physics 122 Lecture 02 October 1st 2013_final

Plasma parameter the picture of debye shielding that

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Unformatted text preview: amp; λD ↑ as T ↑ λD ↓ as n ↑ Quasineutrality If the dimensions L of a system are much larger than λ D , then whenever a localized concentration of charge arises or external potentials are introduced into the system, these are shielded out in a distance short compared with L, leaving the bulk of the plasma free of electric potentials or fields. Outside the sheath on the wall or on an obstacle, ∇ 2 φ is very small, and ni is equal to ne, typically to better than one part in 106. It takes only a small charge imbalance to give rise to potentials of the order of kT/e. the plasma is “quasineutral,” that is, neutral enough so that _ _ one can take ni ~ ne ~ n, where n is a common density called the plasma density; a CRITERION for an ionized gas to be a plasma is that it be dense enough so that λ D is much smaller than L. Plasma Parameter The picture of Debye shielding that we have given above is valid only if there are sufficient particles in the charge cloud. Clearly, if there are only one or two particles in the sheath region, De...
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