Overview

Phase 2 of the experiment will hopefully begin in 2009 if everything goes well. For this phase, the apparatus will be moved to CENPA, at the University of Washington. There, it will be upgraded by adding a dilution refrigerator to it in order to cool it down to 100mK or less, increasing sensativity and scanning speed.

Why Phase 2?

The Upgrade

Phase 2 is an important step forward for the ADMX Project. The addition of the dilution refrigerator will let the apparatus be cooled down to 100mK or less. Whereas adding a SQUID amplifier in Phase 1 allowed the apparatus to scan approximately 60% faster, adding a dilution refrigerator on top of that will let the apparatus scan approximately 400 times faster. Before Phase 1, the apparatus scanned less than 1 megahertz of frequency per day. Since the experiment is scanning a range of over 300 megahertz, it would take well over a year to get the relevant data. With the addition of Phase 2, you can now scan frequencies in a reasonable amount of time.

In addition to that, reducing noise by those amounts makes the apparatus more sensative, letting it scan more areas and more clearly than before. Moreover, since the apparatus' sensativity also partially depends on how long you spend scanning a certain frequency, providing it with faster scanning speed also lets it scan areas better, not just faster.

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The Move to CENPA

The move to CENPA makes working on the ADMX very convenient for the people involved, due to everything that's available on-location. The CENPA site where the ADMX apparatus would go has a crane for lifting heavy objects (the apparatus is very heavy), and cryogenic and gas supply systems.

Furthermore, as the schematic shows, the ADMX apparatus would be located in its own pit area, isolating it from other experiments, which helps people work on it easier without inteference, and also eliminates interference that other experiments might have on the apparatus. In addition to this, CENPA has a long history of success with smaller projects.

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The Dilution Refrigerator

This is the main focus of the ADMX apparatus upgrade for Phase 2 and a crucial piece of the whole experiment. It lets the ADMX scan much faster than before and at a much better sensitivity. So how does a dilution refrigerator work?

It uses Helium-3 and Helium-4, two stable isotopes of Helium (although Helium-3 is very rare), and uses fancy thermodynamics to keep cooling whatever you want to within a few miliKelvin of absolute zero. The long version is, you take Helim-3 and Helium-4, cool them down until they are liquid (each have a different condensation temperature) and mix them together. However, this is like mixing oil and water. A clear divide will form, separating the two Helium components. This is where the fancy thermodynamics comes into play. Theory states that the Helium-4 has to have at least 6% Helium-3 in its mixture to be stable. So, what we do is pump out the Helium-3 it already has, and it will then attemp to suck in the Helium-3 from the other mixture. Since there is a boundary between the two, it takes energy to cross it. Meaning that, whenever the Helium-3 goes over to the Helium-4 mixture, it absorbs some heat from wherever it can, effectively cooling its surroundings. So then we pump out the Helium 3 again and the process repeats.

The dilution refrigerator that will be used for ADMX will be put together from various parts, most of which will likely come from the "Kelvinox 200" from Oxford. You can find a picture of the similar, "Kelvinox 400" on this page.

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