Debye Shielding
Debye shielding is a concept from plasma physics and electrochemistry that describes how electric fields are screened or shielded in a plasma or electrolyte due to the presence of mobile charge carriers. It’s named after the physicist Peter Debye, who first developed the theory to explain how charged particles interact in a medium.
Here’s a detailed explanation of Debye shielding:
1. Basic Concept
In a plasma or electrolyte, when a charged particle (like an electron or ion) is introduced, it creates an electric field around it. This electric field attracts or repels other free charge carriers in the medium, which move in response to the field. As a result, a region around the charged particle becomes "shielded" from the influence of the field, reducing the effective range of the electric field.
2. Debye Length
The Debye length (λ_D) is a measure of the distance over which the electric field of a charged particle is screened out by the surrounding charge carriers. It is defined as:
λD = sqrt of ϵ_0kBT/ ne²
where:
ϵ_o is the permittivity of free space,
k_B is the Boltzmann constant,
T is the absolute temperature,
n is the number density of charge carriers (such as electrons or ions),
e is the elementary charge.
3. Physical Interpretation
Short-Range Effect: Within a distance less than the Debye length, the electric field of a charged particle is significantly reduced by the surrounding charge distribution. Beyond this distance, the field falls off exponentially.
Thermal and Density Effects: The Debye length depends on the temperature and density of the charge carriers. Higher temperatures or lower densities increase the Debye length, implying that the shielding effect is less pronounced.
4. Applications and Implications
Plasma Physics: In a plasma, Debye shielding explains why the electric fields created by individual charges are not felt at large distances. It also affects how plasmas behave under various conditions, such as in fusion reactors.
Electrochemistry: In electrolytes, Debye shielding helps to explain how ions in a solution interact with each other and how charged particles are distributed near electrodes. It’s crucial for understanding processes like electroplating and corrosion.
Semiconductors: The concept of Debye shielding is also relevant in semiconductor physics, where it helps in understanding the behaviour of charge carriers near interfaces or in response to electric fields.
A fundamental characteristics behaviour of a plasma is its ability to shield out electric potential that are applied to it.
Let us suppose two oppositely charged balls connected to a battery are inserted in a plasma as shown in figure.
The positively charged ball attracts the negatively charged particles and thus immediately a cloud of electrons surrounds the positively charged ball. Similarly, a negatively charged ball attracts the positively charged particles and ions surrounds it.
If the plasma were cold there would be just as many charges as in the ball and shielding would be perfect and no electric field would be present in the body of plasma outside the plasma cloud. If the temperature is finite, the potential energy of particles approximately equal to thermal energy of order kT/e can leak into the plasma and cause a finite electric field to exist there.
Let us suppose that the ions have infinite mass as comparable to electron (M/m-> infinite) so that ions do not move but form a uniform background of positive charge and only electron move. Let ϕo be the value of potential ϕ(x) at plane x = 0. Now using poisson's equations in 1D
∇²ϕ = - 4𝜋e (n_i - n_e)
d²ϕ/ dx² = 4𝜋e (n_e - n_i) ...1
where n_i= number of density of ion
n_e = number of density of electron
If the density is far away then
n_i = n_∞ ....2
The electron density is given by M B statistics i.e we have the electron distribution function in the presence of potential energy qϕ is
f(u) = Ae^-1/2 (mu²+ qϕ)/ kTe
f(u) = Ae^-1/2 mu²/ kT_e . e^-qϕ/ kT_e
Here T_e = temperature of electron. For electron q = -e
k= Boltzmann constant
f(u) = Ae^-1/2 mu²/ kTe . e^eϕ/ kTe ...3
Integrating over u then
n_e = ∫ f(u) du = ∫ Ae^-1/2 mu²/ kTe . e^eϕ/ kTe du
n_e = e^eϕ/ kTe ∫ Ae^-1/2 mu²/ kTe du
n_e = n_o e^eϕ/ kTe ......4
Putting this value in equation 1 we get
d²ϕ/ dx² = 4𝜋e (n_o e^eϕ/ kTe - n_o)
=> d²ϕ/ dx² = 4𝜋e n_o [ e^eϕ/ kTe - 1]
In the region where eϕ/ kTe <<1, for x= 0 we can expand the exponential as
e^eϕ/ kTe = 1+ eϕ/ kTe + 1/ 2! (eϕ/ kTe)² + ....
neglecting the higher powers
d²ϕ/ dx² = 4𝜋e n_o ( 1 + eϕ/ kTe -1 )
= 4𝜋e n_o ( eϕ/ kTe )
d²ϕ/ dx² - 4𝜋e² n_o / kT_e ϕ(x) = 0
d²ϕ/ dx² - 1/𝜆²_D ϕ(x) = 0 ...5
𝜆_D = square root kT_e/ 4𝜋e² n_o called Debye length and is measure of shielding distance or thickness of cloud also called sheath.
The solution of equation 5 will be
ϕ (x) = ϕ_o e^-l xl / 𝜆_D....6
Debye length decreases as number density of ion increase but increases as KTe increases.
The electrons are more mobile than ions so that electron temperature, Te is used in the definition of Debye length.
Hence in a crude way, we can say that a charged particle in plasma interacts only with particles less than one Debye length away.
If the dimension L of a system are much larger than 𝜆_D, then, when local charge concentrations or the system arise, these are shielded away in a shorter distance than L, leaving the bulk of plasma free of large electric potentials or fields. This phenomenon is known as Debye Shielding।
Hence a criterion for an ionized gas to be a plasma is that it be dense enough that 𝜆_D << L
The number of particles in a 'Debye sphere'
If there are only few particles in the sheath region, Debye shielding would not be statistically valid concept. Hence for an ionized gas to be a plasma, in addition to 𝜆_D << L , one requires
N_D >> 1
Summary
Debye shielding is a fundamental concept in understanding how charged particles interact within a medium. It describes how the electric field of a charge is reduced due to the redistribution of other charge carriers, leading to a characteristic length scale, the Debye length, which quantifies the extent of this shielding effect. This concept is essential in various scientific fields including plasma physics, electrochemistry, and semiconductor technology.
This note is a part of the Physics Repository