Ion current collection in spacecraft wakes

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Abstract
This work investigates the current–voltage characteristics of a highly negatively biased, isolated probe in the plasma wake of a much larger, unbiased object. The system is investigated for both flowing and stationary plasmas. For the stationary plasma case, spherical probe theory and a simple correction factor to account for the shadow of the plate adequately explain the results. The introduction of a flowing plasma leads to a much more complicated situation. In this case, since the potential field is asymmetric and the ions no longer conserve angular momentum, spherical probe theory is inadequate. The current–voltage characteristics take a much more complicated form than in the stationary case. Indeed, the simulations show that the current–voltage characteristics of varying axial probe positions cross each other. The high complexity of the ion orbits makes theoretical predictions of where the crossover occurs difficult. However, the potential structure around the probe and the ion trajectories are analyzed to explain the current–voltage characteristics. These results show regions can exist in energy–angular momentum space which cause incoming ions to orbit the probe and then escape the simulation. The fraction of particles which do orbit and then escape as a function of probe potential and position determines the shape of the current–voltage characteristic.