Plasma (Part-II)- Lamenting its Traditional Neglect and Potential

Image for representative purpose only.

What is the potential of Plasma? How is the temperature of plasma measured? Comparison between Thermal and Non-Thermal Plasma? What is Plasma Magnetization?

Here we continue with the second part of our blog on plasma. Those who have missed our first blog can read it from Here. It will help to connect with this second part of the blog discussing details about the potential of plasma, measuring its temperature, comparison between thermal and non-thermal plasma and plasma magnetization. Let us explore about all the minute details of plasma, while reading this blog. In words of Hannes Alfvén-

"Students using astrophysical textbooks remain essentially ignorant of even the existence of plasma concepts, despite the fact that some of them have been known for half a century. The conclusion is that astrophysics is too important to be left in the hands of astrophysicists who have gotten their main knowledge from these textbooks. Earthbound and space telescope data must be treated by scientists who are familiar with laboratory and magneto-spheric physics and circuit theory, and of course with modern plasma theory."

What is the Potential of Plasma?

Plasma are very good electrical conductors, electric potentials play an important role. The average potential in the space between charged particles, independent of how it can be measured, is called the "plasma potential", or the "space potential". If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a Debye sheath. The good electrical conductivity of plasma makes their electric fields very small. This results in the important concept of "quasi neutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma on the scale of the Debye length there can be charge imbalance. In the special case that double layers are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. In astrophysical plasma, Debye screening prevents electric fields from directly affecting the plasma over large distances, i.e., greater than the Debye length. However, the existence of charged particles causes the plasma to generate, and be affected by, magnetic fields. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object that separates charge over a few tens of Debye lengths. The dynamics of plasma interacting with external and self-generated magnetic fields are studied in the academic discipline of magneto hydrodynamics.

How is the Temperature of Plasma Measured?

Plasma temperature is commonly measured in kelvins or electron volts and is, informally, a measure of the thermal kinetic energy per particle. High temperature are usually required to bolster ionization, which is a defining feature of a plasma. The degree or the magnitude of plasma ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density), in a relationship called the Saha equation. At low temperatures, ions and electrons tend to reintegrate into bound states atoms and the plasma will eventually become a gas.

In most cases the electrons are close enough to thermal equilibrium that their temperature is relatively well-defined, even when there is a significant deviation from a Maxwellian energy distribution function, for example, due to UV radiation, energetic particles, or strong electric fields. Because of the large difference in mass, the electrons come to thermodynamic equilibrium among themselves much faster than they come into equilibrium with the ions or neutral atoms. For this reason, the ion temperature may be very different from (usually lower than) the electron temperature. This is especially common in weakly ionized technological plasma, where the ions are often near the ambient temperature.

Comparison between Thermal and Non-Thermal Plasma

Based on the relative temperatures of the electrons, ions and neutrals, plasma are classified as "thermal" or "non-thermal" (also referred to as "cold plasma").

• Thermal plasmas have electrons and the heavy particles at the same temperature, i.e. they are in thermal equilibrium with each other.
• Non-thermal plasma on the other hand are non-equilibrium ionized gases, with two temperatures: ions and neutrals stay at a low temperature (sometimes room temperature), whereas electrons are such hotter. A kind of common non-thermal plasma is the mercury-vapor gas within a fluorescent lamp, where the "electrons gas" reaches a temperature of 10,000 kelvins while the rest of the gas stays barely above room temperature, so the bulb can even be touched with hands while operating. A particular and unusual case of "inverse" non-thermal plasma is the very high temperature plasma produced by the Z-machine, where ions are much hotter than electrons.

What is Plasma Magnetization?

Plasma with a magnetic field strong enough to influence the motion of the charged particles is said to be magnetized. A common quantitative criterion is that a particle on average completes at least one gyration around the magnetic field before making a collision, is the "electron gyro frequency" and  is the "electron collision rate". It is often the case that the electrons are magnetized while the ions are not. Magnetized plasmas are anisotropic, meaning that their properties in the direction parallel to the magnetic field are different from those perpendicular to it. While electric fields in plasmas are usually small due to the high conductivity, the electric field associated with a plasma moving in a magnetic field and is not affected by Debye shielding.

Continued in the next blog...