The blackbody BB1000 under consideration is basically an
extended area blackbody for medium temperatures (near ambient
up to 1000 K), which operates under cryo-vacuum conditions.
The BB1000 was designed and manufactured by VEGA International,
Inc. for SDL (Utah) under the contract with SDL/USURF, Subcontract
C920306. BB1000 specifications (obtained in tests) are presented
in Tab.1, in comparison with requirements of the contract.
BB1000 Blackbody Radiator General Concept
In the base of BB1000 radiator design two principal ideas
has been placed:
To use the flat bottom for uniform heat removal.
To use the lateral walls with small emissivity for increasing
the influence of zone with maximal radiation losses.
However, the usage of a flat bottom leads to decrease of
effective emissivity in comparison of inner or outer conical
bottom,especially for coating with significant specular component
of reflection.
It is well known that covering of V-shaped or rectangular
grooves can increase the emissivity of flat surface. But in
this case the cardinal problem is an appearance of the temperature
gradient along groove depth.
We choose the configuration of lateral walls, similar to
one designed by J. Martin and successfully applied to development
of blackbodies of low and medium temperature range . The result
of choice of walls configuration (inner surface generatrix
of BB1000 radiation cavity) is depicted in.
In order to estimate the longitudinal temperature gradient
from bottom to bulge of V-groove, we changed concentric groove
that has a radius which is significantly greater than linear
dimensions of V-groove, on trapezoidal tooth, which is infinitely
extended in direction that perpendicular to its section. We
also neglected transversal temperature gradient. The results
of computation of temperature difference between lower and
upper surfaces of V-grooves are presented in Table 2.
| V-groove temperature |
|
Temperature difference due
to radiative losses |
| 500 K |
|
11 mK |
| 800 K |
|
119 mK |
| 1000 K |
|
301 mK |
The numerical investigation of principal dependencies of
effective emissivity for developed blackbody was performed
by means of Steep3 modeling software, based on the Monte Carlo
algorithm. The main uncertainty of calculation of effective
emissivity is connected with variations of data emissivity
in various reference books and papers. The results of calculation
for two different types of graphite and measured spatial bottom
temperature non-uniformity are shown in Fig.2 and are located
within 0.9907 to 0.9879 range.
BB1000 Blackbody Design
Blackbody BB1000 (see its schematic drawing in Fig.3)
consists of the following elements:
Radiator, formed by the graphite bottom with V-grooves and
cold-plated reflector.
Graphite bottom is equipped with two heaters.
The bottom heaters are located on the bottom of a stainless
steel cylinder,
which is inserted inside graphite bottom.
The additional heaters are located on the cylinder surface
of the
graphite bottom cup for minimize the bottom thermal gradients.
Bottom is equipped with main and additional PRT-100 sensors.
Each sensor has the backup one.
Heat exchanger with copper cryogen circulator, which is mechanically
coupled with BB reflector.
Multi-foil Ni high-temperature insulation.
Multi-foil Al low-temperature insulation.
Blackbody cavity is attached by the cold reflector to
the Aperture Unit via intermediate stainless steel flange.
Blackbody shroud can be attached to the external shroud of
the Aperture Unit.
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