Unformatted text preview: bye shielding would not be a statistically valid concept. We can compute the
number of particles in a “Debye sphere”, ND, as
N D = n π λ D3
3 In addition to λ D << L, “collective behavior” requires N D >>1. Criteria for Plasmas We have given two conditions that an ionized gas must satisfy to be called a plasma. A
third condition has to do with collisions. The weakly ionized gas in a jet exhaust, for example,
does not qualify as a plasma because the charged particles collide so f requently with neutral
atoms that their motion is controlled by ordinary hydrodynamic forces. If ω is the frequency of
typical plasma oscillations and τ is the mean time between collisions with neutral atoms, we
require ωτ > 1 for the gas to behave like plasma rather than a neutral gas. The three conditions
that a plasma must then satisfy are
1. λ D << L
2. ND >>> 1
3. ωτ > 1 Chapter 2 Single Par;cle mo;on Introduc;on As we know when par;cles move [charges] they generate local concentra;ons of +ve and- ve charges giving rise to E ﬁelds. Mo;on of charge...
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This document was uploaded on 01/17/2014.
- Winter '14