Plasmas and Fluids (1986) / Chapter Skim
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3. General Plasma Physics
3. General Plasma Physics
Pages 95-143
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From page 95... ...
Here we examine a number of different applications such as free-electron radiation sources, x-ray lasers, plasma isotope separation, and collective and laser-driven accelerators. Much of this work is motivated and supported by defense-related problems.
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From page 96... ...
The 1970s saw the development of nonlinear plasma physics: the behavior of largeamplitude waves, including particle trapping, saturation, wavebreaking, and turbulence; the discovery of new nonlinear phenomena such as Langmuir and ion acoustic solitons; ponderomotive effects of intense electric fields; parametric instabilities, which are growing wave-wave interactions; tearing modes that cause magnetic lines of force to become braided or to form magnetic islands. The development and widespread use of computational techniques has provided the missing link between theory and experiment.
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From page 97... ...
They were used to produce electron beams, ion beams, and Z-pinch plasmas. The particle beams are of sufficient intensity that the collective self-fields are of decisive importance, which is why the subject has become a part of plasma physics rather than accelerator or particle physics.
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From page 98... ...
It was established that neutralized ion beams could be transported across magnetic field lines, focused, and injected into a tokamak.
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From page 99... ...
The intense soft-x-ray source is of interest for atomic physics of highly stripped ions in plasma and for various applications including lithography, microscopy, and x-ray lasers. Propagation of Charged-Particle Beams in Gas and Plasma During the past 15 years there has been a substantial research effort devoted to propagation of relativistic electron beams and high-current ion beams in gas and plasma.
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From page 100... ...
, and optimization of beam parameters for efficient transport. Other applications of intense beams to collective accelerators, laser accelerators, and coherent-radiation sources have developed sufficiently in the last 10 years that they are discussed in separate sections.
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From page 101... ...
COLLECTIVE ACCELERATORS Collective accelerators make use of the electric and magnetic fields of charged particles in the region of the space where particles are to be accelerated or focused, or both. In principle, very large accelerating and focusing fields are possible, and the fundamental goal is to make use of these large fields to build high-performance accelerators very economically.
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From page 102... ...
An electron ring is formed in a magnetic mirror, and ions are trapped in it. Acceleration takes place by means of an electric field or changing magnetic field along the ring axis.
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From page 103... ...
Electrons are injected to provide charge neutralization and transverse focusing, and transverse magnetic fields prevent electron acceleration. Another form is a cyclic accelerator that is studied at UCI.
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From page 104... ...
A potentially attractive variation of the beat-wave accelerator is the Surfatron. In the Surfatron a transverse magnetic field is externally applied, permitting the accelerated particles to E x B drift in a~direction transverse to the laser propagation direction.
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From page 105... ...
Cyclotron Resonant Accelerator Here an electron beam is injected along a uniform magnetic field together with a parallel propagating laser beam. Because of a self
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From page 106... ...
Problem Areas A number of issues remain to be solved before laser-driven acceleration schemes can become a viable alternative to conventional acceleration mechanisms. Among the most important unsolved problems in this area is that of refocusing of the intense laser beam for multistage acceleration.
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From page 107... ...
COHERENT, FREE-ELECTRON RADIATION SOURCES The possibility of developing lasers and masers in which the active medium is a stream of free electrons has evoked much interest in recent years. The potential advantages are numerous and include continuous frequency tuning through variation of the electron energy, and very high-power operation, since no damage can occur to this lasing medium as can happen in solid, liquid, and gas lasers.
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From page 108... ...
In addition to the above, one can perceive applications in biology and medicine, and as is true for all new advances in technology, the ultimate and most important applications have yet to be identified. The fundamental principle operative in all free-electron radiation sources is electron bunching in the presence of an ambient electromagnetic field.
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From page 109... ...
Electron bunching is in the azimuthal direction and leads to the formation of clusters that rotate about the magnetic field lines. The radiation frequency is approximately equal to the electron-cyclotron frequency.
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From page 110... ...
However, a crash training program at universities would be necessary for the United States to keep abreast of the research and development in foreign countries. Funding for research and development in coherent free-electron radiation sources has come primarily from DARPA, ONR, AFOSR, AFSC, and NSF.
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From page 111... ...
Plasma waves are then generated at the expense of electromagnetic energy, and the plasma is heated by the damping of these waves. The copious production of superthermal electrons (and of fast ions accelerated by the charge separation electric field)
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From page 112... ...
instability discussed below; the former is a stronger eject involving the generation of two plasma waves, preferentially at 45° to the laser beam. The main features of the two-plasmon decay instability were confirmed in a basic experiment in which the decay waves were detected by Thomson scattering.
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From page 113... ...
In summary, the theoretical and experimental discovery of parametric instabilities and the agreement that has been achieved between theory and experiment represent a significant advance in the development of plasma physics in the past decade. Several large problems remain to be solved, notably the general nature of heat transport in the long mean-free-path regime and the nature and effects of self-generated magnetic fields.
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From page 114... ...
The electromagnetic separation processes are of interest since they are insensitive to specific materials, unlike laser-based processes, which depend on the electronic structure and thus are limited to specific materials. One of these approaches is based on the ion cyclotron resonance in a uniform magnetic field.
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From page 115... ...
A major problem that occurs is the formation of a stable arc discharge along and across the magnetic field. It is the radial space-charge electric field crossed with the axial magnetic field that gives rise to the large rotation velocities.
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From page 116... ...
the problem of discovering how a charged particle moves in inhomogeneous electromagnetic fields. The single most significant
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From page 117... ...
Generally, these results show a correction to the quasi-linear estimate. Plasma-physics applications of the above basic theoretical developments have been made to the stochastic heating of plasmas by the absorption of externally launched waves, to the ergodic trajectories of wave packets of plasma waves, to the confinement of particles in the presence of collective fluctuations, to the theory of particle transport in rippled magnetic fields, and to the breakup of nested confining magnetic surfaces in fusion devices (e.g., tokamaks)
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From page 118... ...
Parametric Instabilities Parametric decay of a large-amplitude wave into two daughter waves has been extensively studied in plasmas because of its importance to the wave heating of magnetically confined plasmas and laser-plasma coupling in inertial fusion experiments. Indeed, such parametric wave coupling is the basis of the free-electron laser described in a previous section.
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From page 119... ...
and ensuing anomalous plasma heat loss. In addition to the work on spontaneous reconnection, reviewed above, much notable progress has also resulted from research on forced reconnection, with applications to laboratory plasmas and space plasmas (e.g., knotting of magnetic field lines in turbulent solar convection zones and solar wind-magnetopause interaction)
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From page 120... ...
Finally, our computational tools for examining nonlinear plasma phenomena have greatly expanded as a result of the development of innovative new numerical algorithms and concepts. PLASMA THEORY DEVELOPMENTS RELATED TO MAGNETIC CONFINEMENT More than half of the federal funding of plasma-physics theory over the past decade has been in connection with the magnetic-confinement approach to controlled thermonuclear fusion.
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From page 121... ...
Further, magnetic-fiux coordinates are developed from these results and, since the motion of single particles within magnetically confined plasmas is quite complex and determined by the direction of the magnetic field and its gradients, they are utilized for almost all of magnetic fusion theory. Finally, criteria have been developed for when magnetic-flux surfaces cannot be defined because the magnetic field lines become stochastic (at least in some region)
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From page 122... ...
Two-dimensional magnetized plasma equilibria can often be calculated analytically, but fully three-dimensional high-pressure equilibria usually are calculated numerically or utilizing expansion parameters, usually weak toroidicity or long-thin cylinder approximations. In addition, recently some primarily numerical models have been developed for anisotropic-pressure (perpendicular pressure different from that parallel to magnetic field)
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From page 123... ...
Macroscopic Instabilitie~Resistive Magnetohydrodynamics In toroidal magnetic-confinement systems, magnetic field lines can close back on themselves after an integer number of transits around the torus, thereby forming a rational surface. The finite (i.e., nonzero)
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From page 124... ...
While some of these models seem to be on the right track, in that they can clarify a number of generic features of anomalous transport, there is not yet any fully satisfactory theoretical model of the anomalous radial electron heat transport process in tokamaks. Summary Over the last decade, theoretical and computational tools for understanding plasma confinement and heating in magnetic systems have developed tremendously, to the level where they can now, at least in many areas (Coulomb collisional effects, equilibrium, ideal and resistive MUD global modes)
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From page 125... ...
Improved diagnostics of magnetic fusion plasmas were obtained by the use of newly measured magnetic dipole transitions in highly ionized iron group elements. Charge exchange from hydrogen into highly excited states of impurity ions has opened new possibilities for spatially resolved spectroscopic measurements in tokamaks.
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From page 126... ...
The most promising laboratory x-ray laser scheme should then be selected and be pursued vigorously to demonstrate significant gain. For the magnetic fusion program, sources will have to be developed to produce neutral beams of energies '500 keV.
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From page 127... ...
Most likely this facility would be centered around a high-power laser, but other high-power plasma devices should be considered as well. · Encourage the full utilization of national experimental or theoretical users facilities (Texas Tokamak TEXT, University of Rochester Laser Facility, Magnetic Fusion Energy Computer Center)
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From page 128... ...
Of this amount, well over half is available in national laboratories and at most 20 percent each in universities and industrial laboratories. Recommended Funding Levels In view of the critical importance and strong leverage of atomic physics in plasma research, a significant increase over present funding levels is recommended.
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From page 129... ...
This constitutes a demand for an immense breadth and depth of diagnostic techniques. The problems and techniques are common to most magnetically confined plasmas.
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From page 130... ...
The first is impurity concentration the density of ions aside from hydrogen-and the second is the inferences that can be drawn from the Doppler effect on-line shape the temperature and velocity of the ion. Among the major developments in the past decade have been the discovery and cataloging of comparatively strong forbidden magnetic dipole lines in the spectra of highly ionized impurities.
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From page 131... ...
A neutral may exchange its electron with a hot ion, and the resulting energetic neutral, no longer confined by the magnetic field, may escape the plasma. Analysis of the energy of these neutrals indicates the temperature of the confined, hot ions.
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From page 132... ...
In the best cases, diagnostics are now sufficiently complete to imply values for transport rates, the thermal conductivities of ions and electrons in a particular experiment. Heavy-Ion Diagnostics One of the most obvious characteristic parameters of an electrically conducting medium like a plasma is the electric potential or electric field within the medium, but this has been notoriously difficult to determine for hot plasmas.
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From page 133... ...
Such fluctuations and instabilities have been detected in many ways. In the easiest cases, they can be seen as oscillations in the magnetic field outside the plasma or they are coupled to an electromagnetic wave, which can propagate through the plasma and be detected outside.
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From page 134... ...
At the top of this list are local current density and magnetic field within a plasma, for which measurements of both average and fluctuating quantities are sorely needed. A technique to measure the electron velocity distribution function would be extremely useful, as would a more generally applicable method for measuring plasma potential and electric field, the heavy-ion-beam probe being impractical for many plasmas.
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From page 135... ...
The only budget specifically allocated to diagnostic development is a small program within the Division of Applied Plasma Physics in the Office of Fusion Energy (DOE)
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From page 136... ...
At sufficiently high density the coulombic correlations become so strong that the fluid undergoes a first-order phase transition to a lattice. Much of the matter in the universe is in the strongly coupled plasma state since stellar interiors are highly ionized and often at very high density.
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From page 137... ...
Conceptually the OCP limit, however, plays the same role for strongly coupled plasmas that the hard-sphere fluid plays for the theory of neutral liquids. Cluster expansion methods used for dense gases and liquid-state theory proved in the 1960s to be not particularly useful for strongly coupled plasmas, and numerical simulation methods were developed.
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From page 138... ...
In the late 1970s, it was found experimentally that electrons could be trapped on the surface of liquid helium and thus confined to motion on a two-dimensional surface. It was found that the electron density could be increased so that the resulting two-dimensional plasma could be produced in weak coupling and then as a strongly coupled plasma, which finally exhibited a two-dimensional phase transition.
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From page 139... ...
The experimental measurement of several lines of the Lyman series has given a reasonable test of current line-broadening theories and the usefulness of the Monte Carlo microfield for predicting spectral line shapes from highly ionized strongly coupled plasmas. Outlook for the Next 10 Years Future improvement on present-day understanding of ionic matter will probably come about by a fully quantum treatment of strongly coupled plasmas.
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From page 140... ...
NONNEUTRAL PLASMAS A nonneutral plasma is a collection of charges that satisfy the usual many-body criterion to be a plasma but in which there is not overall charge neutrality. These systems usually have intense self-electric fields and may also have intense self-magnetic fields.
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From page 141... ...
In confinement geometries for which the confining electric and magnetic fields have cylindrical symmetry, general stability and confinement theorems prove that a pure electron plasma (or a pure ion plasma) simply cannot escape.
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From page 142... ...
In a magnetically insulated transmission line, the electric fields are so large that copious field emission of electrons takes place; however, with suitable magnetic fields the electrons are confined and do not short out the line. Magnetic insulation (confinement)
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From page 143... ...
The occurrence of nonneutral plasma was recognized in many important applications. We expect that plasma physics will be broadened and enriched by the study and application in the next decade of nonneutral plasmas.
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