Simultaneous measurement of temperature and emissivity of lunar regolith simulant using dual-channel millimeter-wave radiometry

Abstract

Millimeter wave (MMW) radiometry can be used for simultaneous measurement of emissivity and temperature of materials under extreme environments (high temperature, pressure, and corrosive environments). The state-of-the-art dual channel MMW passive radiometer with active interferometric capabilities at 137 GHz described here allows for radiometric measurements of sample temperature and emissivity up to at least 1600 °C with simultaneous measurement of sample surface dynamics. These capabilities have been used to demonstrate dynamic measurement of melting of powders of simulated lunar regolith and static measurement of emissivity of solid samples. The paper presents the theoretical background and basis for the dual-receiver system, describes the hardware in detail, and demonstrates the data analysis. Post-experiment analysis of emissivity versus temperature allows further extraction from the radiometric data of millimeter wave viewing beam coupling factors, which provide corroboratory evidence to the interferometric data of the process dynamics observed. These results show the promise of the MMW system for extracting quantitative and qualitative process parameters for industrial processes and access to real-time dynamics of materials behavior in extreme environments.

High Temperature Millimeter Wave Radiometric and Interferometric Measurements of Slag-Refractory Interaction for Application to Coal Gasifiers

“…This "active" returned thermal probe signal then reflects off the sample with an amplitude that depends on the sample reflectivity (and hence emissivity) [7]. By appropriate combination of the signals from a dual-receiver system, the emissivity can be continuously monitored as a function of temperature, as been recently described [8].…”

“…At normal incidence, the signal reflects off the object of interest, returns via the same channel through the horn and into a mixer where it is converted to a direct current (DC) signal ("video"), is routed through a lock-in amplifier to detect the chopper-modulated part, then monitored by a computer through a general purpose interface bus (GPIB). A more detailed description of the dual-receiver MMW system can be found elsewhere [8]. figure) through the ceramic, brass, and aluminum waveguides.…”

“…The analytical basis for the thermal return reflection (TRR) method for resolving emissivity and temperature in radiometric measurements has been previously discussed for singlereceiver [7] and dual-receiver [8] systems. Briefly, the radiometer, operating in the MMW region, is in the classical limit of blackbody radiation, where the power radiated is linearly related to the temperature.…”

“…1) [8,9]. The heterodyne receiver system consists of four channels: two receiver channels and two "video" channels.…”

“…The expressions used to compute the sample temperature and emissivity have been derived elsewhere [8]. Briefly, the sample emissivity (ε s ) is given by…”