Charging efficiency (solid lines) and current density (dashed lines) as a function of the mass loading in pebbles, for both 1D filaments (blue lines) and 2D sheets (red lines), for a characteristic structure width of 3000 km. The charging efficiency is high at low pebble densities when the positrons can escape freely from the structure, but falls for high pebble densities due to self-absorption of positrons within the structure. The current behaves oppositely: at low pebble densities the current is low, since the structure is thin to its own positron emission, while the current density plateaus around the characteristic value of 3.6 × 10^-14 C m^-2 s^-1 for high pebble densities.