Photos of an RT-MLI composed of stacked Styrofoam layers. Each layer has a diameter of 210 mm and a thickness of 3 mm. The total number of layers shown in the left photo is 17.
Layout showing the principle behind RT-MLI. Exchanges of thermal radiation between the layers are balanced. The thermal loads conducted from the top surface to the bottom surface in each layer are also balanced with the exchanged radiation on each surface.
Layout for measuring the basic performance of the RT-MLI in reducing room temperature radiation. Five thermometers were located at the positions marked by solid dots, a–e. We monitored the top and bottom surface temperatures of the RT-MLI as well as the temperatures on the absorber array and the radiation shield. Thermal loads passing through the RT-MLI were measured by obtaining the temperature difference between thermometers d and e.
[Top panel]: Surface temperatures of RT-MLI as a function of the number of Styrofoam layers; the solid and dashed lines indicate the top side (location a in Fig. 3 ) and the bottom side (location b in Fig. 3 ) of the RT-MLI, respectively. Surface temperatures of a thick Styrofoam block (thickness of 100 mm) are also shown for comparison. Several layers of the RT-MLI with a total thickness of ∼20 mm achieved a performance equivalent to that of the thick Styrofoam block. A simulation with an emissivity of 0.72 almost exactly reproduces the temperature gradient of the RT-MLI. [Middle panel]: Temperature difference between the two thermometers on the bottom surface of the RT-MLI in the center (b in Fig. 3 ) and on the edge (c in Fig. 3 ). The principle of the RT-MLI guarantees uniform surface temperatures (see main text for details: Sec. II B ). [Bottom panel]: Thermal loads passing into the absorber array for each configuration. The thermal radiation estimated from the bottom temperature of the RT-MLI is also shown; it should be the dominant source of the thermal loads. A prediction based on the simple 1/(N + 1) law (Eq. (7) ), regarding the behavior of a conventional MLI is overlaid for comparison. The RT-MLI exhibits a lower slope than the simple 1/(N + 1) law because of a contribution of the second term in Eq. (5) .
Transmittance of RT-MLI at room temperature. Using an FTS, we measured the transmittances of four different configurations distinguished by the number of layers 1, 6, 12, and 24. We also measured the transmittance of a Styrofoam block for comparison. Below the 220 GHz region, the transmittance of the 24-layer sample was also measured using signal generators. The transmittance was roughly proportional to the Nth power of the number of layers, i.e., the transmittance of an N-layer RT-MLI was approximately 0.997 N at a frequency of 200 GHz.
Layout of the setup used to demonstrate the effects of RT-MLI. This setup emulates a radiowave receiver with a 200-mm-diameter aperture. Instead of a detector array, we used an absorber array. The temperatures obtained at each location (A, B, ⋅⋅⋅, E) are summarized in Table III .
Photograph of the setup shown in Fig. 6 .
[Top]: Temperatures attained at each layer as shown in Fig. 6 and Table III . [Bottom]: Thermal loads passing into absorbers measured using temperature differences between C and D in Fig. 6 and Table III . Estimated thermal radiation from the Nylon filter is also shown. The difference between the measured load and the estimated radiation (≈0.1 W) indicates that effects of reflected radiations from the upper layers. A setup that has tight shielding to prevent reflection is expected to reduce the difference.
Physical properties of Styroace-II Styrofoam at room temperature. a
Temperatures (K) achieved at each thermometer location (see Fig. 3 ).
Temperatures (K) obtained the setup in Fig. 6 ; the additional radiation shielding took the form of no shielding, installation of the Styrofoam blocks, and installation of RT-MLI (12 layers above the PTFE filter and 5 layers above the Nylon filter).
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