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Long Hold-Time Cryostat
Overview
TopHat in Antarctica
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In order to maintain the TopHat detectors at 0.3 K for a long duration balloon flight, we have had to push long hold time cryogenic technology to its limits. The completed TopHat cryostat is shown in Figure 1.

 

TopHat Cryostat
TopHat Long-Holdtime Cryostat
Figure 1.
Figure2.


The circular port on the lower left is for the optical beam, and is shown with a solid Aluminum blank. The cryogenic plumbing and electrical connectors are located at the top. The following table summarizes the current, as-tested, characteristics of the cryostat. Some areas are still being improved to increase our operating margins for a 10-day flight.

Item
Volume
(liters)
Mass
(kg)
Cryogen
Capacity
(kg)
Hold
Time
(days)
Vacuum Jacket
1 8 .
4 . 0
-
-
Liquid Nitrogen Tank
2 . 8
  1 . 9  
2 . 3
-
Isothermal Shield
-
0 . 6 7
-
-
4He Tank
1 . 8  
1 . 1
0 . 2 5 0
17
Condensing Dewar
0 . 3 3 0
0 . 2 9
0 . 0 4 0
-
3He Tank
0 . 0 3 0
-
0 . 0 0 3
14

The total mass of the dewar together with the cryogen is 11 kilograms. The entire hold time for the dewar when it is in full operation is 14 days.

How does it work?

This cryostat keeps the detectors and optics at 0.25 K (-273 C) for 2 weeks. The cold temperatures are reached by evaporating liquid 3He (a rare isotope of Helium) which has the lowest boiling point of any material. In this case the boiling point is further reduced by pumping on the 3He to about 4x10-7 of atmospheric pressure with a Zeolite adsorption pump. This effect is what makes water boil at a lower temperature at high altitudes (lower pressure). We use about 1 mole (3 g) of 3He, which allows us to keep the volumes small and the mass low. This is one of the keys to this cryostat. By miniaturizing, we can dramatically reduce mass and therefore the size of the support structures, thus minimizing parasitic conducted heat and maximizing hold time. Figure 2 shows the main mechanical parts of the cryostat.

Main Mechanical Parts of Cryostat
Figure 3.

It contains a total of four internal tanks, one vapor cooled isothermal shield, a vacuum jacket, and small stainless steel support suspensions. An aluminum can kept at 2.7 K by 4He ("normal" Helium) surrounds the 3He tank. At atmospheric pressure 4He boils at 4 K but to reduce the heat load on the 3He stage we reduce the pressure on the 4He to about 1/3 atmospheric pressure. During flight, no pump is needed since the external pressure is low enough to provide this effect. See Figure 3 for a photograph of the 4He tank.

4He Tank
Figure 4.

Notice the helical fill/vent tube that provides a long thermal path in a small space. Surrounding the 4He tank with an aluminum can reduces its heat load. 4He gas "boiling" out from the 4He tank cools this can, also called the isothermal shield. The isothermal shield is, in turn, surrounded by a can cooled with liquid nitrogen (77 K). The nitrogen tank has 50 layers of superinsulation to reduce the radiation from the warm vacuum shell.

Layers of Superinsulation
Figure 5.

The main job of the vacuum shell is to keep air out. Even a small amount of air leaking into the cryostat would quickly condense on the cold surfaces and conduct large quantities of heat into the dewar. What's special about this 3He cryostat? This cryostat is unique in its ability to remain cold *unattended* for over two weeks at balloon altitudes, while being very small and light.

Why is it needed?

The detectors used in TopHat are bolometers. These bolometers detect incoming radiation by measuring temperature changes when radiation is absorbed on the detector. Since the only requirement on the bolometer is that it absorb the radiation, these detectors are inherently very wide band devices and very suitable for continuum measurements in the far-infrared. Similar technology has been used from 1 cm wavelength microwaves to X-rays. Keeping the bolometers very cold has two major advantages: 1) It reduces the thermal noise inherent in all warm materials, and 2) At low temperatures the heat capacity of the detectors are greatly reduced, making the temperature excursions much larger for the same input energy (and thus making them more sensitive).

Technological Improvement.

Although cryostats have been around for a long time this design has a longer hold time than any of comparable weight, and a lower weight than any of comparable hold time. The features that make this possible are: 1. The doubly reentrant pump tube on the 3He stage that has lower flow impedance and higher thermal impedance than can be obtained with a single surface tube. The inner section of the tube is only 0.003" thick. The "tank enclosing a tank" approach is used throughout to reduce the radiation loads and to build a strong and modular design. Each tank is supported with eight small stainless steel wires to reduce the thermal conduction between stages. 2. The fill and vent tubes are arranged in a double helix to allow them to be long and thin and still fit within a small volume. For example, the 4He fill/vent tube is 36" long even though the entire dewar is only 10" in diameter and 17" high.

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