Ion and plasma systematics during the first KATRIN neutrino mass measurements

The Karlsruhe Tritium Neutrino (KATRIN) experiment is targeted to determine the
effective mass of the electron antineutrino with an unprecedented sensitivity of 200$\,$meV/$c^2$ (90\% C.L.).
This will be achieved by measuring the beta-spectrum of tritium close to the kinematic endpoint at 18.6$\,$keV.
Gaseous tritium is contained in the Windowless Gaseous Tritium Source (WGTS) at an activity of $10^{11}\,$Bq in a magnetic field of 2.5$\,$T and at a temperature of 100$\,$K or less, depending on the operation mode.
Inside the WGTS a cold, magnetized plasma is formed due to the continuous creation of ions and electrons via beta-decay and subsequent inelastic scattering of beta-electrons off gas molecules.
The space charge potential of this plasma defines the reference energy scale for the neutrino mass measurement and therefore stringent requirements regarding homogeneity and temporal stability have to be met in order to reach the KATRIN design sensitivity.
In addition, the neutrino mass sensitivity potentially could be worsened by ions leaving the WGTS and inducing background events.
This thesis presents experimental studies of these effects in the course of various KATRIN measurement campaigns performed in the years 2017 to 2019.
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By operating a dedicated hardware setup for ion blocking, removal and detection, the ion-induced background could be minimized.
Extensive investigations of the interplay between the plasma potential and various hardware devices - most importantly the rear wall disc mounted at the WGTS rear end - allowed for the determination of optimum operational parameters to minimize systematic plasma effects during the first neutrino mass measurements.