Electromagnetic design of the large-volume air coil system of the KATRIN experiment

Ferenc Glück,Alexander Osipowicz,Nancy Wandkowsky,Benjamin Leiber,Jan Reich,Susanne Mertens,Guido Drexlin

doi:10.1088/1367-2630/15/8/083025

Abstract

The Karlsruhe Tritium Neutrino (KATRIN) experiment is designed to determine the absolute neutrino mass scale with a sensitivity of 200 meV (90% confidence level) by measuring the electron energy spectrum close to the endpoint of molecular tritium β 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.

Highlights

The magnetic field inside the main spectrometer is of key importance to minimize the cosmic ray μ-induced background
Previous investigations performed with the Mainz neutrino mass spectrometer [28, 29] and the Karlsruhe Tritium Neutrino (KATRIN) PS [30, 31] have revealed that the background is smaller when the magnetic field inside the spectrometer is higher
-6 -4 -2 0 2 4 6 z (m) with the low field correction system (LFCS) coils this asymmetry becomes smaller. It is noticeable in table 2 that for the two minima configuration the central coils 7 and 8 have to be operated with rather large current values, because in this case the magnetic field is designed to have a local maximum at the center (z = 0) of the off-axis field lines

Summary

Introduction

The magnetic field inside the main spectrometer is of key importance to minimize the cosmic ray μ-induced background. Secondary electrons emitted at the inner surface of the spectrometer and electrodes cannot move perpendicularly to magnetic field lines (they move much easier parallel to these field lines). For higher values of the magnetic field inside the spectrometer volume the shielding is more efficient, as for example the flux tube is farther away from the inner tank and electrode surface. A higher magnetic field at the analyzing plane reduces the energy resolution, thereby making the transmission function broader. Electron tracking simulations indicate that the background could depend on the magnetic field shape in the main spectrometer (e.g. one minimum or two minima with local maximum) [32]

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Results

Conclusion