Measurements of the sheath potential in low density plasmas

The authors have measured the sheath potential around a probe in a range of different plasma conditions in the UMIST quadrupole GOLUX and in a related experiment in which the plasma expands freely to supersonic velocity. In the latter case, the sheath potential agrees well with an appropriately modified form of the usual expression for a field-free plasma, for both hydrogen and argon plasmas. In GOLUX, however, the sheath potential is found to be significantly less than the accepted value, even when the magnetic field is taken into account. For the slow-moving plasma in the outer part of the quadrupole confining field, they present both theoretical and experimental results showing that the reduction is due to truncation of the electron velocity distribution as the probe drains electrons from a closed flux-tube faster than they can be replaced. In the central hot plasma, however, this explanation cannot apply. Here the plasma is moving at about sonic speed and magnetic effects are weak. Nevertheless the results are significantly different from those in the field-free experiment.