Screened Poisson equation explained

In physics, the screened Poisson equation is a Poisson equation, which arises in (for example) the Klein–Gordon equation, electric field screening in plasmas, and nonlocal granular fluidity^[1] in granular flow.

Statement of the equation

The equation is$$\backslash left[\backslash Delta\; -\; \backslash lambda^2\; \backslash right]\; u(\backslash mathbf)\; =\; -\; f(\backslash mathbf),$$

where

\Delta is the Laplace operator, λ is a constant that expresses the "screening", f is an arbitrary function of position (known as the "source function") and u is the function to be determined.

In the homogeneous case (f=0), the screened Poisson equation is the same as the time-independent Klein–Gordon equation. In the inhomogeneous case, the screened Poisson equation is very similar to the inhomogeneous Helmholtz equation, the only difference being the sign within the brackets.

Electrostatics

In electric-field screening, screened Poisson equation for the electric potential

\phi( r) is usually written as (SI units)

$$\backslash left[\backslash Delta\; -\; k\_0^2\; \backslash right]\; \backslash phi(\backslash mathbf)\; =\; -\; \backslash frac,$$

where

k ^{-1} _{0} is the screening length, \rho _{\rm} ( r) is the charge density produced by an external field in the absence of screening and \epsilon _{0} is the vacuum permittivity.This equation can be derived in several screening models like Thomas–Fermi screening in solid-state physics and Debye screening in plasmas.

Solutions

Three dimensions

Without loss of generality, we will take λ to be non-negative. When λ is zero, the equation reduces to Poisson's equation. Therefore, when λ is very small, the solution approaches that of the unscreened Poisson equation, which, in dimension

n =3 , is a superposition of 1/r functions weighted by the source function f:

$$u(\backslash mathbf)\_\; =\; \backslash iiint\; \backslash mathrm^3r\text{'}\; \backslash frac.$$

On the other hand, when λ is extremely large, u approaches the value f/λ², which goes to zero as λ goes to infinity. As we shall see, the solution for intermediate values of λ behaves as a superposition of screened (or damped) 1/r functions, with λ behaving as the strength of the screening.

The screened Poisson equation can be solved for general f using the method of Green's functions. The Green's function G is defined by

$$\backslash left[\backslash Delta\; -\; \backslash lambda^2\; \backslash right]\; G(\backslash mathbf)\; =\; -\; \backslash delta^3(\backslash mathbf),$$where δ³ is a delta function with unit mass concentrated at the origin of R³.

Assuming u and its derivatives vanish at large r, we may perform a continuous Fourier transform in spatial coordinates:

$$G(\backslash mathbf)\; =\; \backslash iiint\; \backslash mathrm^3r\; \backslash ;\; G(\backslash mathbf)\; e^$$

where the integral is taken over all space. It is then straightforward to show that

$$\backslash left[k^2\; +\; \backslash lambda^2\; \backslash right]\; G(\backslash mathbf)\; =\; 1.$$

The Green's function in r is therefore given by the inverse Fourier transform,

$$G(\backslash mathbf)\; =\; \backslash frac\; \backslash ;\; \backslash iiint\; \backslash mathrm^3\backslash !k\; \backslash ;\; \backslash frac.$$

k _{r} :

$$G(\backslash mathbf)\; =\; \backslash frac\; \backslash ;\; \backslash int\_0^\backslash infty\; \backslash mathrmk\_r\; \backslash ;\; \backslash frac.$$

This may be evaluated using contour integration. The result is:

$$G(\backslash mathbf)\; =\; \backslash frac.$$

The solution to the full problem is then given by

$$u(\backslash mathbf)\; =\; \backslash int\; \backslash mathrm^3r\text{'}\; G(\backslash mathbf\; -\; \backslash mathbf\text{'})\; f(\backslash mathbf\text{'})=\; \backslash int\; \backslash mathrm^3r\text{'}\; \backslash frac\; f(\backslash mathbf\text{'}).$$

As stated above, this is a superposition of screened 1/r functions, weighted by the source function f and with λ acting as the strength of the screening. The screened 1/r function is often encountered in physics as a screened Coulomb potential, also called a "Yukawa potential".

Two dimensions

In two dimensions:In the case of a magnetized plasma, the screened Poisson equation is quasi-2D:$$\backslash left(\backslash Delta\_\backslash perp\; -\; \backslash frac\; \backslash right)u(\backslash mathbf\_\backslash perp)\; =\; -f(\backslash mathbf\_\backslash perp)$$with

\Delta _{\perp=\nabla ⋅ \nabla} _{\perp} and \nabla _{\perp = \nabla - (B)/(B)} ⋅ \nabla , with B the magnetic field and \rho is the (ion) Larmor radius.The two-dimensional Fourier Transform of the associated Green's function is:$$G(\backslash mathbf)\; =\; \backslash iint\; d^2\; r~G(\backslash mathbf\_\backslash perp)\; e^.$$The 2D screened Poisson equation yields:$$\backslash left(k\_\backslash perp^2\; +\backslash frac\; \backslash right)\; G(\backslash mathbf\_\backslash perp)\; =\; 1\; .$$The Green's function is therefore given by the inverse Fourier transform:$$G(\backslash mathbf\_\backslash perp)\; =\; \backslash frac\; \backslash ;\; \backslash iint\; \backslash mathrm^2\backslash !k\; \backslash ;\; \backslash frac.$$This integral can be calculated using polar coordinates in k-space:$$\backslash mathbf\_\backslash perp\; =\; (k\_r\backslash cos(\backslash theta),\; k\_r\backslash sin(\backslash theta))$$The integration over the angular coordinate gives a Bessel function, and the integral reduces to one over the radial wavenumber k _{r} :$$G(\backslash mathbf\_\backslash perp)\; =\; \backslash frac\; \backslash ;\; \backslash int\_0^\backslash infty\; \backslash mathrmk\_r\; \backslash ;\; \backslash frac\; =\; \backslash frac\; K\_0(r\_\backslash perp\; \backslash ,\; /\; \backslash ,\; \backslash rho).$$

Connection to the Laplace distribution

The Green's functions in both 2D and 3D are identical to the probability density function of the multivariate Laplace distribution for two and three dimensions respectively.

See also

• Yukawa interaction

Notes and References

1. Kamrin . Ken. Koval. Georg . Nonlocal Constitutive Relation for Steady Granular Flow. Physical Review Letters. 26 April 2012. 108. 17. 10.1103/PhysRevLett.108.178301. 2012PhRvL.108q8301K. 22680912. 178301. 1721.1/71598. free.