PARTICLE ASTROPHYSICS

High magnetic fields could be useful in space-based detectors designed to analyze high-energy charged particles in cosmic rays. The experiment AMS-2, which was deployed on the International Space Station (ISS) in May 2011, employs a 1,200 kg Nd2Fe14B permanent magnet with a field strength of around 0.125 T. Original plans called for a superconducting magnet using NbTi wire, which would have had a field strength about five times higher and which would have enabled the study of particles with proportionately higher energy. However, difficulties were encountered with the superconducting magnet, particularly a poorly understood heating effect, which would have increased the cryogenic cooling load and shortened the running time for the experiment. Consequently, the decision was made to employ the permanent magnet instead.

Presumably, the development of improved magnets, perhaps at much higher fields, could significantly increase the capability of a future space-based detector. However, a new detector is unlikely to be undertaken in the near future, for reasons unrelated to the magnet issue. The power requirements for the experiment are beyond the capabilities of any existing space platform other than the ISS, and AMS-02 was already too large to be carried by any vehicle other than the U.S. space shuttle, now retired.

High magnetic fields may also have a role in ground-based experiments to search for axion or axion-like particles as a possible constituent of cold dark matter. Strong magnetic fields are supposed to convert a small fraction of axions into observable photons. The Axion Dark Matter Experiment (ADMX), based at the University of Washington, employs a superconducting magnet that generates 8 T in a region that is 1 m long and 0.5 m in diameter. The experiment is designed to convert axions into microwave photons and would be sensitive to axions with a mass in the range of 2 to 20 μeV. Planned upgrades to this experiment, which should greatly increase sensitivity, do not involve stronger magnetic fields but rather involve improvements in the design of the microwave detectors and lowering of the temperature of the microwave cavity, by means of a dilution refrigeration system. However, stronger magnetic fields could have an important role in future experiments of this kind.

Another experiment, the CERN Axion Solar Telescope (CAST), is designed to look for axions produced in the core of the sun, with masses up to 104 eV. This experiment, which started operating at CERN in 2002, employs a magnet approximately 10 meters long, with a maximum field of 9.6 T, originally designed for the LHC (Aalseth et al., 2002). For masses below 0.02 eV, CAST has set an upper limit to the axion-photon coupling constant gαγ of <8.8 × 10-11 GeV-1, with larger values for higher masses (Collar et al., 2012). Again, stronger magnetic fields will be important for achieving greater sensitivity in future experiments.



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