A waveguide-coupled thermally-isolated radiometric source

TL;DR

This paper presents a novel waveguide-coupled thermal source with high emissivity and thermal isolation, suitable for cryogenic millimeter and sub-millimeter wave sensors, demonstrating reliable performance over a broad frequency band.

Contribution

The design introduces a thermally isolated, waveguide-mounted dielectric taper source with high emissivity and mechanical reliability for cryogenic applications.

Findings

Achieved emissivity of 0.999 over 33-50 GHz band

Thermal time constant of 40 seconds at 15 K

Effective thermal isolation using Kevlar threads

Abstract

The design and validation of a dual polarization source for waveguide-coupled millimeter and sub-millimeter wave cryogenic sensors is presented. The thermal source is a waveguide mounted absorbing conical dielectric taper. The absorber is thermally isolated with a kinematic suspension that allows the guide to be heat sunk to the lowest bath temperature of the cryogenic system. This approach enables the thermal emission from the metallic waveguide walls to be subdominant to that from the source. The use of low thermal conductivity Kevlar threads for the kinematic mount effectively decouples the absorber from the sensor cold stage. Hence, the absorber can be heated to significantly higher temperatures than the sensor with negligible conductive loading. The kinematic suspension provides high mechanical repeatability and reliability with thermal cycling. A 33-50 GHz blackbody source demonstrates an emissivity of 0.999 over the full waveguide band where the dominant deviation from unity arrises from the waveguide ohmic loss. The observed thermal time constant of the source is 40 s when the absorber temperature is 15 K. The specific heat of the lossy dielectric MF-117 is well approximated by $C_v(T)=0.12\,T\,^{2.06}$ mJ g$^{-1}$ K$^{-1}$ between 3.5 K and 15 K.