A negative potential barrier on the low-potential side of a moving double layer gives rise to a current limitation in a collisionless plasma terminated by a positively biased cold collector plate in a $Q$ machine. The double layer is produced in front of the plasma source and moves toward the collector during the limitation. When the double layer arrives at the collector, the barrier dissolves and the current increases, causing a rise in potential along the whole plasma column. The double layer then reforms at the source.

An emissive probe has been used in the edge region of the CASTOR tokamak in order to test the possibility of direct measurements of the plasma potential. The difference between the floating potential of a cold probe and that of an emissive probe has been found to be approximately 1.3 times the electron temperature, which is less than predicted by the probe theory. Several possible reasons to explain this discrepancy are offered, such as secondary electron emission, uncertainties in the ion temperature, different collecting areas for electrons and ions, etc. The possible impact of a space charge formed by the emitted electrons is also discussed.