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The precision in reconstructing events detected in a dual-phase time
projection chamber depends on an homogeneous and well understood electric field
within the liquid target. In the XENONnT TPC the field homogeneity is achieved
through a double-array field cage, consisting of two nested arrays of field
shaping rings connected by an easily accessible resistor chain. Rather than
being connected to the gate electrode, the topmost field shaping ring is
independently biased, adding a degree of freedom to tune the electric field
during operation. Two-dimensional finite element simulations were used to
optimize the field cage, as well as its operation. Simulation results were
compared to ${}^{83m}\mathrm{Kr}$ calibration data. This comparison indicates
an accumulation of charge on the panels of the TPC which is constant over time,
as no evolution of the reconstructed position distribution of events is
observed. The simulated electric field was then used to correct the charge
signal for the field dependence of the charge yield. This correction resolves
the inconsistent measurement of the drift electron lifetime when using
different calibrations sources and different field cage tuning voltages.
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