  The KATRIN experiment is designed to determine the absolute neutrino mass
scale with a sensitivity of 200 meV (90 % CL) by measuring the electron energy
spectrum close to the endpoint of molecular tritium beta decay. Electrons from
a high-intensity gaseous tritium source are guided by a strong magnetic field
of a few T to the analyzing plane of the main spectrometer where an integral
energy analysis takes place in a low field region (B<0.5 mT). An essential
design feature to obtain adiabatic electron transport through this spectrometer
is a large volume air coil system surrounding the vessel. The system has two
key tasks: to adjust and fine-tune the magnetic guiding field (Low Field
Correction System), as well as to compensate the distorting effects of the
earth magnetic field (Earth Field Compensation System). In this paper we
outline the key electromagnetic design issues for this very large air coil
system, which allows for well-defined electron transmission and optimized
background reduction in the KATRIN main spectrometer.
