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A waveguide-coupled thermally isolated radiometric source
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Image of FIG. 1.
FIG. 1.

Photograph of the source in a cryogen-free refrigeration system. The tapered absorber is centered within and thermally isolated from a square electro-formed copper waveguide using a kinematic suspension.

Image of FIG. 2.
FIG. 2.

Sketch of the source. The absorber is bonded to a copper pin (colored green) that reaches a depth of 1 in. into the absorber to effect isothermal response. The bobbin-absorber is inserted into a sleeve, centered within the waveguide with shims, and bonded to the sleeve.

Image of FIG. 3.
FIG. 3.

Detailed sketch of the kinematic suspension. The spring preload ensures tension is maintained throughout thermal cycling. The magnitude of the preload (60 N) is at 20% of the tensile strength of Kevlar. The kinematic suspension is rigid in the x-y plane and does not allow rotation about the z axis. The only degree of freedom is translation in the z direction, which is less than 0.5 mm. Approximate dimensions of the angles are as follows: ϕ = 30°, θ = 60°, and ψ = 340°.

Image of FIG. 4.
FIG. 4.

Thermal circuit of the source when installed in a cryogenic system. The heat capacity of the source is represented by a lumped element that is dominated by the lossy dielectric MF-117 absorber. The thermal conductivity of the copper wires that attach to the sleeve is much larger than the Kevlar threads. The source lengths L 3.5K = 26 cm and L 1K = 6 cm can be tailored to meet the desired experimental thermal time constant in other cryogenic systems.

Image of FIG. 5.
FIG. 5.

Measured dependence of the source temperature and time constant on heater power (filled squares). The solid line is a power law fit to the data of the form , where K = 0.072 mW Kn and n = 2.11 are the fit parameters, P is the heater power, T is the source temperature, and T B = 3.5 K is the bath temperature.

Image of FIG. 6.
FIG. 6.

Measurements of the specific heat of Eccosorb MF-117. The solid line is a fit to the data (filled squares). The specific heat of Eccosorb CR-110 and CR-124 are from Refs. 2 and 19 , respectively.

Image of FIG. 7.
FIG. 7.

(a) Sketch of the conical taper that functions as a lossy adiabatic impedance transformer when placed in a square waveguide housing. (b) Sketch of the waveguide assembly employed for the power reflection measurement of the absorber. The reflectance is measured with and without a quarter-wave delay shim inserted at the calibration plane.

Image of FIG. 8.
FIG. 8.

Room temperature measurements of the reflected power from the absorbing cone shown in Fig. 7 . The waveguide cut-off frequency is f c = 26.35 GHz. The residual error of the calibration in the waveguide band is given by the black dashed line. The reflectance is experimentally observed and characterized by varying the absorber insertion position, d.

Image of FIG. 9.
FIG. 9.

Available single-mode power from the thermal source in frequency bands defined by single-mode rectangular waveguide between 1.3f c and 1.9f c . The available power drops rapidly below a cut-off temperature T c = 12f c h2 k B . The coupling loss due to flange reflection has been set to zero.


Generic image for table
Table I.

Summary of source flux errors.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A waveguide-coupled thermally isolated radiometric source