A case is made for the existence of permeability of current sheets in electromagneticpulsed plasma thrusters. Permeability refers to the leakage of plasma or neutral par-ticles behind the sheet and could cause a significant effect on the performance of thethruster. Analytical calculations are presented which demonstrate the mismatch be-tween experimental velocity profiles and theoretical snowplow performances. Electrondensity measurements show a significant plasma wake following the sheet. Argumentsare also presented based on these data, that in some cases the accelerating plasma sheetentrains less than one third of the propellant. An index of permeability is defined asa ratio of sheet-trailing mass to encountered propellant mass. It may be quantifiedeither by directly measuring the trailing mass, or by measuring the mass entrained bythe sheet. The diagnostics needed to measure this index are discussed.I. INTRODUCTION