# Coulomb collision

Coulomb collision

A Coulomb collision is a binary elastic collision between two charged particles interacting through their own Electric Field. As with any inverse-square law, the resulting trajectories of the colliding particles is a hyperbolic Keplerian orbit. This type of collision is common in plasmas where the typical kinetic energy of the particles is too large to produce a significant deviation from the initial trajectories of the colliding particles, and the cumulative effect of many collisions is considered instead.

## Mathematical Treatment for Plasmas

In a plasma a Coulomb collision rarely results in a large deflection. The cumulative effect of the many small angle collisions, however, is often larger than the effect of the few large angle collisions, so it is instructive to consider the collision dynamics in the limit of small deflections.

We can consider an electron of charge -e and mass me passing a stationary ion of charge +Ze and much larger mass at a distance b with a speed v. The perpendicular force is (1/4πε0)Ze2/b2 at the closest approach and the duration of the encounter is about b/v. The product of these expressions divided by the mass is the change in perpendicular velocity:

$\Delta m_e v_\perp \approx \frac{Ze^2}{4\pi\epsilon_0} \, \frac{1}{vb}$

Note that the deflection angle is proportional to 1/v². Fast particles are "slippery" and thus dominate many transport processes. The efficiency of velocity-matched interactions is also the reason that fusion products tend to heat the electrons rather than (as would be desirable) the ions. If an electric field is present, the faster electrons feel less drag and become even faster in a "run-away" process.

In passing through a field of ions with density n, an electron will have many such encounters simultaneously, with various impact parameters and directions. The cumulative effect can be described as a diffusion of the perpendicular momentum. The corresponding diffusion constant is found by integrating the squares of the individual changes in momentum. The rate of collisions with impact parameter between b and (b+db) is nv(2πb db), so the diffusion constant is given by

$D_{v\perp} = \int \left(\frac{Ze^2}{4\pi\epsilon_0}\right)^2 \, \frac{1}{v^2b^2} \, nv (2\pi b\,{\rm d}b) = \left(\frac{Ze^2}{4\pi\epsilon_0}\right)^2 \, \frac{2\pi n}{v} \, \int \frac{{\rm d}b}{b}$

Obviously the integral diverges toward both small and large impact parameters. At small impact parameters, the momentum transfer also diverges. This is clearly unphysical since under the assumptions used here, the final perpendicular momentum cannot take on a value higher than the initial momentum. Setting the above estimate for $\Delta m_e v_\perp$ equal to mv, we find the lower cut-off to the impact parameter to be about

$b_0 = \frac{Ze^2}{4\pi\epsilon_0} \, \frac{1}{m_e v^2}$

We can also use πb02 as an estimate of the cross section for large-angle collisions. Under some conditions there is a more stringent lower limit due to quantum mechanics, namely the de Broglie wavelength of the electron, h/(mev).

At large impact parameters, the charge of the ion is shielded by the tendency of electrons to cluster in the neighborhood of the ion and other ions to avoid it. The upper cut-off to the impact parameter should thus be approximately equal to the Debye length:

$\lambda_D = \sqrt{\frac{\epsilon_0 k T_e}{n_e e^2}}$

## Coulomb logarithm

The integral of 1/b thus yields the logarithm of the ratio of the upper and lower cut-offs. This number is known as the Coulomb logarithm and is designated by either lnΛ or λ. It is the factor by which small-angle collisions are more effective than large-angle collisions. For many plasmas of interest it takes on values between 5 and 15. (For convenient formulas, see pages 34 and 35 of [1] of the NRL Plasma formulary.) The limits of the impact parameter integral are not sharp, but are uncertain by factors on the order of unity, leading to theoretical uncertainties on the order of 1/λ. For this reason it is often justified to simply take the convenient choice λ = 10.

The analysis here yields the scalings and orders of magnitude. For formulas derived from careful calculations, see page 31 ff. in the NRL Plasma formulary.

Rutherford scattering

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Coulomb collision — noun the interaction between two moving electric charges at close range …   Wiktionary

• Coulomb (disambiguation) — Coulomb or Coulombs may refer to: Charles Augustin de Coulomb (1736–1806), French physicist, also: Coulomb, a unit of electric charge Coulomb s law, in electrostatics Coulomb blockade, an increased resistance of certain electronic devices Coulomb …   Wikipedia

• Coulomb excitation — is a technique in experimental nuclear physics to probe the electromagnetic aspect of nuclear structure. In coulomb excitation, a nucleus is excited by an inelastic collision with another nucleus through the electromagnetic interaction. In order… …   Wikipedia

• Collision response — In the context of classical mechanics simulations and physics engines employed within video games, collision response deals with models and algorithms for simulating the changes in the motion of two solid bodies following collision and other… …   Wikipedia

• Collision cascade — A classical molecular dynamics computer simulation of a collision cascade in Au induced by a 10 keV Au self recoil. This is a typical case of a collision cascade in the heat spike regime. Each small sphere illustrates the position of an atom, in… …   Wikipedia

• Electrothermal instability — NOTOC The electrothermal instability (also known as the ionization instability or Velikhov instability in the literature) is a magnetohydrodynamic (MHD) instability appearing in magnetized non thermal plasmas used in MHD converters. It was first… …   Wikipedia

• Plasma (physics) — For other uses, see Plasma. Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation. The colors are a result of relaxation of electrons in excited states to lower energy states after they have recombined… …   Wikipedia

• Fusor — The Farnsworth–Hirsch Fusor, or simply fusor, is an apparatus designed by Philo T. Farnsworth to create nuclear fusion. It has also been developed in various incarnations by researchers including Elmore, Tuck, and Watson, and more lately by… …   Wikipedia

• Polywell — The polywell is a plasma confinement concept that combines elements of inertial electrostatic confinement and magnetic confinement fusion, intended ultimately to produce fusion power. The name polywell is a portmanteau of polyhedron and potential …   Wikipedia

• Gyrokinetic ElectroMagnetic — GEM is short for Gyrokinetic ElectroMagnetic. GEM is a gyrokinetic plasma turbulence simulation that uses the delta f particle in cell method. It is used to study waves, instabilities and nonlinear behavior of tokamak fusion plasmas. Information… …   Wikipedia