The condition for the formation of a sheath in front of an electrode in a plasma is reviewed.The treatment is extended to cases with negative ions, high energy electrons and secondary electrons. The case when the positive ion temperature is higher than the electron temperature is also treated.The plasma dispersion relation is discussed from the point of sheath formation.
Experimental results show that the sheath in a rf plasma processing discharge will expand and contract with the osicllating potential. Moreover, in SiHi discharges the surrounding plasma is by nature a dusty plasma containing negative ions. Computer simulation indicates that such negative ions alone will introduce a weak field throughout the plasma and a double layer at the sheath edge.The substrate etching rate is critically controlled by the ion injected through this sheath from the ‘plasma’ and thus a more complete understanding of this is essential for plasma processing technology.
Formation of the sheath and presheath in edge plasmas of magnetic confinement fusion devices is briefly discussed.One important aspect of a plasma flowing to the wall that must be included in the analysis is the presheath mechanism provided by the Lorentz force in a magnetic field oblique to the wall or in a spatially varying magnetic field.Another aspect is existence of energetic electrons which has dramatic effects both on potential formation and on heat transport through self-consistent separation of two electron species and induction of secondary electron emission.
We review the sheath which is produced around a moving object in space.The sheath in space is grouped from different point of view; one is ambient plasma density, another is potential of the object. In terms of the ambient plasma density, neutral particles collide with electrons inside the sheath in the lower ionosphere below 90km. Shape of the sheath which is formed around moving object are distorted which is totally different from stationally laboratory plasma.In the high altitude, where ambient plasma density is less than 103els/cc, the satellite is surrounded by photoelectron in the sunlight. When we look at the sheath from the view point of the magnitude of the voltage applied to it, the sheath is divided into two groups; low voltage and high voltage regimes. Low voltage regime is a conventional satellite sheath. High voltage regime is encountered in bombardment of the spacecraft by auroral electrons under low density plasma or in electrodynamic tether system. Study about the highly biased large electrode is important for the future solar power station.
Plasma processing including ion processing is reviewed. The advantage of plasma processing, compared with usual thermal processing, is described.In the plasma processing, the plasma-surface interaction for thin film formation is addressed as a key issue.As the example of plasma processing, carbonization experiments based on plasma CVD and nitridation experiments using ECR plasma are briefly introduced.
Computed tomography studies for plasma imaging are reviewed from a standpoint of numerically coping the sparseness in projection measurement. After a brief note on the failure of the standard methods of reconstruction, the methods analytical with series expansions and algebraic with discrete formulations are summarized with an aspect of linear and nonlinear regularizations of least squares solutions.The usefulness of neg-entropies like AIC and GCV for improving the obtained image is emphasized with successful examples in experiment.
It is shown that the plasma equilibrium equation of Grad-Shafranov type is obtained in the system with a closed helical magnetic axis, however only when its torsion is kept constant along the axis. The simple analytical solution of the equation is derived and the method for high beta plama confinement is suggested. The solution is compared with that of a tokamak system in order to clarify the characteristic feature of the system with a helical magnetic axis.
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