How many types of plasma are there

Plasma research: the fourth state of matter

The fourth state of matter is commonly referred to as plasma. What many do not know: plasmas can have very different properties. While the hot plasmas at low pressure have been well researched, the plasmas at high pressure and low temperature - as they are believed to be in the interior of the large planets - still pose great puzzles. The new FAIR facility at GSI will be dedicated to researching these puzzles.

As a student, you usually learn quite early that there are three states of aggregation: solid, liquid and gaseous. These states can easily be demonstrated in the classroom using water. In addition to these aggregate states - and not all students experience this any more - there is another one: plasma. Matter reaches this state if one adds so much energy in the form of pressure and temperature that individual or all electrons are torn from the electron shell of the atoms. The result is a structure (that's the literal translation from the Greek) of free, negatively charged electrons and positive ions. Matter in this state has completely new physical properties: For example, plasmas are usually very conductive and can be strongly influenced by magnetic fields.

In our world we often encounter plasma states without realizing them. For example, candle flames or thunderstorms are partly made of plasma. This condition is very common in space: Scientists suspect that more than 99 percent of the visible matter in the cosmos is plasma. The stars and possibly the interior of planets are made up of plasmas.

Plasmas can be made very differently: Depending on the temperature and pressure, they exist in different forms. For example, hydrogen exists as a relatively thin plasma in the photosphere of the sun, as a metallic liquid in the center of large planets, or as a hot, high-density fusion plasma inside stars. Such a fusion plasma also prevails inside our sun, in which hydrogen is fused to form helium.

The theoretically predicted phase diagram of hydrogen

In addition to these naturally occurring plasmas, there are numerous ways to produce the plasma in the laboratory. On the one hand, scientists generate it through strong electrical current discharges in a gas or through the use of strong laser beams. Another way is the use of heavy ion beams. Here, high-intensity beams of heavy ions are shot at different elements. The bombarded matter is strongly stimulated by this energy supply, so that the atoms lose their conventional structure and the electrons are set free. This type of “bombardment” enables particularly uniform, larger plasmas of high density to be generated.

Inside large planets such as Saturn or Jupiter, scientists now expect a special form of plasma, the so-called strongly coupled plasma, in which elements known to us as gaseous such as hydrogen, iodine or xenon change into a metallic plasma state. This means that the atomic nuclei are bound in a lattice structure and the electrons move in an electron gas - just as it is known from metals such as iron on earth. The physicists want to investigate this form of plasma, which is created at comparatively low temperatures but high pressures, in the new FAIR facility at GSI. In this facility, by bombarding matter with heavy ion beams pulsed in quick succession, extreme pressures in the megabar range are achieved, which corresponds to about a million times our air pressure.

In addition to this method of producing plasmas, the GSI also has the option of producing plasmas with a particularly powerful laser. Compared to heavy ion beams, this Phelix laser will generate plasmas with lower density but higher temperatures and thus make other areas of the transition into the plasma state (phase transition) explorable.

Photo of a plasma in a frozen xenon crystal

From the worldwide unique combination of heavy ion and laser beams, physicists expect new results for plasma research that have not been possible through previous experiments. On the one hand, the laser and heavy ion beams make different areas in the phase diagram accessible. On the other hand, they enable new types of experimentation and observation techniques. The latter are mainly necessary because the plasma states in the laboratory are only very short-lived (less than a nanosecond). A plasma generated by heavy ion beams is analyzed by a laser beam injected from another direction at the same time - and vice versa. For such experiments, new special beam handling techniques (synchronization, beam focusing, and so on) are necessary for both laser and heavy ion beams.

The experiments made possible by FAIR also open up the prospect of exploring the physical fundamentals of so-called inertial fusion. Some scientists see this as the future of energy supply for mankind. Small hydrogen capsules are to be compressed by bombardment with heavy ion beams in such a way that the fusion process to form helium is set in motion with the release of usable energy.