Plasma [physics]

The plasma is the highest-energy state hot ionized gas consisting of approximately equal numbers of positively charged ions and negatively charged electrons. Like gases, plasmas have no fixed shape or volume and are less dense than solids or liquids. But unlike ordinary gases, plasmas are made up of atoms in which some or all of the electrons have been stripped away and positively charged nuclei, called ions, roam freely.

The characteristics of plasmas are significantly different from those of ordinary neutral gases so that plasmas are considered a distinct “fourth state of matter.“ For example, because plasmas are made up of electrically charged particles, they are strongly influenced by electric and magnetic fields and conducts electricity. In contrast, most gases are electrical insulators.

An example of such influence is the trapping of energetic charged particles along geomagnetic field lines to form the Van Allen radiation belts. When plasma is exposed to a magnetic field, it may assume structures, including layers, filaments, and beams. An excellent example of some of these structures can be observed in a plasma ball.

In addition to externally imposed fields, such as the Earth’s magnetic field or the interplanetary magnetic field, the plasma is acted upon by electric and magnetic fields created within the plasma itself through localized charge concentrations and electric currents that result from the differential motion of the ions and electrons. The forces exerted by these fields on the charged particles that make up the plasma act over long distances and impart to the particles’ behavior a consistent, collective quality that inert gases do not display. (Despite the existence of localized charge concentrations and electric potentials, a plasma is electrically “quasi-neutral,“ because, in aggregate, there are approximately equal numbers of positively and negatively charged particles distributed so that their charges cancel).

It is estimated that 99% of the matter in the observable universe is in the plasma state hence the expression “plasma universe.“ (The phrase “observable universe“ is an important qualifier: roughly 90% of the mass of the universe is thought to be contained in “dark matter,“ the composition and state of which are unknown). Stars, stellar and extragalactic jets, and the interstellar medium are examples of astrophysical plasmas. In our solar system, the Sun, the interplanetary medium, the magnetospheres and/or ionospheres of the Earth and other planets, as well as the ionospheres of comets and certain planetary moons all consist of plasmas.

The plasmas of interest to space physicists are extremely tenuous, with densities dramatically lower than those achieved in laboratory vacuums. The density of the best laboratory vacuum is about 10 billion particles per cubic centimeter. In comparison, the density of the densest magnetospheric plasma region, the inner plasmasphere, is only 1000 particles per cubic centimeter, while that of the plasma sheet is less than one particle per cubic centimeter.

The temperatures of space plasmas are very high, ranging from several thousand degrees Celsius in the plasmasphere to several million degrees in the ring current. While the temperatures of the “cooler“ plasmas of the ionosphere and plasmasphere are typically given in degrees Kelvin, those of the “hotter“ magnetospheric plasmas are more commonly expressed in terms of the average kinetic energies of their constituent particles measured in “electron volts.“

An electron volt (eV) is the energy that an electron acquires as it is accelerated through a potential difference of one volt and is equivalent to 11,600 degrees Kelvin. Magnetospheric plasmas are often characterized as being “cold“ or “hot.“ Although these labels are quite subjective, they are widely used in space physics literature. As a rule of thumb, plasmas with temperatures less than about 100 eV are “cold,“ while those with temperatures ranging from 100 eV to 30 keV can be considered “hot.“ (Particles with higher energies – such as those that populate the radiation belt – are termed “energetic“).

Plasma confinement

When properly configured, external magnetic fields cause confinement of the plasma by exploiting a force entirely analogous to that which induces mutual attraction of parallel current-carrying wires in the same direction. Plasma configurations are generally called striations. Lack of homogeneity in the external magnetic field or presence of electric fields due to induction effects cause in the plasma separate flows of charges of the two signs which tend to destroy the confinement.

The problem of plasma confinement can be summarized as follows: to inject a globally neutral plasma in a magnetic field of such intensity that all particles have helical trajectories with small radii of curvature compared to the size of the container to avoid that they immediately touch the walls; the applied magnetic fields must have lines that close on themselves to avoid that the particles, spiraling along the lines of force, escape the confinement; reached the confinement, it is necessary that this corresponds to a state of stable equilibrium. Very intense fields with particular configurations are used for this purpose.

The plasma configurations used on fusion are generally toroidal in shape. As a result of the coupling between the motion of a plasma and the surrounding field, the plasma is the seat of wave motions ranging from elastic waves, at frequencies lower or of the order of those of collision between particles, to electromagnetic waves, propagated as if the plasma were a medium with refractive index less than 1 with a characteristic frequency, called plasma frequency, which is the frequency of natural oscillation of the electrons of the plasma even in the absence of an external magnetic field. An isotropic isothermal plasma with a Maxwellian distribution of velocities does not exhibit instabilities. These, on the other hand, are typical of plasmas produced in the laboratory and contained magnetically.

Microscopic and macroscopic plasma instabilities

Plasma instabilities can be roughly divided into macroscopic and microscopic. Macroscopic instabilities are those consisting of large plasma motions aimed at destroying its configuration and are, in general, similar to the hydrodynamic instabilities observed when fluids of increasing density are placed on top of each other within a downward-facing gravitational field.

Microinstabilities lead to fluctuations in the density and electric field within the plasma and to a transport of particles and energy out of the magnetic field that should trap them. Whichever category they belong to, instabilities tend to destroy plasma confinement and are therefore the subject of study. However, even in the absence of instabilities, the plasma can evolve to a more disordered state due to particle collisions.

The predominant phenomenon in collisions in a hot plasma is the so-called Rutherford scattering which involves both trajectory deviation and energy exchange; such collisions establish an upper limit to the plasma lifetime. From the collision processes depend three important modes of irradiation by the plasma, irradiation that is the direct route to its cooling. The first one consists in the production of X-rays (electron-ion collision); the second one is consequent to the electron-electron collision; the third one is the irradiation by excitation, due to the de-excitation of ions, partially deprived of orbital electrons following the collision with electrons.

Important methods of measurement of plasma parameters (concentration of charged particles, degree of ionization, temperature, electrical and thermal conductivity) are based on the use of electrical probes. These, historically used first, are usable only when the experimental conditions allow to introduce an electrode in the plasma; other techniques, based on optical and microwave methods, have the advantage of not altering the physical state of the plasma.

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