Penning discharge in the KATRIN pre-spectrometer

The KArlsruhe TRItium Neutrino (KATRIN) experiment is a next-generation, large-scale tritium β-decay experiment to determine the neutrino mass by investigating the kinematics of tritium β-decay with a sensitivity of 200 meV/c² using the MAC-E filter technique. In order to reach this sensitivity a low background level of 10⁻² counts per second (cps) is required. A major background concern in MAC-E filters is the presence of Penning traps. A Penning trap is a special configuration of electromagnetic fields that allows the storage of electrically charged particles. This paper describes the mechanism of Penning discharges and the corresponding measurements performed at the test setup of the KATRIN pre-spectrometer. These investigations led to the conclusion that the observed electric breakdown, strong discharges and extremely large background rates were due to discharges caused by Penning traps located at both ends of the pre-spectrometer. Furthermore, the paper describes the design of a new set of electrodes (modified ground electrodes and new ``anti-Penning'' electrodes) to successfully remove these traps. After the installation of these electrodes in the pre-spectrometer, the measurements confirmed that the strong Penning discharges disappeared. The experience gained from the pre-spectrometer was used to design the electrode system of the main spectrometer. Recent measurements with the main spectrometer showed no indications of Penning trap related backgrounds.