrical furnace as infrared source confirmed that the ICOTOM signals decrease with their own temperature and that effect is especially significant for the B1 band. Large differences can then be observed on the output electrical signal depending on the housing tem-perature.
Experiments with plasmas: Embedded in a sub-sonic inductively-coupled plasma wind tunnel, the radiometers were exposed to CO2 plasma at various pressures and global specific enthalpies. The experi-ments carried out on the wind tunnel SOUPLIN [3] allowed to expose the ICOTOM to CO2 inductively-coupled plasmas. Pressures were included in the range 1.2-12 kPa whereas global specific enthalpies were included in the range 5.5-13 MJ/kg. The radiometers were placed in a cooled holder, perpendicularly to the plasma jet, on the border of it. The cooling system allowed to controlled the ICOTOM temperatures. In order to analyze, the radiation received by the ICOTOM inside the plasma, complementary meas-urements were carried out in order to characterize the plasma in terms of densities and temperatures.
The radiative heat flux received by the ICOTOM radiometers depends mainly on the dissociation rate of CO2. That parameter controls the temperature of the plasma as well as the CO and O densities, and then the radiative transfer from the plasma to the detectors. So, the analysis of the ICOTOM signal includes the re-building of that signal through the measurements of CO and O densities and temperatures. In order to per-form a mapping of the plasma (assumed to be ax-isymmetric), laser measurements (spontaneous Ra-man scattering and two-photon laser-induced spec-troscopy) were conducted within the view cone of the ICOTOM signal.
Results: The results obtained show a significant in-crease of the ICOTOM signals with pressure especially in the lower part of the pressure range. The signals also increase with the global specific enthalpy but very slightly, following the opposite effect of a higher tem-perature and a higher CO2 dissociation. The ratio B1/B2 strongly decreases with pressure while it slightly decreases with global specific enthalpy. The signals and the ratio stabilize at higher pressures.
Calculations: Some calculations were carried out with CDSD4000 and HITEMP databases at equilibri-um in order to compare the measured ratio with refer-ence conditions and black body rediation. In order to rebuild the ICOTOM signals, measurements were in-cluded in a radiative transfer code. Final comparison between calculations and experiences are on progress.