The kinematics of the plasma sheath in two very low energy plasma focus devices (70 and 300J) is studied by means of non-perturbative optical diagnostics. Experiments are performed in deuterium at the filling pressure where neutron emission is maximum on each generator. In both devices, the sheath movement is monitored close to the anode surface, from where their thicknesses and their average propagation velocities are obtained. It is found that the sheath thicknesses are closer to or lower than 1mm, which is significantly smaller than those measured in larger devices. Moreover, it is found that the sheaths remain attached for a period of time close to 20-30% of the quarter period of the discharge, and it coincides with estimations using theoretical models of sheath formation. These results suggest that the usual modeling tools used in plasma focus need further revision given that some discarded parameters such as plasma resistance and detachment time are important in very low energy devices, which is not necessarily true for high-energy devices. Some calculations and comparison with models are presented and discussed.
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