In plasma physics, polarization refers to two primary phenomena: the dielectric response of the plasma medium itself to an external electric field, leading to charge separation, and more commonly, the specific orientation and phase relationship of the electric field oscillations of electromagnetic radiation (light) emitted from or propagating through the plasma.
This second aspect, the polarization of emitted radiation, is particularly significant as it offers a powerful diagnostic tool. It reveals the internal dynamics and anisotropic properties of the plasma, acting as a window into its microscopic state.
Understanding Polarization of Electromagnetic Waves
Before diving into plasma specifics, it's helpful to understand polarization in general. Electromagnetic waves, such as light, consist of oscillating electric and magnetic fields perpendicular to each other and to the direction of wave propagation. Polarization describes the orientation of the electric field's oscillation in the plane perpendicular to the wave's direction of travel.
- Linear Polarization: The electric field oscillates along a single line.
- Circular Polarization: The electric field vector rotates in a circle as the wave propagates.
- Elliptical Polarization: The electric field vector traces an ellipse.
Plasma as a Polarizable Medium
Just like any dielectric material, plasma can exhibit a form of polarization. When an external electric field is applied to a plasma, the lighter electrons and heavier ions respond differently:
- Charge Separation: Electrons are displaced in one direction, and ions in the opposite direction, creating a net electric dipole moment within localized regions of the plasma.
- Internal Field Generation: This separation of charge generates an internal electric field that opposes the external applied field.
- Dielectric Response: This collective behavior of charges contributes to the plasma's dielectric constant or permittivity, which dictates how the plasma responds to and screens electric fields. This property significantly influences the propagation of electromagnetic waves through the plasma, affecting their speed and direction.
For more on plasma's dielectric properties, see Plasma dielectric constant.
Polarization of Radiation Emitted from Plasma: A Diagnostic Window
A crucial aspect of polarization in plasma physics involves the electromagnetic radiation (light, X-rays, microwaves) that the plasma itself emits. The polarization of radiation emitted from plasma directly reflects the anisotropic properties of the plasma, especially the angular anisotropic distribution of electron velocities. This means that if the electrons in a plasma are moving preferentially in certain directions rather than isotropically (uniformly in all directions), the light they emit will often have a specific polarization state.
This phenomenon is invaluable for researchers:
- Anisotropy Detection: By analyzing the polarization of emitted light, scientists can infer the presence and degree of anisotropy in the electron velocity distribution, which might be caused by strong magnetic fields, collisions, or specific heating mechanisms.
- Magnetic Field Measurement: In magnetized plasmas, the emission from electrons and ions is often polarized. The degree and direction of this polarization can be used to map the strength and orientation of the internal magnetic fields, which are critical for magnetic confinement fusion devices.
- Impurity Ion Line Observations: Polarization has been observed on impurity ion lines from various experimental devices. For example, observations have been made on such lines from the WT-3 tokamak and the GAMMA 10 tandem mirror device, providing insights into their complex plasma dynamics.
Factors Influencing Emitted Polarization:
Several factors can lead to polarized emission from plasma:
- Magnetic Fields: Strong magnetic fields (e.g., in tokamaks or space plasmas) cause electrons to gyrate, leading to cyclotron and synchrotron radiation that is highly polarized. The Zeeman effect in atomic lines also induces polarization.
- Electron Velocity Anisotropy: If electrons have a preferred direction of motion (e.g., a "beam" of electrons or a non-Maxwellian distribution), their emitted radiation will be polarized.
- Collisional Anisotropy: Anisotropic collisions between plasma particles can also lead to polarization of emitted radiation.
- Plasma Density and Temperature Gradients: These can influence the propagation and emission of polarized light within the plasma.
Applications and Significance (Polarimetry)
The study and measurement of polarization in plasma physics, known as polarimetry, is a fundamental diagnostic technique. It provides non-invasive ways to characterize various plasma parameters that are difficult to measure directly.
Key applications include:
- Fusion Research:
- Measuring current profiles in tokamaks by detecting Faraday rotation, a magnetic field-induced rotation of linearly polarized light.
- Diagnosing electron density and temperature distributions.
- Understanding energetic particle confinement and transport.
- Astrophysics:
- Studying solar flares and the solar corona.
- Analyzing radiation from pulsars, active galactic nuclei, and other cosmic sources where strong magnetic fields and energetic plasmas are present.
- Space Plasma Physics:
- Investigating the Earth's ionosphere and magnetosphere.
- Characterizing solar wind interactions with planetary magnetic fields.
By carefully measuring changes in the polarization state of light, researchers can deduce critical information about the plasma's density, magnetic field, and particle distributions, advancing our understanding of this ubiquitous state of matter.
