Why does the Alpha Magnetic Spectrometer (AMS) on the ISS use carbon dioxide for coolant?

Question

Spaceflight Now's Astronauts finish repairs to space station cosmic ray detector says:

Originally designed to operate for just three years, the AMS chalked up eight years of operation before being sidelined by the sequential failures of four carbon dioxide coolant pumps. The coolant system is what allows the AMS detectors to achieve the required levels of sensitivity.

To repair the system and extend the instrument’s life through the end of the station program, engineers developed a four-spacewalk plan to install a custom-built 350-pound module containing four pumps and a reservoir of fresh carbon dioxide coolant.

The AMS was not designed to be serviced in space, making the work particularly challenging. But during spacewalks last Nov. 15, 22 and Dec. 2, Morgan and Parmitano opened up the AMS, removed insulation, snipped eight coolant lines, installed the new pump module and spliced, or “swaged,” the coolant lines back into place.

Also see Astronauts complete 4-spacewalk marathon to fix space station's $2 billion antimatter detector

Questions:

1. Is the carbon dioxide used to cool detectors, or the superconducting magnet, or both, or neither?
2. Are these pumps gas compressors, or is there actual liquid CO2 flowing?
3. Why carbon dioxide? This sounds a bit exotic to me. Freon or ammonia can be used for general cooling and refrigeration and liquid helium is usually used for superconducting magnets.

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Cropped from How the Antimatter-Hunting Alpha Magnetic Spectrometer Works (Infographic)

Answer 1

1) There is no superconducting magnet in AMS-2. This would have required cooling with liquid helium resulting in a limited life time of only 3 years because of helium evaporating. Instead, they used a normal, rare-earth magnet. It has a lower field strength and is heavier, but does not require any power or cooling. The cooling in AMS-2 is only for the electronics and detectors. They all don't need special temperatures, typically operating slightly below room temperature. It's likely that such an instrument on Earth could be operated by air-cooling only.

2) The CO2 cooling for AMS-2 is a two phase system - liquid CO2 is pumped into the detector, evaporating as it heats up and transporting the heat out to the radiators.

3) CO2 is a well-known coolant, used in the very first refrigeration devices. For a long time it wasn't used to great extent, but since many of the traditional synthetic coolants are phased out nowadays it got a quite promising choice for alternative coolants. The huge advantage of CO2 is that it is one of the very few non-toxic refrigerants available.

In many places CO2 provides a more efficient and compact cooling compared to e.g. ammonia. One disadvantage is, that it has to be run at high pressure (CO2 doesn't get liquid below 5 bar pressure and 30 and more bar are common). On the other hand, the heat transport capability is high, and viscosity is low, resulting in a small size.

There is a 2002 publication detailing some of the advantages of using CO2 as a coolant: Feasibility demonstration of a mechanically pumped two-phase CO2 cooling loop for the AMS-2 tracker experiment

Answer 2

Supplementary answer to this excellent one because a) I like schematics, and b) downloaded a bunch of info a while back in anticipation of AMS EVA questions that haven't yet been asked.

The cooling system:

Explanation of the CO2 cooling cycle:

From AMS02 TRACKER THERMAL CONTROL SYSTEM OVERVIEW AND SPIN-OFF FOR FUTURE SPACECRAFT COOLING SYSTEM DEVELOPMENTS