Plasma parameter
From Wikipedia, the free encyclopedia
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:
where
- n is the number density of particles,
- λD is the Debye length.
[edit] 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) [1].
[edit] Plasma properties and Λ
The magnitude of Λ can be summarised below [2]:
| Description | Plasma parameter magnitude | |
| Λ<<1 | Λ>>1 | |
| Coupling | Strongly coupled plasma | Weakly coupled plasma |
| Debye sphere | Sparsely populated | Densely populated |
| Electrostatic influence | Almost continuously | Occasional |
| Typical characteristic | Cold and dense | Hot and diffuse |
| Examples | Solid-density laser ablation plasmas Very "cold" "high pressure" arc discharge White dwarfs / neutron stars atmospheres Plasma ball |
Ionospheric physics Astrophysical plasmas Nuclear fusion Space plasma physics |
