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TheTG wave then couples to helicon waves to form a combined TG-helicon wave. TheTG wave parametrically decays into electron cyclotron waves, as mentioned above,and the latter heats the electrons by collisional damping. Though the TG wave is anessential part of the RF coupling, it is not easily detected because it is normallylocalized to a thin layer near the surface. By lowering the B-field to thicken thelayer, and measuring the RF current rather than the RF B-field, the TG wave can bedetected, as shown in Fig. 9.13. The current profile shows large peaks due to the TGmode, but these are not seen in the B-field profile.9.6Plasmas in SpaceAs we leave the earth, we first encounter plasmas in the ionosphere, where aurorasare born about 100 km above the earth.
Further out are the Van Allen radiationbelts, filled with plasma from the solar wind. Further yet is the magnetosphere3509 Special Plasmaswhere the B-fields of the sun and the earth collide, and, on the night side, themagnetotail, where reconnection takes place (Fig.
8.31). The sun has importantplasma effects in its prominences, flares, and sunspots. The outer planets haveplasma in their atmospheres and rings. Space plasma physics is the study of theseplasmas in the solar system. Plasma astrophysics, on the other hand, concernsplasmas in the rest of the universe.
Galaxies contain plasmas in stars, gas clouds,and black holes. Pulsars, quasars, and active galactic nuclei are all astrophysicalplasmas, some with extreme properties. Neutron stars, for instance, have densitiesof order 4 1017 kg/m3. A sugar-cube sized piece (1 cm3) of this material wouldweigh as much as 1.3 million Eiffel towers.Auroras are produced by ions and electrons of energies of about 100–1000 eVwhich reach down to altitudes of about 100 km. They excite the oxygen andnitrogen atoms in the atmosphere, giving off green and brown light from oxygenand blue and purple light from nitrogen.
Striking displays of Aurora Borealis areobserved in northern latitudes. Aurora Australis also occurs in Antarctica but is seenmostly by penguins. In outer space all that we can see must be in the plasma state,but this is only a tiny fraction of a universe containing dark energy, dark matter, andblack holes. Nonetheless, astrophysics cannot be studied without the language ofplasma physics.9.7Atmospheric-Pressure PlasmasWhen a voltage is applied between two electrodes in air, a spark will form betweenthe electrodes, forming a plasma, when the voltage is high enough. This breakdown,studied in 1889 by F.
Paschen, was perhaps the first plasma experiment. Thebreakdown voltage depends on the gas, the pressure p, and the distanced between the electrodes. Figure 9.14 shows this relationship in a modern experiment in argon, where contaminants such as moisture are absent. When pd is small,electrons’ mean free paths are longer than d, and large voltages are required toaccelerate ions to energies that can release secondary electrons from the surface.When pd is large, electrons lose energy in collisions with the neutral gas, and thevoltage rises again. Thus there is a minimum in the Paschen curve.
After breakdownbetween parallel plates, the current is not uniform but tends to flow in streamers,each reaching its own equilibrium.In such an equilibrium, each time an ion strikes the negative plate (the cathode),γ electrons are released, γ being the secondary electron emission coefficient. Theseelectrons are accelerated by the sheath into the plasma and collide with neutrals toform new electron-ion pairs. Each new ion will strike the cathode to release moresecondary electrons, thus creating an avalanche. The density will exponentiate. Letα be the probability for an ionizing collision in a unit distance. Clearly α isproportional to 1/λmfp, where λmfp is the mean free path for ionization. One electronwill generate eαd ion-electron pairs in a distance d. Upon colliding with the cathode,9.7 Atmospheric-Pressure Plasmas351Fig.
9.14 A Paschen curve for argonthese ions will generate γ(eαd 1) new electrons. In equilibrium this has to reproduce exactly the original electron. Thus steady state requiresγ eαd1 ¼ 1:ð9:47ÞThis is called a Townsend discharge.
The symbols α and γ are also used to denotetwo regimes of RF discharges, a low-current and a high-current regime, with adiscontinuous jump between the two.9.7.1Dielectric Barrier DischargesTo prevent sparking at atmospheric pressures, a dielectric barrier can be insertedbetween the two electrodes. In Fig.
9.15, the electrodes are covered with insulatingdielectric, and high-voltage pulses create the plasma by capacitive coupling. InFig. 9.16, a dielectric barrier separates the electrodes, but the E-field extends intothe space above to ionize the plasma. The substrate to be treated is passed horizontally through the plasma.Dielectric barrier discharges are used in xenon lamps and for the pixels in plasmadisplay panels (TV screens), for instance.3529 Special PlasmasFig.
9.15 A dielectric barrier discharge of Type 1Fig. 9.16 A dielectricbarrier discharge of Type 29.7.2RF Pencil DischargesAnother type of discharge that operates at atmospheric pressure is in the shape of apencil, as shown in Fig. 9.17. The plasma is excited with RF or microwaves toprevent arcing. In one commercial application, a mixture of helium and oxygen isinjected, and 200 W of RF at the industrial frequency of 13.56 MHz is appliedbetween the center tube and ground.
A plasma beam about two inches long isformed with density up to 1013 cm 3, compared to normal air density of3 1019 cm 3. Since no vacuum system is required, these beams can be usedmedically for cauterizing skin and for some dental procedures. The beam can also9.7 Atmospheric-Pressure Plasmas353Fig. 9.17 Schematic of a pencil-type atmospheric plasmabe scanned, line by line, over a large surface that requires plasma treatment, such ascleaning or deposition.
To treat a wire-shaped object, an atmospheric plasma can bemade inside a cylinder through which the wire is passed. Conversely, a catheter canbe sterilized with a plasma created with a wire inside it.A duodenoscope is a small medical instrument inserted into the small intestine totreat such conditions of the bile ducts and main pancreatic duct. These devises havevery small openings which are difficult to sterilize. In 2015, numerous deaths werecaused by a superbug known by the acronym CRE which survives normal sterilization methods. Since bacteria would be killed by a plasma at 1 or 2 eV,duodenoscopes could be cleaned with an atmospheric plasma, since the smallmean free path of electrons in air would allow them to enter very small cavities.In recent years atmospheric pencil plasmas have become widely used in the medicalprofession.Problems9.1 A C60 pair-ion plasma is created with a temperature KT ¼ 0.3 eV.
Describe thesheath at the end walls that intercept the magnetic field lines.9.2 Modify the R- and L-wave dispersion relations for propagation at an angle θto B.Chapter 10Plasma Applications10.1IntroductionThe effect of plasmas had been noticed as early as 1901, when Marconi found thatradio waves could cross the Atlantic in spite of the curvature of the earth. We nowknow that the waves were reflected by the ionosphere. The study of plasmasprobably began with Irving Langmuir’s experiments on sheaths in 1928, and itwas he who coined the name plasma in a blood-free context.
Practical use of plasmasbegan in the late 1940s with E.O. Lawrence’s invention of the calutron (named forthe University of California) for the separation of U235 from U238 for use in atomicbombs. It was the effort to tame the H-bomb into a steady source of electricity—hydrogen fusion—that spawned modern plasma physics. More on that later.Today, many objects used in everyday life are made or treated with plasmas.These are not the fully ionized plasmas needed for fusion but are partially ionizedplasmas with electron temperatures below about 4 eV, the so-called low-temperature plasmas.
About 12 % of electricity generated in the U.S. is used for lighting,and over 60 % of lamps involve low-temperature plasmas. Fluorescent lights use anargon or neon plasma containing mercury to generate invisible ultraviolet light,which then excites a phosphor coating that glows visibly. Fluorescents are beingreplaced by more efficient LEDs (light-emitting diodes), which contain solid-stateplasmas in p-n junctions. The latter are also at the heart of solar cells.
Semiconductors in electronic devices are made with the use of plasma etching and deposition. Plastic sheets are made hydrophylic or hydrophobic depending on whetherthey are to be printed on, such as in food packaging. The pixels in our TVs andcomputer screens are etched with plasmas. Windows are glazed with plasmas totransmit or reflect specific wavelengths. Heat barriers in jet engines are made withplasma deposition. Thrusters in spacecraft are plasma ejectors (Sect. 10.4). InThe original version of this chapter was revised.
An erratum to this chapter can be found at https://doi.org/10.1007/978-3-319-22309-4_11© Springer International Publishing Switzerland 2016F.F. Chen, Introduction to Plasma Physics and Controlled Fusion,DOI 10.1007/978-3-319-22309-4_1035535610Plasma Applicationsmedicine, plasmas are used for sterilization and for hardening or knee implants byion implantation, for instance. The subject of plasma chemistry has been developedfor these applications. All lasers contain plasmas, ranging from the huge megajoulelasers at Livermore in the U.S., in Osaka in Japan, and at Bordeaux in France, to thelaser pointers used in lectures.Many phenomena in basic physics either involve plasmas or may involve plasmaswhen finally solved.
Lightning strikes are gas discharges between charged clouds orbetween a cloud and ground. Ball lightning is a glowing sphere of plasma that is onthe ground and lasts many seconds. Because of its unpredictability, it is unexplained.The geodynamo inside the earth that creates its magnetic field involves motions ofliquids, but probably not gaseous plasmas. The BICEP2 experiment (BackgroundImaging of Cosmic Extragalactic Polarization) seeks to find evidence of primordialgravitational waves from the Big Bang in the cosmic microwave background. Dust isbelieved to be involved.
Dusty plasmas have been added to this edition.10.2Fusion EnergyAll of the world’s energy depends on the sun. Our fossil fuels come from trees thatgrew in sunlight millions of years ago. Sunlight causes evaporation and makes clouds,and the rain or snow from them gives us hydropower. To minimize the CO2 releasedby burning coal, oil, or gas, efforts are made to develop solar and wind power, both ofwhich derive energy from the sun.
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