Force balance and density criteria for double sheaths in plasma

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Abstract
Two aspects of double sheaths (nonneutral regions) in plasmas are studied via analytic models. First, the motion of the structure through the plasma is analyzed by considering the force balance among the various species. A general proof is presented which shows the net acceleration of the sheath is zero in the absence of dissipative processes. A specific example is given which shows that a pressure difference between the two sides of the sheath results in the enhancement of the beam current into the low pressure side rather than acceleration of the sheath. The effect of beam‐plasma interactions is also discussed. The second part of the analysis is the derivation of separate (though nearly identical) density criteria for sheath nucleation and sheath maintenance in equilibrium. These are compared with experimental results of Lutsenko, Sereda, and Kontsevoi and reasonable agreement is obtained.