Disclosed are ion cyclotron resonance (ICR) cells and other ion-trapping cells with plural groups of multiple trapping electrodes for shaping (e.g., flattening) the radial electric field within the ICR cell. Also disclosed are methods for controlling the electric field to diminish effects of de-phasing. The diminished effects are achieved by decreasing space-charge contributions by increasing the length of the ion-oscillation path along the z-axis of the ICR cell. The methods and devices enhance the time-domain signal of a Fourier-transform ion-cyclotron resonance mass spectrometer (FTICR-MS) and provide enhanced resolution and accuracy of mass measurements.