Experimental & Theoretical Plasma Physics

Information for Newcomers & Media

Short story

Plasmas are hot electrically conducting gases found in many different situations and having an extremely wide range of densities and temperatures. Examples of plasmas are the solar corona, neon lights, the Earth’s ionosphere and magnetosphere, and the astrophysical jets associated with new-born stars. The Bellan group has plasma experiments that behave in ways analogous to the solar corona and to astrophysical jets and so these experiments provide a means for gaining new insights regarding these space plasmas. The group also has a dusty plasma experiment where the dust consists of micron size grains of water ice; this is relevant to noctilucent clouds, some of Saturn’s rings, and accretion disks around new-born stars.

Long story

Everyday examples of plasma are neon signs, fluorescent lights, lighting, Northern Lights, and welding arcs. More exotic plasmas are the solar corona and astrophysical jets. Because a gas turns into a plasma when sufficiently heated, plasmas are sometimes called the “fourth state of matter”. Atoms are somewhat like miniature solar systems since the nucleus which contains a certain number of protons is orbited by the same number of electrons. The number of protons and their associated electrons defines each element of the periodic table, for example, hydrogen has one electron and one proton while oxygen has sixteen electrons and sixteen protons. If the atom is jiggled up somehow, say by collision with another atom or with an errant free electron or proton, an electron can be knocked off so the atom is missing an electron and now has more protons than electrons so it is positively charged. This is called an ion. A plasma is a gas consisting of free electrons and free ions. The plasma may also have neutral atoms and in some special situations discussed separately, it could also have some dust particles. Plasmas can be categorized many ways but a good way is according to whether the plasma is weakly ionized (has many more neutral than electrons and ions) or is strongly ionized (has essentially no neutrals).

Plasmas are electrically conducting and the electric currents in a plasma can both create magnetic fields and experience forces because of magnetic fields. This provides another categorization scheme, namely whether magnetic effects are important or not. In a strongly magnetized plasma the magnetic forces are much more important than the force associated with a pressure gradient, while in an unmagnetized plasma the magnetic field is so small as to be unimportant.

When a plasma becomes hotter its electrical conductivity increases substantially which is quite the opposite behavior of ordinary matter. This means that hot plasmas are quite good conductors and in many situations can be thought of as behaving like superconductors. This means that the magnetic field is frozen into the plasma so if the magnetic field lines move, the plasma moves with these field lines.

There are various levels of description of a plasma. The simplest is called magnetohydrodynamics and this treats the plasma as a superconducting magnetized gas. Electric currents flow in the plasma and these currents create magnetic fields and also interact with magnetic fields. The plasma is pushed around by this interaction but moves in such a way that the magnetic field remains frozen into the plasma.

More detailed levels of description include two-fluid and Vlasov theories where the plasma is considered as a gas of ions and electrons. Two-fluid theory assumes the electrons have come to thermal equilibrium with each other and so can be assigned an electron temperature with the same assumption holding for ions so there is an ion temperature. The electron and ion temperatures might differ and the electrons and ions might behave in quite different manners. Vlasov theory drops the assumption that electrons or ions have come to thermal equilibrium and so provides an even more complex point of view.

The magnetohydrodynamic description does a good job of characterizing much of the behavior of the solar corona,   astrophysical plasmas, Earth’s magnetosphere, and  fusion plasmas such as spheromaks and tokamaks.  However,  this description misses many fine scale behaviors which require going to one of the more detailed theories.

The equations of plasma physics have no intrinsic scale; this contrasts with solid state physics for example where the intrinsic scale is the separation distance between atoms in a solid. Because plasmas have no intrinsic scale, the same set of equations can govern two systems having extremely different scales. For example, the same equations describe behavior in a lab experiment having characteristic length of less than a meter and the behavior of the solar corona, a magnetohydrodyamic plasma having a chacteristic scale of millions to hundreds of millions of meters.  When the length scale changes, the time scale also changes so what takes place in microseconds in the lab might take minutes on the sun.

It is difficult to measure properties of the solar corona plasma in detail and the plasma is dynamic and each eruption or motion is somewhat different. The lack of detailed data and the irreproducibility makes it difficult to develop theoretical models that explain observed behavior. Similar issues arise for astrophysical and magnetospheric plasmas. However, because of the scalability of plasmas it is often possible to create a laboratory plasma that has similar, but scaled down, behavior. If the lab plasma is reproducible and well-diagnosed then it becomes possible to study a complex phenomenon by having it repeat over and over again the same way and then using sophisticated diagnostics to measure each aspect. This can then be reconciled with a theoretical model and the theory can then be scaled up.

The Bellan plasma group has several magnetohydrodynamically governed plasma experiments. These have various geometric configurations and are relevant to solar, astrophysical and fusion plasmas.

Plasmas have much more complex structure than a neutral gas such as the air we breathe and in particular can support the propagation of wide variety of waves having frequencies ranging over many orders of magnitude.

Dusty plasma involve an extra species, namely charged dust grains, so the plasma consists of electrons, ions, dust grains, and often neutrals.  The addition of dust grains brings in an additional degree of complexity. Dusty plasmas exist naturally in several different situations including the noctilucent clouds existing in polar regions at extremely high altitudes, some of Saturn’s rings, and certain types of accretions disks that orbit new-born stars.

Plasma diagnostics are extremely varied and include spectroscopy, probe measurement of particle fluxes and fields, photography, and wave measurements.

Plasmas are also of industrial importance. Weakly-ionized, unmagnetized plasmas are used to etch the integrated circuits used in modern electronics while highly ionized magnetized plasmas are used in electric arc furnaces that melt down scrap steel to make new steel. Plasma thrusters are also being used as rocket engines in some space missions and require much less fuel than a conventional chemical thruster.

Plasma parameters vary enormously from one type of plasma to another. Plasmas are  chacterized by their density,  temperature (electrons and ions can have different temperatures), and  magnetic field. Densities vary over thirty orders of magnitude going from the most rarified space plasmas to the most dense inertial confinement fusion plasmas. Temperatures and magetic fields can range over twelve orders of magnitude from microgauss in some place plasmas to megagauss in some lab plasmas. Despite this wide range, the same equations apply to these different situations and so a person who studies plasma physics can work in all these different areas.