Thomson scattering

Thomson scattering is a specific way that electromagnetic radiation interacts with matter. It describes the elastic scattering of light by a free charged particle. In this process, the particle does not change its kinetic energy. Similarly, the frequency of the photon remains exactly the same. Scientists call this the low-energy limit of Compton scattering. This limit applies when the photon energy is much smaller than the mass energy of the particle. It also applies if the wavelength of light is much greater than the Compton wavelength of the particle. For an electron, this means using wavelengths longer than hard x-rays.

The mechanism of Thomson scattering follows a very specific sequence of events. First, an incident plane wave hits a free charged particle. This wave contains an electric field component. As long as the particle's speed is much less than the speed of light, this electric field is the main cause of acceleration. The magnetic field component can usually be ignored in a first approximation. The electric field causes the charged particle to accelerate and move in the direction of the oscillating field. This movement results in electromagnetic dipole radiation. The particle then emits new radiation of the same frequency as the original wave.

This scattering process has distinct characteristics regarding direction and polarization. The moving particle radiates most strongly in a direction perpendicular to its acceleration. This radiation is also polarized along the direction of the particle's motion. An observer might see different levels of polarization depending on their location. We can divide the electric fields into two types: tangential and radial. Tangential components are perpendicular to the plane of the diagram and are not affected. Radial components lie in the plane. The amplitude of the outgoing wave is proportional to the cosine of the angle between the incident and scattered waves. Consequently, the intensity is diminished by a factor of the cosine squared of that angle.

Physicists have used mathematical models to define how much energy is scattered. This is often described using an emission coefficient. This coefficient measures the energy scattered by a volume element into a specific solid angle. For unpolarized incident light, there are two separate emission coefficients. These correspond to radially polarized light and tangentially polarized light. The Thomson differential cross section relates to the sum of these emissivity coefficients. This cross section depends on the charge of the particle and its mass. It also involves the permittivity of free space. A notable feature is that the cross section is independent of the photon frequency.

The history of this phenomenon is tied to the physicist J. J. Thomson. He was the first to explain how electromagnetic radiation scatters from a free particle. His work provided a classical description of this interaction. Today, Thomson scattering remains a vital concept in the field of plasma physics. It helps scientists understand how light and charged particles behave in different environments. Understanding these classical limits allows researchers to build better models for more complex interactions.

We can see Thomson scattering in action through several notable examples in space. One example is the solar K-corona, which is visible during a solar eclipse. This corona is the result of solar radiation scattering off electrons in the solar corona.

To study this, the ESA and NASA SOHO mission and the NASA STEREO mission use three separate satellites. These missions generate three-dimensional images of the electron density around the Sun. Another example is found in the cosmic microwave background. This background contains a small linearly polarized component known as E-modes. This specific component was first detected by DASI in 2002.

Thomson scattering also plays a critical role in modern laboratory science and technology. In fusion devices like tokamaks or ICF targets, it is used for plasma diagnostics. Scientists use high-intensity laser beams to measure electron temperatures and densities with high accuracy. For instance, the Wendelstein 7-X stellarator uses an upgraded Thomson scattering system. This system uses Nd:YAG lasers to emit multiple pulses in quick succession. These pulses can occur at intervals between 2 ms and 33.3 ms. This allows for up to twelve consecutive measurements. Additionally, models used in X-ray crystallography are based on the principles of Thomson scattering.