Why is the spectrometer so big in KATRIN experiment? I've been studying KATRIN experiment for a popularizing science project and I was wondering, why is the spectrometer so big ? I mean the thing is so huge that they had to carry it over 8000km on the sea, it is 24meters long and had a diameter of 10m. But in all the publications related I didn't find any hint, only that the precision of the filter is given by:
$$\frac{\Delta E}{E} = \frac{B_{\text{min}}}{B_{\text{max}}}$$
which involves only magnitude of magnetic field. They also want the magnetic field to vary adiabatically which involves:
$$\nabla B/ B << 1$$
That can explain the length, but why this ovoid shape ?
I'm not an expert of this field if someone can give a clear and precise answer I'll be very grateful !
Thank you,

 A: I'm assumming you obtained your information from the page on the katrin.kit.edu website where the measurement principle of a MAC-E-filter is discussed
The objective is to obtain the highest resolution that current technology can obtain.
The electrons entering the main filter have already undergone multiple stages of pre-filtering, second to last in a smaller (but still very large) MAC-E-filter, so the electrons entering the main filter already have an energy very close to each other.
I don't see explicit mention anywhere of why the main chamber is so large.
I assume that if the "main" chamber would be of similar size to the chamber of the pre-filter the "main" chamber wouldn't add all that much.
I assume that the larger the chamber of the main filter the higher the resolution of the cut-off that the setup is designed to obtain.

From the 2004 KATRIN design report:
Section 2.1.1 (page 36) describes that it is also possible to build the setup in such a way that it also support a time-of-flight mode.
"By measuring the time-of-flight of the electrons those of interest can be distinguished fromthe high energy electrons and the MAC-E-Filter transforms from an energy high-passfilter into an energy band-pass filter of the same energy resolution and nearly the sameluminosity as the MAC-E-Filter without time-of-flight measurement"
When set up to operate in time-of-flight mode the separation is based on duration of the flight. The bigger the chamber, the longer the total duration of the flight, the more time available for separation.

As you know, the researchers are interested only in the electrons that carry away so much energy from the decay event that the neutrino formed in that event received only the energy that is the minimum it needs to be formed at all. All electrons that carry away less than that maximum need to be washed away. The electrons that even make it to the main chamber will have energies very, very close to each other. That of course makes separation very, very hard to obtain.
