1629373397-425d4de58b7aea127ffc7c337418ea8d (Introduction to Plasma Physics and Controlled Fusion Francis F. Chen), страница 6

Research wascarried out by each individual country until 2007, when the ITER project wasstarted. ITER stands for International Thermonuclear Experimental Reactor, a largeexperiment being built in France and funded by seven countries. Serendipitously,ITER is a Latin word meaning path or road. It is a road that mankind must take tosolve the problems of global warming and oil shortage by 2050. Of all the“magnetic bottles” presented at Geneva, the USSR’s TOKAMAK has survived as theleading idea and is the configuration for ITER.

First plasma in ITER is scheduledfor about the year 2021.141 IntroductionThe fuel is heavy hydrogen (deuterium), which exists naturally as one part in6000 of water. The principal reactions, which involve deuterium (D) and tritium(T) atoms, are as follows:D þ D !3 He þ n þ 3:2 MeVD þ D ! T þ p þ 4:0 MeVD þ T !4 He þ n þ 17:6 MeVThese cross sections are appreciable only for incident energies above 5 keV.Accelerated beams of deuterons bombarding a target will not work, because mostof the deuterons will lose their energy by scattering before undergoing a fusionreaction. It is necessary to create a plasma with temperatures above 10 keV so thatthere are enough ions in the 40-keV range where the reaction cross sectionmaximizes.

The problem of heating and containing such a plasma is responsiblefor the rapid growth of the science of plasma physics since 1952.1.7.3Space PhysicsAnother important application of plasma physics is in the study of the earth’s environment in space. A continuous stream of charged particles, called the solar wind,impinges on the earth’s magnetosphere, which shields us from this radiation and isdistorted by it in the process. Typical parameters in the solar wind are n ¼ 9 106 m3,KTi ¼ 10 eV, KTe ¼ 12 eV, B ¼ 7 109 T, and drift velocity 450 km/s.

Theionosphere, extending from an altitude of 50 km to 10 earth radii, is populated by aweakly ionized plasma with density varying with altitude up to n ¼ 1012 m3. Thetemperature is only 101 eV. The solar wind blows the earth’s magnetic field into along tail on the night side of the earth. The magnetic field lines there can reconnectand accelerate ions in the process. This will be discussed in a later chapter.The Van Allen radiation belts are two rings of charged particles above theequator trapped by the earth’s magnetic field. Here we have n 109 m3,KTe 1 keV, KTi ’ 1 eV, and B ’ 500 109 T.

In addition, there is a hot component with n ¼ 103 m3 and KTe ¼ 40 keV, and some ions have 100s of MeV.Exploration of other planets have revealed the presence of plasmas. ThoughMercury, Venus, and Mars have little plasma phenomena, the giant plants Jupiterand Saturn and their moons can have plasma created by lightning strikes. In 2013The Voyager 1 satellite reached the boundary of the solar system. This wasascertained by detecting an increase in the plasma frequency there (!).1.7.4Modern AstrophysicsStellar interiors and atmospheres are hot enough to be in the plasma state.

Thetemperature at the core of the sun, for instance, is estimated to be 2 keV;1.7 Applications of Plasma Physics15thermonuclear reactions occurring at this temperature are responsible for the sun’sradiation. The solar corona is a tenuous plasma with temperatures up to 200 eV. Theinterstellar medium contains ionized hydrogen with n ’ 106 m3 (1 per cc). Variousplasma theories have been used to explain the acceleration of cosmic rays.Although the stars in a galaxy are not charged, they behave like particles in aplasma; and plasma kinetic theory has been used to predict the development ofgalaxies.

Radio astronomy has uncovered numerous sources of radiation that mostlikely originate from plasmas. The Crab nebula is a rich source of plasma phenomena because it is known to contain a magnetic field. It also contains a visual pulsar.Current theories of pulsars picture them as rapidly rotating neutron stars withplasmas emitting synchrotron radiation from the surface. Active galactic nucleiand black holes have come to the forefront.

Astrophysics now requires an understanding of plasma physics.1.7.5MHD Energy Conversion and Ion PropulsionGetting back down to earth, we come to two practical applications of plasmaphysics. Magnetohydrodynamic (MHD) energy conversion utilizes a dense plasmajet propelled across a magnetic field to generate electricity (Fig. 1.5). The Lorentzforce qv B, where v is the jet velocity, causes the ions to drift upward andthe electrons downward, charging the two electrodes to different potentials.

Electrical current can then be drawn from the electrodes without the inefficiency of aheat cycle.A more important application uses this principle in reverse to develop enginesfor interplanetary missions. In Fig. 1.6, a current is driven through a plasma byapplying a voltage to the two electrodes. The j B force shoots the plasma out ofthe rocket, and the ensuing reaction force accelerates the rocket. A more moderndevice called a Hall thruster uses a magnetic field to stop the electrons while a highvoltage accelerates the ions out the back, giving their momenta to the spacecraft.A separate discharge ejects an equal number of warm electrons; otherwise, thespace ship will charge to a high potential.Fig.

1.5 Principle of the MHD generator161 IntroductionFig. 1.6 Principle ofplasma-jet engine forspacecraft propulsion1.7.6Solid State PlasmasThe free electrons and holes in semiconductors constitute a plasma exhibiting thesame sort of oscillations and instabilities as a gaseous plasma. Plasmas injected intoInSb have been particularly useful in studies of these phenomena. Because of thelattice effects, the effective collision frequency is much less than one would expectin a solid with n ’ 1029 m3. Furthermore, the holes in a semiconductor can have avery low effective mass—as little as 0.01 me—and therefore have high cyclotronfrequencies even in moderate magnetic fields.

If one were to calculate ND for a solidstate plasma, it would be less than unity because of the low temperature and highdensity. Quantum mechanical effects (uncertainty principle), however, give theplasma an effective temperature high enough to make ND respectably large. Certainliquids, such as solutions of sodium in ammonia, have been found to behave likeplasmas also.1.7.7Gas LasersThe most common method to “pump” a gas laser—that is, to invert the populationin the states that give rise to light amplification—is to use a gas discharge. This canbe a low-pressure glow discharge for a dc laser or a high-pressure avalanchedischarge in a pulsed laser. The He–Ne lasers commonly used for alignment andsurveying and the Ar and Kr lasers used in light shows are examples of dc gaslasers.

The powerful CO2 laser has a commercial application as a cutting tool.Molecular lasers such as the hydrogen cyanide (HCN) laser make possible studiesof the hitherto inaccessible far infrared region of the electromagnetic spectrum. Thekrypton fluoride (KrF) laser uses a large electron beam to excite the gas.

It has therepetition rate but not the power for a laser-driven fusion reactor. In the semiconductor industry, short-wavelength ultraviolet lasers are used to etch ever smallertransistors on a chip. Excimer lasers such as the argon fluoride laser with 193 nmwavelength are used, with plans to go down to 5 nm.1.7 Applications of Plasma Physics1.7.817Particle AcceleratorsIn high-energy particle research, linear accelerators are used to avoid synchrotronradiation on curves, especially for electrons. The 3-km long SLAC accelerator atStanford produces 50-GeV electrons and positrons. Plasma accelerators generateplasma waves on which particles “surf” to gain energy.

This technique has beenapplied at SLAC to double the energy from 42 to 84 GeV. A 31-km InternationalLinear Collider is being built to generate 500 GeV colliding beams of electrons andpositrons. In principle, plasma waves can add 1 GeV per cm.1.7.9Industrial PlasmasAside from their use in semiconductor production, partially ionized plasmas of highdensity have other industrial applications. Magnetrons are used in sputtering, amethod of applying coatings of different materials.

Eyeglasses can be coated withplasmas. Arcs, such as those in search lights, are plasmas. Instruments in medicalresearch can be cleaned thoroughly with plasmas.1.7.10 Atmospheric PlasmasMost plasmas are created in vacuum systems, but it is also possible to produceplasmas at atmospheric pressure. For instance, a jet of argon and helium can beionized with radiofrequency power. This makes possible small pencil-size devicesfor cauterizing skin. Industrial substrates can be processed by sweeping such a jet tocover a large area.

Large atmospheric-pressure plasmas can also be produced forroll-to-roll processing.Problems1.8. In laser fusion, the core of a small pellet of DT is compressed to a density of1033 m3 at a temperature of 50,000,000 K. Estimate the number of particlesin a Debye sphere in this plasma.1.9. A distant galaxy contains a cloud of protons and antiprotons, each withdensity n ¼ 106 m3 and temperature 100 K. What is the Debye length?1.10. (Advanced problem) A spherical conductor of radius a is immersed in auniform plasma and charged to a potential ϕ0.