Complex plasma

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A complex or dusty plasma consists of a physical plasma in which there are also particles up to about 100 μm in size. These microparticles, often referred to as dust because of their small size , are hit by the ions and electrons of the plasma and are charged electrically according to the physical conditions in the plasma. In space under UV irradiation as electrons by the photoelectric effect from the dust, the dust invites to positive. Another effect predominates in the laboratory: Since the electrons in a plasma normally have a much higher thermal speed than the ions, they hit the dust particles more frequently, so that their (negative) charge - depending on the size of the particles - between a few and many thousands Electron charges lies.

As in a normal plasma, this charge is shielded by the mobile charge carriers in the plasma, i.e. the ions and electrons. If the dust particles are close enough to one another at a sufficiently high density, they will still feel the charges on the other particles and begin to interact with one another. The complex plasma can then, depending on the choice of plasma parameters, occur in the gaseous, liquid or solid state ( plasma crystal ), as well as in intermediate states. It is therefore in different aggregate states.

When examining complex plasmas in the laboratory, the unique advantage is that the dust particles can be made individually visible by illuminating with a laser and observing them with a camera. In this way, the movement of the microparticles can be tracked and evaluated individually. Fundamental processes, such as phase transitions and wave propagation, can be made visible, so to speak, with the naked eye, on the kinetic, i.e. the fundamental level, which is difficult to do with real crystals on the atomic level. The forces acting can be inferred directly from the trajectories and the known masses of the microparticles.

On earth, the dust particles are held in suspension (levitated) in the electrical field of the plasma edge layer , as otherwise they would fall to the floor of the plasma chamber under the action of gravity . An exception are nanometer-sized particles, in which case the influence of the gravitational pull is small compared to other forces. Despite the levitation, mostly only a few layers of dust systems form on the earth. Therefore experiments under microgravity, e.g. B. during parabolic flights or on the International Space Station ISS . In 2001 the 'PKE-Nefedov' plasma crystal experiment at MPE Garching was the first ever physical experiment on the ISS.

Various forces act on the dust particles in the plasma. In addition to gravitation and electrical or magnetic forces, the dust particles are z. B. hit by the neutral gas particles. The ions also exert a significant force on the dust particles. In contrast, the friction with the electrons can mostly be neglected. In addition, a strong laser can be used to exert a force on the dust particles. Another role is played by the thermophoretic force , which drives the microparticles to the colder side in the presence of a temperature gradient . In order to better study the weak forces on the particles, experiments under weightlessness are also helpful.

Complex plasmas exist in nature in many conditions; they play a role in the earth's atmosphere, the planetary ring (e.g. rings of Saturn ) and the tail of comets . Many types of dusty plasmas exist in space in particular, since 99% of the matter is present as plasma, which frequently comes into contact with interstellar or interplanetary dust . Research into complex plasmas in the laboratory provides important information on how planets arise from a disk of dust and gas around a young star .

Dust in the plasma is not always desirable. In the production of microchips z. B. it destroys the sensitive structures. The (disruptive) effects of dust in the planned ITER fusion reactor are currently being discussed. In many cases (e.g. soot) particles in hot combustion gases are also undesirable. Methods that were developed, among other things, to deal with complex plasmas can help here.

Web links

• Research group complex plasmas - DLR Oberpfaffenhofen

literature

• H. Thomas, GE Morfill, V. Demmel et al: Plasma Crystal - Coulomb crystallization in a dusty plasma. In: PHYSICAL REVIEW LETTERS. 73, 1994, pp. 652-655, doi : 10.1103 / PhysRevLett.73.652 .
• HM Thomas, GE Morfill: Melting dynamics of a plasma crystal. In: NATURE. 379, 1996, pp. 806-809, doi : 10.1038 / 379806a0 .
• GE Morfill, HM Thomas, U. Konopka and others: Condensed plasmas under microgravity. In: PHYSICAL REVIEW LETTERS. 83, 1999, pp. 1598-1601, doi : 10.1103 / PhysRevLett.83.1598 .
• GE Morfill, HM Thomas, U. Konopka et al: The plasma condensation: Liquid and crystalline plasmas. In: PHYSICS OF PLASMAS. 6, 1999, pp. 1769-1780, doi : 10.1063 / 1.873435 .

supporting documents

1. ↑ Michael Kretschmer: MPE: Plasmakristall - Current. In: www2011.mpe.mpg.de. Retrieved November 20, 2016 . 
2. ↑ TW Hartquist, O. Havnes, and GE Morfill, The effects of charged dust on Saturn's rings, A&G (2003) 44 (5): 5.26-5.30, doi : 10.1046 / j.1468-4004.2003.44526.x .
3. ^ CJ Mitchell et al .: Saturn's Spokes: Lost and Found . Science , March 17, 2006, Vol. 311. No. 5767, pp. 1587-1589, doi : 10.1126 / science.1123783 .
4. ↑ Qualification and quality assurance of wall material - ITER fusion reactor - LABO ONLINE. In: www.labo.de. Retrieved November 20, 2016 . 
5. ↑ Malizia, A .; Poggi, LA; Ciparisse, J.-F .; Rossi, R .; Bellecci, C .; Gaudio, P. A Review of Dangerous Dust in Fusion Reactors: from Its Creation to Its Resuspension in Case of LOCA and LOVA. Energies 2016, 9, 578 doi : 10.3390 / en9080578 .