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Gravity Probe B

Testing Einstein's Universe

The Dewar's Porous Plug—Liquid In, Vapor Out

As we mentioned in last week's highlights, the GP-B mission is in the final weeks of science data collection before the helium in the Dewar is exhausted. Our tests and calculations indicate that the liquid helium will completely evaporate in early September. All told, this will give us more than eleven months of relativity data to test EinsteinÕs theory about curved spacetime. It is therefore appropriate to look back at one of the critical pieces of equipment that has allowed the mission to collect so much data for so long -- the porous plug.

The porous plug is a small device located near the top of the 650-gallon Dewar. Its primary purpose is to provide a controlled vent for helium vapor while retaining the liquid helium inside for as long as possible. It does this by taking advantage of the superfluid nature of the liquid helium.

Superfluidity is a characteristic of liquid helium that only appears when it is 2.18 kelvin or colder (our liquid helium is holding steady at 1.8K). When liquid helium is superfluid, it has no viscosity. This allows it to move about on a surface or through a porous substance without friction. A remarkable effect of this is that if superfluid helium was left in an short open beaker in a supercooled environment, the liquid would crawl up the inside of the beaker and down the outside until the beaker was empty!

(For more details, check out GP-B's Scientific Papers under "The Engineering Story" or NASA's GSFC Introduction to Liquid Helium or CERN's Teaching Materials .)

The role of the porous plug is to control this flow of superfluid helium just enough to let some of it evaporate without letting the liquid leak out. A simple vent would not work because in a zero-g environment, the superfluid helium would simply migrate out of any opening in the Dewar, no matter how small we made it or how quickly we opened and closed it. Instead, we needed to make a porous plug.

The plug itself is made of stainless steel, but it has gone through a “sintering” process that makes it slightly porous (like a sponge). Sintering is a process of powdering a material into tiny granules and then heating the granules until they coalesce into a porous mass, without heating them so high that they melt together. The pores in the resulting mass are extremely small (in fact, invisible to the naked eye), but they are large enough for the superfluid helium to find its way through the plug.

Before the oozing superfluid helium leaks completely through the plug, it reaches a point where it begins to evaporate (called the liquid-vapor interface). When it evaporates, the helium vapor takes heat energy out of the liquid helium. Now the liquid helium near the outside of the plug is colder and it sinks back into the Dewar. At least, this is how it is supposed to work.

It was a tricky challenge to choose the right size for the porous plug. If it was too small, the liquid-vapor interface would be below the plug, which would choke the plug and prevent the release of helium vapor. If the plug was too large, the interface would be above the plug and the liquid helium would ooze out of the plug before it had evaporated and would be lost. The proper size turned out to be 6.9 cm disk, 0.635 cm thick with a permeability of 3.8 x 10^-10 cm squared. The material for the plug is a low-carbon version of stainless steel provided by the Mott Corporation in Connecticut (which also produced our sintered titanium filters to purify the helium gas and the sintered titanium blades for the cryopump -- See June 24 Highlight).

Now that we have gone to the trouble of making a sophisticated porous metal plug, why are we doing this? Why is the porous plug so critical to the life of the mission? Because without finding some way of continuously releasing the helium vapor that builds up inside the Dewar, the pressure and temperature of the science instrument would rise to unacceptable levels.

The challenge begins with the fact that the Dewar and satellite, while orbiting the Earth, are warmed up by radiation from the Sun and Earth. The Dewar has thermal barriers to insulate the helium from this radiation, but some residual heat does enter the Dewar and heats up the liquid helium. If left alone, the liquid helium would gradually transition into helium vapor, thereby increasing the pressure and temperature inside the Dewar. So we need to regularly vent the Dewar.

By releasing the vapor, we do two things. First, we prevent the pressure from rising and maintain it at a steady level. Second, when the helium evaporates out of the Dewar, it takes heat energy with it. Therefore, any heat energy that has leaked into the liquid helium through the Dewar is released out of the liquid helium. This is the same effect that happens when your sweat evaporates; your skin is releasing heat energy and it cools your body off.

By using the porous plug, we have been able to maintain a steady pressure and temperature inside the Dewar. The liquid helium has stayed within a few millikelvin of 1.8K ever since we launched sixteen months ago. This has allowed the science instrument to operate properly at superconducting temperatures and has given us more than eleven months of relativity data to test Einstein. Without this sophisticated plug, the mission would have been over before it had even started.


We recently updated our NASA Factsheet on the GP-B mission and experiment. You'll now find this 6-page document (Adobe Acrobat PDF format) listed as the last navigation link under "What is GP-B" in the upper left corner of this Web page. You can also click here to download a copy.

Drawings & Photos: The layered composite photos of the GP-B spacecraft orbiting the Earth and starlight from IM Pegasi entering the spacecraft's telescope, as well as the GP-B experiment diagram were created by GP-B Public Affairs Coordinator, Bob Kahn using Adobe Photoshop and Adobe Illustrator. Mr. Kahn also took the photos of the Greenwich Observatory and the photos of Dr. Anne Kinney from NASA Headquarters visiting GP-B. The photo of the gyroscope housing and the SQUID readout diagrams are from the GP-B Image Archive here at Stanford. The sky chart image, showing the guide star, IM Pegasi was generated by the Voyager III Sky Simulator from Carina Software. Click on the thumbnails to view these images at full size.


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