Introduction

The plasma parameter is a number, denoted by capital Lambda, Λ, which measures the average number of electrons contained within a Debye sphere (a sphere of radius the Debye length) in a plasma (but note that the word parameter is usually used in plasma physics to refer to bulk plasma properties in general: see plasma parameters). It is defined as N_D = (4π n λ_D³) /3, where n is the number density of particles, and λ_D is the Debye length.

The plasma approximation

One of the criteria which determines whether a collection of charged particles can rigorously be termed a plasma is that Λ>>1. When this is the case, collective electrostatic interactions dominate over binary collisions, and the plasma particles can be treated as if they only interact with a smooth background field, rather than through pairwise interactions (collisions) J.D. Callen, University of Wisconsin-Madison, Draft Material for Fundamentals of Plasma Physics book: Collective Plasma Phenomena //homepages.cae.wisc.edu/~callen/chap1.pdf PDF.

Plasma properties and Λ

The magnitude of Λ can be summarised below See The plasma parameter lecture notes from Richard Fitzpatrick:

Plasma physics