In short, it's an experiment to directly measure the mass of the electron-type neutrino. Very occasionally, when tritium beta-decays, the electron carries away all the energy. If the neutrino has mass (and we are rather certain [1] that it does), then there's a little less energy to carry away. The giant MAC-E spectrometer (actually a chain of them, of which the big one is the last) is the only known way of resolving the 13,000 electron-volt electron to less than an electron-volt.
It's a very hard experiment, but it's important, and nobody knows an easier terrestrial path to the same knowledge (cosmology limits are equal to or better than KATRIN, but there's a big difference between the cosmos and something you can do on Earth).
(Edit: and for you vacuum jocks out there, that entire volume operates at UHV. It's electropolished internally, and for their vacuum test, it pumped to below 10^-7 Torr on a single 6-8" turbo before bakeout.)
To followup: neutrinos have been established to have mass from neutrino oscillation experiments but those experiments have left a fairly large range of possible masses for neutrinos. Experiments like KATRIN will further pin down the properties of neutrinos as well as their rest mass.
http://www.katrin.kit.edu/
and the wikipedia article:
http://en.wikipedia.org/wiki/KATRIN
In short, it's an experiment to directly measure the mass of the electron-type neutrino. Very occasionally, when tritium beta-decays, the electron carries away all the energy. If the neutrino has mass (and we are rather certain [1] that it does), then there's a little less energy to carry away. The giant MAC-E spectrometer (actually a chain of them, of which the big one is the last) is the only known way of resolving the 13,000 electron-volt electron to less than an electron-volt.
It's a very hard experiment, but it's important, and nobody knows an easier terrestrial path to the same knowledge (cosmology limits are equal to or better than KATRIN, but there's a big difference between the cosmos and something you can do on Earth).
(Edit: and for you vacuum jocks out there, that entire volume operates at UHV. It's electropolished internally, and for their vacuum test, it pumped to below 10^-7 Torr on a single 6-8" turbo before bakeout.)
[1] http://en.wikipedia.org/wiki/Neutrino#Mass