Click for next page ( 185


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 184
6 Basic Plasma Science INTRODuCTION In the preceding chapters, the committee described studies of many funda- mental plasma phenomena as they relate to research in a particular topical area. In this chapter, it describes complementary plasma studies, where the primary goal is to isolate and study in detail fundamental plasma phenomena. These studies echoes principal themes of the report, focusing on the discovery and exploration of new plasma regimes and testing our understanding of the underlying principles of plasma science. The phenomena of interest span a vast range. Of particular interest, for example, are the six fundamental processes highlighted in Chapter 1: multiphase effects in plasmas; explosive instabilities; particle acceleration mecha- nisms; turbulence and turbulent transport, magnetic reconnection and magnetic self-organization; and the effects of strong particle correlations in plasmas. These and many other important plasma effects manifest themselves in a wide range of situations, from dusty plasmas to HED plasmas. While the primary goal is to explore these and other important phenomena in detail, there is a close connection to the broad range of other investigations in this report, from fusion, to space and astrophysics, to HED and low-temperature plasmas. These advances in our fundamental understanding are crucial for innovat- ing technologies that use plasmas. Just as developing and validating fundamental theories of the band structure of semiconductors necessarily preceded transistors, developing and validating fundamental theories of the basic behavior of plasmas necessarily precedes exploiting plasma technologies fully for energy, national se- curity, and economic competitiveness. 14

OCR for page 184
Basic Plasma science 1 Such scientific inquiry frequently leads to the discovery of qualitatively new phenomena and new plasma regimes. Recent examples include states of true ther- mal equilibrium in single-component plasmas, the creation of a wide range of HED and ultracold plasmas, and creation of the first stable neutral antimatter (antihy- drogen). In each case, new physical situations and phenomena have been discovered that allow us, in turn, to test and expand our fundamental understanding in new ways. This research provides strong intellectual ties to other areas of science and engineering, including fluid dynamics, atomic physics, nonlinear dynamics, soft condensed matter physics, and solid-state plasmas. The research is typically done on the smallest scale the problem admits, so that there is the flexibility to make changes quickly and economically as the sci- ence unfolds. The complementary roles of theory and computation are critical. This is particularly true in plasma science, where nonlinear and nonequilibrium phenomena in many-body systems are of central importance. These research activities serve a critical function in educating and training scientific and technical personnel. Typical research efforts are small, university- scale activities. As such, they provide excellent opportunities to train students in a variety of disciplines and techniques that are critical not only to plasma science but also to many other areas of modern science and technology. Such projects al- low young researchers to participate in all facets of the research, from planning, to conducting experiments and calculations, to disseminating research results. These small-scale research projects produce a very significant fraction of the U.S. Ph.D.’s in plasma science. RECENT PROgRESS AND FuTuRE OPPORTuNITIES As our knowledge of plasma science has grown over the past decade, so has our appreciation of the vast range of plasma phenomena. Plasmas of interest span enormous ranges of parameters—more than 22 orders of magnitude in density (i.e., 1022), 15 orders of magnitude in temperature, and 19 orders of magnitude in magnetic field. Plasmas at the extremes include the tenuous ISM, laser-cooled plas- mas, relativistic laser-driven plasmas, stellar interiors, and the magnetospheres of pulsars. Understanding the fundamentals of plasma behavior over such enormous ranges of parameters presents huge challenges. Here the committee discusses progress and future opportunities in eight topics: • Nonneutral and single-component plasmas, • Ultracold plasmas, • Dusty plasmas, • Laser-produced and HED plasmas,

OCR for page 184
Plasma science 1 • Microplasmas, • Turbulence and turbulent transport, • Magnetic fields in plasmas, and • Plasma waves, structures, and flows. The first five topics are unique or special physical situations in which research is yielding a wealth of scientific progress and new opportunities (Box 6.1). An analogy can be drawn with condensed matter physics, where different materi- als exhibit vastly different phenomena, from quantum dots to carbon nanotubes to high-temperature superconductors; study of each physical system is yielding important new science. Access to these new regimes of plasma science has been made possible by developments in other fields as well as by improved techniques within basic plasma science itself. For example, techniques developed in atomic, molecular, and optical science for cooling, trapping, and working with ultracold atoms and molecules have contributed to basic plasma science studies. Similarly, the development of ultra-short-pulse, high-power lasers (as described in Chapter 3) has opened a window on fundamental physics studies of HED plasmas in the laboratory. The final three topics—turbulence, magnetic fields, and plasma waves, struc- tures, and flows—are three of the six key science themes highlighted in Chapter 1. The science benefits greatly from the many synergies between these themes. Studies of ordering in pure ion plasmas are relevant to dusty plasmas and HED plasmas. Understanding turbulence and its consequences is furthered by experi- ments in nonneutral as well as neutral plasmas. Studies of structure and self-or- ganization benefit from a range of experimental and theoretical efforts. Progress in one area can often be validated quickly and used in another. This complemen- tary approach—perhaps more than ever before—is central to rapid and efficient progress. Two crosscutting physics concepts further unify the research—the concept of strong and weak coupling and the concept of plasma self-organization (Box 6.2). Whether a plasma is strongly or weakly coupled is determined by the ratio, BOX 6.1 The Dynamic Forefront of Research—New Opportunities Many of the current forefront areas in basic plasma research (dusty plasmas, HED plasmas, micro- plasmas, and ultracold plasmas) were virtually below the scientific radar screen at the time of the last decadal study. Recent studies have extended by orders of magnitude the range of plasma parameters amenable to study, identified new phenomena, motivated new theory, and led to new understandings of plasma behavior, providing a wealth of exciting new research opportunities.

OCR for page 184
Basic Plasma science 1 BOX 6.2 Strong and Weak Coupling and Quantum Effects One important crosscutting theme in plasma science is the commonality of phenomena in weakly coupled plasmas and strongly coupled plasmas. The defining quantity is the Coulomb coupling parameter, Γ, which is the ratio of the average interparticle Coulomb potential energy divided by the kinetic energy of a plasma particle, namely Γ ≡ e2/akBT, where a = [(3/4πn)]1/3 is the average interparticle spacing, n is the plasma density, T is the plasma temperature, and kB is the Boltzmann constant. Weakly coupled plasmas correspond to Γ < 1; they typically exhibit waves and nonlinear phenom- ena, instabilities, turbulence, and a lack of spatial ordering (as in a gas). Weak coupling effects dominate in space plasmas and magnetic confinement fusion plasmas, such as those in tokamaks. Strongly coupled plasmas are characterized by Γ > 1, where Γ ~ 1 corresponds to a liquid and Γ ≥ 200 corresponds to crystalline ordering. In the solid phase, the crystalline structure can dominate physical properties, and transport typically occurs via the diffusion of defects. Examples in which strongly coupled plasma phenomena are important and frequently dominant include pure ion plasmas, ultracold plasmas, dusty plasmas, and laser-produced HED plasmas. A further distinction is the regime in which quantum mechanical effects are important. Quantum effects in the particle energy distributions are important at high densities and low temperatures when the - - Fermi energy is greater than the plasma temperature, namely n > (3π2)–1(2mkBT/h2)3/2, where h is Planck’s - constant. Quantum effects are important for waves and oscillations when hω ≥ kBT, where ω is the oscil- lation frequency. The boundaries between strongly and weakly coupled plasma phenomena and those in which quantum effects are important were shown schematically in Figure 1.2. Γ, of the Coulomb potential energy to the plasma temperature. Strongly coupled plasmas (Γ >> 1) are characterized by very strong Coulomb correlation effects that ultimately lead to crystalline order. Examples include dusty plasmas, ions in electromagnetic traps, and neutron stars. Weakly coupled plasmas (Γ < 1) include most laboratory plasmas and fusion plasmas. These plasmas are much more likely to exhibit nonlinear wave phenomena and turbulence. The second crosscutting theme is self-organization, which can dominate plasma behavior. While the spatial ordering discussed above is analogous to ordering in ordinary liquids and solids, weakly coupled plasmas in a magnetic field, for example, undergo much more extensive topological changes as a result of the reconnection and rearrangement of the field. This, in turn, can produce qualitative changes in the shape of the plasma, the nature of particle orbits, and other plasma proper- ties. Such self-organization phenomena are important, for example, in magnetic confinement fusion and in space and astrophysical plasmas, where they can create a range of behaviors, including explosive events, shocks, and large-scale flows. Nonneutral and Single-Component Plasmas Typical plasmas discussed in this report are approximately electrically neutral and have roughly equal densities of positive and negative charges. However, there

OCR for page 184
Plasma science 1 is an important special class of plasmas for which this is not the case, so-called nonneutral plasmas, the extreme case being a plasma of a single sign of charge, a so-called single-component plasma. In this case, a uniform magnetic field can be used to restrict the plasma radially, and electrostatic voltages used to confine particle motion along the magnetic field. While these plasmas exhibit phenomena similar to electrically neutral electron-ion plasmas, single-component plasmas can be confined indefinitely. This permits studies of a wide range of plasma phenomena with high precision, including critical plasma processes that are not understood (described in Chapter 1), such as strong correlation and turbulence. Single-component plasmas have remarkable properties. Examples include pure ion, electron, positron, and antiproton plasmas. They can evolve to true states of thermal equilibrium uncommon in other plasmas. Magnetized electron plasmas behave as ideal, two-dimensional fluids, with electron density playing the role of fluid vorticity. This has enabled new studies of vortex turbulence and led to the discovery of novel vortex crystal states, illustrated in Figure 6.1, which motivated a new theory of the turbulence. FIGURE 6.1 Evolution of vortex turbulence in a pure electron plasma. These magnetically confined plasmas flow across the magnetic field in direct analogy to the flow of an incompressible fluid with an unusually small viscosity. Recently, these plasmas were used for tests of theories of the behavior of two-dimensional flows in ideal fluids not possible in other physical systems. The experiments demonstrated surprising new phenomena. Electron density, which is the exact analogue of vorticity in an ordinary fluid, can relax (above) to a vortex crystal or (below) to one large-scale vortex. Courtesy of C.F. Driscoll, University of California at San Diego.

OCR for page 184
Basic Plasma science 1 Crystal formation in pure ion plasmas has a long and distinguished history that began in the 1980s with work on ion plasmas in Penning and radio-frequency traps carried out in parallel with complementary work on cold ion plasmas in stor- age rings. Recent investigations of nonneutral and single-component plasmas have explored with precision the details of such crystal formation. It had long been pre- dicted that an infinite homogeneous Coulomb crystal would have a body-centered cubic structure, and this has now been confirmed experimentally—the ultimate result of strong correlation when Γ ≥ 200. Recent theory for relatively thin plasmas with only a few crystal planes predicted a series of structural phase transitions due to an intricate interplay between surface and bulk free energy. The spectacularly successful test of this theory is shown in Figure 6.2 for a cold ion plasma at a tem- perature ~3 mK and Γ > 500. Other important recent results include the creation of antiproton and positron antimatter plasmas, studies of energy transport through long-range collisions, and studies of the intrinsic thermodynamics of these systems. One long-term goal is study of relativistic electron-positron plasmas, which are of astrophysical interest, for example, in the magnetospheres of pulsars. side-view images 2 plane axial position (z /a) 1 0 -1 rhombic hexgonal - -2 1 2 3 4 5 (b) areal charge density (σa 02 ) (a) FIGURE 6.2 Spatial ordering in pancake-shaped strongly correlated plasmas with a small number of crystal planes (Γ > 500). Left: top-view, in-plane image of a hexagonal crystal. Right, above: side-view images of the crystal planes. Right, below: the phase diagram as a function of in-plane charge density, showing the phase changes and introduction of new crystal planes. Lines are the theoretical predictions, illustrating superb agree- ment. Courtesy of J.J. Bollinger, National Institute of Standards and Technology.

OCR for page 184
Plasma science 10 Recently, a method was discovered to compress nonneutral plasmas radially across the confining magnetic field (the so-called rotating-wall technique, which employs a rotating electric field). Now a standard tool around the world, it enables plasma confinement for essentially infinite times and the plasma density to be precisely controlled and varied over orders of magnitude. Potential applications include long-term storage of antimatter, particle-antiparticle traps, and commer- cial positron beam sources for materials analysis. Application of this technique to antimatter plasmas was critical to the recent success, described below, in creating the first cold antihydrogen atoms. Owing to the unique confinement properties of single-component plasmas and the fact that they can reach thermal equilibrium, plasma transport processes can be studied in them with a precision not possible in other situations. This is done by making controlled departures from equilibrium and observing the relaxation of the plasma back to the equilibrium state. While the simplest nonneutral plasmas are cylindrically symmetric with no regions of localized particle trapping, the ef- fects of asymmetries have been observed but are not yet understood. This offers the opportunity to bridge the gap between our understanding of nonneutral plasmas and conventional electron-ion plasmas. For example, plasma rotation, which is a zeroth-order effect in single-component plasmas owing due to their space charge, is known to play an important role in confinement in tokamak plasmas. ultracold Neutral Plasmas Ultracold plasmas provide qualitatively new opportunities for plasma science, ranging from the study of spatial ordering in new plasma regimes, to the study of novel atomic physics processes, to the development of techniques to produce and study antihydrogen. Research in this area resides at the boundary between atomic physics and plasma physics. These novel plasmas provide the opportunity to push plasma physics into new regimes in parameter space. Aided by the powerful tools of laser cooling and laser manipulation and imaging of the plasma ions (techniques similar to those used to form Bose condensed gases of alkali atoms), studies of ultracold plasmas provide new tests of our understanding of plasma phenomena and new scientific opportunities. For example, ultracold plasmas can be used to study regimes where correlation effects are important and situations in which the electron and ion temperatures are vastly different. Typical ultracold plasmas are formed from cold gases of atoms at ~10 µK, photoionized to produce electrons with energies of a few kelvin. The resulting ultracold, unmagnetized plasma expands freely into vacuum, driven by the pres- sure of the electron gas. In these unusually cold plasmas, the dominant collisional mechanism is three-body recombination forming highly excited (Rydberg) atoms. Recombination rates increase rapidly as the temperature is lowered and can be

OCR for page 184
Basic Plasma science 11 exceedingly large, with as much as 30 percent of the plasma converting to Rydberg atoms. When the laser frequency is tuned below the ionization limit, a gas of ul- tracold Rydberg atoms is formed that, in turn, quickly forms an ultracold plasma through atom-atom collisions. These ultracold plasmas serve as laboratories for studies of the statistical mechanics and thermodynamics of elementary plasma systems. For instance, the electrons gain almost all the energy from ionization. They rapidly come to thermal equilibrium at a higher temperature than the ions. The random positions of the electrons and ions following ionization induces disorder heating. As the plasma expands, there is competition between expansion cooling, in which the electrons transfer their energy to ion expansion, and recombination-induced heating, in which excess energy is carried away by the free electrons. The electrons are weakly correlated, while correlation of the ions is important (Γ ~ 4). Temporal oscillations of the kinetic energy are observed that provide a clear signature for the importance of these correlations. One outstanding issue is how the approach to (quasi-) equi- librium proceeds in a system in which the density and, possibly, the temperature change by many orders of magnitude. One popular topic in ultracold plasma research is the creation and study of the stable, neutral antiatom antihydrogen, which is the bound state of a positron and an antiproton. There is keen interest in making precise comparisons between the properties of such antimatter and those of matter to test fundamental symmetries of nature. Examples include tests of invariance with respect to charge conjugation, parity, and time reversal (the so-called CPT theorem) and precise tests of the gravi- tational attraction of matter to antimatter. Recently, two groups at the antiproton decelerator at CERN in Geneva produced the first neutral, low-energy antimatter (weakly bound antihydrogen atoms) by mixing cryogenic positron and antiproton plasmas. Data from one of these experiments are shown in Figure 6.3. A quantitative understanding of the plasma processes involved in antihydrogen formation will be required to raise production and trapping efficiency. The cur- rent technique requires overlapping of the positron and antiproton charge clouds. Understanding how to improve the production efficiency as well as how to trap the antihydrogen without instabilities is an important subject for research. Other outstanding problems include developing a method to trap the neutral antihy- drogen atoms in shallow magnetic-gradient traps and to drive the highly excited (Rydberg-state) atoms to the ground state so that their properties can be studied with precision. Dusty Plasmas Dusty plasmas are ionized gases containing small (i.e., micron-size) particles of solid material. The “dust” can be virtually any material, dielectric or conducting,

OCR for page 184
Plasma science 12 (a) (b) FIGURE 6.3 Antihydrogen in the laboratory: (a) image of antiproton decays as neutral antihydrogen atoms are formed in an antiproton-positron plasma and hit the plasma-confining electrodes and (b) modulation of the antihydrogen production rate by varying the positron plasma temperature. Such production in the laboratory of the first stable, neutral antimatter depends critically upon creating and manipulating cold, antimatter plasmas. Courtesy of the Athena Collaboration, via J. Hangst, University fig 6-3 a and b of Aarhus, Denmark. from precision microspheres introduced deliberately into the plasma to dust parti- cles grown in situ by aggregating atoms from the ambient neutral gas. A particularly important feature of dusty plasmas is that the dust particles become highly charged. A 10-micron particle can have a charge of ~104 electrons. As a result, particles can be levitated against the force of gravity by electric fields that occur naturally in the plasma. Because the dust particles repel one another, they often become strongly coupled with values of Γ >> 1. This produces strong spatial correlations of the dust particles, so that they often exhibit liquid- or solidlike behavior. They scatter light efficiently, so it is possible to track particle motion in real time using video imaging, which allows comparison of experiment and theory with a precision not possible in other plasma and condensed matter systems. An acoustic wave and a shock wave in a dusty plasma are shown in Figure 6.4. A decade ago, billions of dollars’ worth of semiconductor manufacturing yield was being lost as a result of particles that grew in situ in the processing plasmas. Techniques making use of the new understanding of dusty plasmas were developed to control this contamination. Another area of great practical importance is dust in tokamak fusion plasmas, where sputtered materials can condense to form dust particles. These particles can accumulate in the reactor, where they can contribute to the absorption of large amounts of tritium. Such tritium retention is a serious engineering issue in the design and operation of ITER. In strongly coupled dusty plasmas, the crystalline and liquid phases and the melting transition have been studied in detail. By levitating dust particles in the

OCR for page 184
Basic Plasma science 1 FIGURE 6.4 Waves and instabilities in dusty plasmas. Charged dust particles introduce unique po- tential structures in plasmas. They alter significantly the short- and long-range forces and can affect the ordering and dynamics of these dust grains. As an example, the dust introduces a slow timescale into the plasma dynamics. Shown here are (left) a dust-acoustic wave with centimeter per second speed (slower by five orders of magnitude than typical laboratory plasmas) and (right) Mach cone of a dusty-plasma shock wave. Courtesy of J. Goree, University of Iowa. sheath of a plasma discharge, it has become possible to create an interacting, two- dimensional plasma crystal and a two-dimensional liquid dust plasma. The equi- librium configurations, transport properties, and wave propagation in this novel system have been studied, and new theories of the liquid state have been developed. Work in this area has considerable synergy with soft condensed matter physics. The area is relatively young. As a consequence, there are many opportunities to improve instrumentation that, in turn, will enable new experimental studies. Dusty plasmas offer a new regime for the study of particle and energy transport in plasmas (Box 6.3). Experiments are needed to test recent predictions for such quantities as the coefficients of diffusion and viscosity, relevant, for example, in industrial processes. Another important issue is the nature of waves and transport in dusty plasmas of astrophysical interest. Finally, study of dusty plasmas in large magnetic fields would enable tests of theoretical predictions for new classes of dusty plasma phenomena. BOX 6.3 Dusty Plasmas Dusty plasmas are important in many areas of science and technology. Fundamental studies include ordering and transport in many-body systems; cometary tails and planetary rings in space plasmas; and dust in the ISM: Practical applications include high-tech materials processing, spray coating technology, and other industrial processes.

OCR for page 184
Plasma science 14 Laser-Produced and HED Plasmas There has been dramatic progress in our ability to create, study, and use laser- produced plasmas. Ultraintense, ultrafast lasers, ranging in size from those that require enormous buildings to those compact enough to fit on a tabletop, have revolutionized this field. A vast range of important phenomena can be studied with these systems, and applications abound, including advanced lithographic techniques for nanoscale electronics, simulation of astrophysical phenomena, and a range of issues related to national security. Research at large HED facilities is described in Chapter 3. Small-scale systems can now produce many terawatts of peak power (see subsection on plasma-based electron accelerators in Chapter 3 for more discussion). Owing to such reductions in size, many investigations can be conducted in university- or intermediate-scale experiments. This section describes recent progress and the wealth of opportunities that exist for future research. Several of these examples illustrate the synergistic relationship between pure and applied research—for one thing, novel plasma phenomena are frequently being used as innovative research tools in many areas of science and engineering: • Beam physics. Whereas plasmas in thermal equilibrium are Maxwellian distributions, relativistic beams are typically non-Maxwellian in that dif- ferent temperatures exist in the perpendicular and parallel directions. Fur- thermore, the Debye length for a relativistic beam is usually much greater than the radius of the beam itself. Despite these apparent differences, and although a beam is typically nonneutral, it can exhibit many plasmalike phenomena. The propagation of an intense particle beam through a fo- cusing channel, for example, involves many concepts from plasma physics. Examples include the plasma frequency, which is used to quantify the forces due to space charge; the beam emittance, which is a beam-physics measure of temperature; and the utilization of self-consistent field descriptions of collective behavior. • Plasma optics. Because plasmas have an unlimited damage threshold, they are ideal media with which to control very intense light fields, similar to plasma switches that are the method of choice to turn very large electrical currents on and off. Using small-scale lasers, plasmas can be made to act as novel optical elements. The use of plasma wake fields to accelerate electrons is detailed in Chapter 3. Such acceleration techniques, including a newly discovered bubble acceleration mechanism, can be combined with plasma optical elements to enable a new generation of plasma experiments and de- vices. Examples include preformed plasma lenses, ion channels, and plasma channels, such as that shown in Figure 6.5. Applications include x-ray lasers,

OCR for page 184
Plasma science 20 discussed below, a new generation of reconnection experiments at larger scale will be critical to further progress in this important area. Magnetic Self-Organization Plasmas frequently rearrange their large-scale magnetic structure spontane- ously. Although the specifics vary, the underlying self-organization mechanisms ap- pear to be common to laboratory, space, and astrophysical plasmas. Here the com- mittee discusses two important consequences of the self-organization. The critical issue for magnetic fusion of controlling such events is discussed in Chapter 4. Momentum Transport. Many toroidal plasmas are observed to rotate in the to- roidal direction, thereby developing toroidal angular momentum. During magnetic self-organization events, this angular momentum can be transported radially. The leading theoretical explanation of the transport is that the momentum is altered by a magnetic Lorentz force due to MHD instabilities, but other models have also been proposed, such as momentum transport along stochastic magnetic fields. The next decade promises important tests of these flow-driven instabilities in liquid- metal experiments, such as those described above. Ion Heating. Frequently the plasma ions heat during magnetic reconnection. Examples where this is an important effect include reversed-field-pinch plasmas and spherical-tokamak plasmas and when plasmas are merged. While this ion heat- ing is well documented in experiments, the underlying heating mechanism has yet to be understood and remains a challenge. Plasma Waves, Structures, and Flows The focus of this section is recent studies of fundamental plasma processes such as particle acceleration and plasma instabilities, which can drive plasma waves, structures, and flows. Experiments can now provide measurements of relevant quantities, including the electrical potential, density, magnetic field, and particle distribution functions—all at thousands of spatial locations and at very high data acquisition rates to allow comparison with new theories and a new generation of plasma simulations. Phenomena believed to trigger the instabilities, such as the explosive instabilities highlighted in Chapter 1, can be varied in a controlled fash- ion and thresholds determined. The experiments described here contribute to our understanding in different ways depending on the nature of the topic under study. In fortuitous cases, experiments can be conducted that can be scaled to a situation of direct, practical relevance—for example, in a space or astrophysical plasma. More often, fundamental insights can be gained that are of benefit to both the particular

OCR for page 184
Basic Plasma science 20 application and our general understanding of plasma behavior. Finally, for many important problems, theory and simulations can be tested and benchmarked. Laser-induced fluorescence (LIF) has recently been used to study weakly damped low frequency modes that are not adequately described by either col- lisional or collisionless models. These studies could have implications in many areas of plasma physics. Great progress has been made recently in understanding the roles played by Alfvén waves in laboratory plasmas and naturally occurring plasmas such as those in the solar wind and fusion devices (Box 6.4). Shown in Figure 6.13 is one such example where Alfvén waves were created by the currents generated when a dense plasma expands into a less dense magnetized plasma. This is similar to the process that occurs in coronal mass ejections. Alfvén waves with fine cross-field structure can produce heating and cross-field energy transport. A theory of Alfvén waves with large transverse wave numbers has been developed and its predictions verified in experiments. Alfvén waves can also play an important role in generating turbulence at small spatial scales (through a cascade of waves to short wavelength). The details of this Alfvén-wave cascade have been explored theoretically and computationally in the last decade using an MHD formalism. The cascade often continues to length scales where an MHD description is not valid, motivating simulations that are now able to calculate the fluctuation spectrum and turbulent heating. Future research will focus on comparing the re- sults of detailed laboratory experiments with new theory and simulations. There is now a wealth of new opportunities for laboratory experiment and complementary theory and modeling. The following are some key examples: • Particle acceleration by waves. Particle distribution functions frequently contain particles that have experienced nonlocal acceleration processes, which can now be studied in detail. The physics of charged-particle beams is closely related to that of plasmas in a moving reference frame. This provides opportunities to address outstanding questions in charged-particle-beam BOX 6.4 Alfvén Waves Alfvén waves are oscillations of the field lines in a magnetized plasma. While ubiquitous, they are difficult to study in the laboratory owing to their relatively large spatial scales. Alfvén waves have now been studied in detail for the first time in laboratory experiments, including Alfvén-wave maser action. Applications include understanding the aurora, the solar wind, coronal mass ejections from the Sun, and fusion plasmas.

OCR for page 184
Plasma science 20 (a) (b) (c) FIGURE 6.13 Laser-produced plasma expanding, from right to left, into a lower-density background plasma. (a) Current density in a plane near the generation point. (b) Magnetic field of expansion-driven Alfvén waves downstream. These data illustrate the state of the art in high-resolution, multiparameter, multiple-point measurements that can now be brought to bear on a wide variety of important plasma problems. (c) Overview of the expansion of the laser-produced plasma. Courtesy of W. Gekelman, LAPD Plasma Laboratory, University of California at Los Angeles.

OCR for page 184
Basic Plasma science 20 physics—for example, in simplified geometries such as radio-frequency traps. • Turbulent resistivity. Frequently, the resistivity due to turbulence is much greater than that due to Coulomb collisions. This can now be studied, even on the timescale of electron motion. • Structure in plasmas. Opportunities here include the study of magnetic, field-aligned density perturbations, filaments of enhanced temperature and/or potential, and the effects of localized beams. • Plasma flows. A variety of wave phenomena can be driven by plasma flows. This will be an important area for future work exploiting the synergies between laboratory and space plasma studies. • Expanding, high-density plasmas. A new generation of high-power, high- repetition-rate lasers offers great potential for studying transient processes where high-density plasma expands into a magnetized background plasma. Important phenomena include collisionless shocks, collision of flowing plasmas, magnetic field generation, and magnetic reconnection. IMPROvED METHODOLOgIES FOR BASIC PLASMA STuDIES A number of developments over the past decade hold much promise for future progress. Experimental and technical capabilities continue to expand. New sensors and new optical and laser systems enable experiments unheard of a decade ago. There has been progress in the optimization of many probes of plasma proper- ties. LIF has become a valuable diagnostic of ion temperature. Experiments have benefited greatly by the revolutionary progress made in computing power and data collection capabilities. Massive amounts of data can now be collected at high rates and analyzed and stored cheaply. Experiments can be done with much higher precision and greatly improved spatial and temporal resolution, frequently in three spatial dimensions. Examples include the magnetic reconnection data in Figure 6.12 and the study of Alfvén waves shown in Figure 6.13. In the future, microelectromechanical systems technology will offer the pos- sibility of a qualitatively new generation of microprobes with sub-Debye-length spatial resolution (tens of GHz) and sufficient temporal resolution to resolve electron motion. Analyzers could be arranged in clusters to directly measure the three-dimensional particle distribution functions. In principle, thousands of these probes could be placed in a plasma and complete spatial and temporal data ac- quired without perturbation of the system. On the theory front, great changes have come from improved computational technology and algorithms and the development of new theoretical models. The ability to carry out realistic simulations of actual experiments has improved simi- larly, so that detailed and accurate comparisons can be carried out in a wide variety of situations. Examples include the phase transitions in the three-dimensional ion

OCR for page 184
Plasma science 210 crystals shown in Figure 6.2 and the comparison of turbulent drift-wave spectra in a toroidal plasma device in Figure 6.9. However, considerable challenges remain—for example, in modeling multi- scale problems such as magnetic reconnection—due to the enormous range of spatial scales involved. New embedding techniques are needed to deploy kinetic models in regions of a large-scale computation where simpler fluid models fail. Resources dedicated to developing such models need to be a priority if the model- ing of large-scale plasma phenomena is to be successful. On a related theoretical front, it is the observation of many in the plasma com- munity and members of the committee that the past decade has seen a significant decline in activity in areas of mathematical physics relevant to plasma science. While this probably reflects a shift in activity to computation and simulation as those capabilities continue to improve, the importance of continued develop- ment of new plasma-science-related mathematical physics techniques cannot be overestimated. The field would benefit greatly if the plasma community and the federal agencies would consider carefully how this growing deficiency might be remedied. Finally, there is the important issue of coordinating basic research activities. In areas such as fast reconnection, for example, satellite measurements, dedicated laboratory experiments, and a new generation of theoretical and computational models have brought significant advances to our understanding. Such coordinated efforts are essential in optimizing progress in many areas, including understanding dynamos, magnetic reconnection and self-organization, plasma turbulence, and turbulent transport. In the past decade, there has been an increased appreciation by members of the plasma community of complementary and related activities in other areas of the field, and this has led to many productive synergies and success- ful collaborative efforts. To optimize future progress, it will be very important for this positive trend to continue and grow. CONCLuSIONS AND RECOMMENDATIONS FOR THIS TOPIC Many important new research opportunities in basic plasma science come about from progress and new discoveries in the last decade. Such opportunities ex- ist for studies in dusty plasmas, a new generation of laser-driven and HED plasmas, and micro- and ultracold plasmas, in addition to studies of new and fundamental aspects in areas such as Alfvén-wave physics and magnetic reconnection and self- organization. However, there are two potential roadblocks to progress: • Access to support for basic plasma science investigations. • The need for intermediate-scale experimental facilities for basic plasma studies.

OCR for page 184
Basic Plasma science 211 Addressing both of these concerns would be aided greatly by this report’s principal recommendation, namely that there is need for the Office of Science to assume stewardship for plasma science. As pointed out in a 1995 report,2 plasma science is a fundamental discipline similar, for example, to condensed-matter phys- ics, fluid mechanics, or chemistry. The diversity of scales of research in plasmas, from university laboratory to space missions and billion-dollar-class megascience projects, has hindered the articulation of scientific themes that unite research in plasma science and engineering across a campus or even a geographic region. university-Scale Investigations Conclusion: Basic plasma science—often university-based research and at a small scale—is a vibrant field of research through which much new under- standing of plasma behavior is being developed. Basic plasma science offers compelling research challenges for the next decade because it has extended by orders of magnitude the range of plasma parameters amenable to study, identified new phenomena, and developed new theoretical, computational, and experimental methods. There has been a considerable shift in the funding of university-scale basic plasma investigations in the last decade. The committee now gives a brief overview of the changes. Further details can be found in Appendix D. Partly in response to recommendations made in the 1995 NRC report, the joint NSF/DOE Partnership in Basic Plasma Science and Engineering was created in 1997. Typically proposals have been solicited triennially. This joint program between NSF and the DOE Office of Fusion Energy Sciences (OFES) has been funded at approximately $6 million per year. The program has become a critical source of funding for basic plasma research and is responsible for much of the progress described in this chapter. In parallel, OFES created a General Science Program to fund basic research at DOE laboratories and a very successful Young Investigator Program to fund research by junior faculty at colleges and universities. In addition, DOE and NSF recently sup- ported the creation of the Center for Magnetic Self-Organization of Laboratory and Astrophysics Plasmas. Programs such as these have had a strong, positive influence on the development of basic plasma science in the last decade. The emerging programmatic support at DOE’s NNSA in the past decade, through the Stockpile Stewardship Academic Alliance program, has provided a new level of stewardship of the growing area of laboratory explorations of HED plasmas. Paradoxically, during the same period (1995-2006), a vital and effective 2 NRC, Plasma Science: From Fundamental Research to Technological Applications, Washington, D.C.: National Academy Press, 1995.

OCR for page 184
Plasma science 212 program for basic plasma research at the Office of Naval Research, funded at $4 million per year, was terminated when U.S. Navy priorities changed. Conclusion: The collaborative partnership for basic plasma science and engineering between the National Science Foundation and Department of Energy has been critical to progress in basic plasma science. Focusing on single-investigator and small-scale research and aided by an effective sys- tem of peer review, it is an efficient and effective instrument to fund basic plasma research. Recent solicitation for the partnership program has had very high proposal pressure—in part owing to the triennial rather than an- nual solicitation schedule. The NSF/DOE Partnership in Basic Plasma Science and Engineering has been effective in terms of important research progress as judged, for example, by pub- lication in premier scientific journals such as Physical Review Letters. It has also contributed greatly to the education of new scientific and technical personnel for the field as judged by the number of Ph.D.’s granted in plasma science. It has made important connections with other areas of science and has achieved greater recog- nition for plasma science in the broader scientific community. The program is also very effective for providing research support for tenure-track faculty. It is the opinion of this committee that the success of this program is limited by the relatively small funding base. In the latest round of solicitations, only 20 percent of the proposals were funded, with the average grant size at $100,000 per year. A second limitation is the current emphasis on a triennial solicitation cycle for proposals to the Partnership. Simply put, science does not proceed on a 3-year cycle. Opportunities are lost if a new research project must wait several years to be considered for funding. This can be a particularly critical problem for young investigators and those in competition with foreign researchers. It is also a great impediment in maintaining momentum in an established research program. Years can be lost before a proposal is considered, and more delay if the first proposal has a correctable flaw that further postpones funding pending revision and resub- mission. For a university assistant professor, who typically has 6 years to establish a research program before a tenure decision is made, loss of even 1 or 2 years of funding can be a critical event. Recommendation: To realize better the research opportunities in basic plas- ma science, access to timely and adequate funding is needed. The Partner- ship for Basic Plasma Science and Engineering between the National Science Foundation and the Department of Energy should be expanded by going from the present triennial solicitation of proposals to an annual schedule. As discussed in Chapter 1, there is great potential for the Department of En- ergy to play a greater role in furthering all of plasma science, including its most fundamental aspects.

OCR for page 184
Basic Plasma science 21 Conclusion: Basic plasma science has benefited significantly from the in- creased stewardship of plasma science provided in the last decade by the Office of Fusion Energy Sciences of the Department of Energy. It would be further improved by even more comprehensive stewardship by that office. The intellectual synergies between basic plasma science and the subfields of plasma research would be greatly enhanced by leveraging more of the infrastructure that the subfields have in common. The committee believes that the DOE Office of Science would provide a natural environment in which to accomplish this objec- tive. Two areas of critical importance to DOE’s mission are low-temperature and HED plasmas. As discussed in this chapter and elsewhere in this report, these areas offer a wealth of opportunities and challenges for basic plasma science. A broader framework would, for the first time, create a structure that promotes the scientific kinship of these areas. HED and magnetic fusion plasma science would benefit from the closer connections to other plasma science areas. Such a framework would also serve as a common gateway for researchers from other fields whose interests bring them into contact with plasma science researchers. It would, for example, enhance the intellectual connections between the basic plasma science community and NASA-supported space and astrophysical missions, providing NASA program managers and scientists with a natural mechanism to interact more effectively with the basic plasma science research community. Intermediate-Scale Facilities The appropriate size for a basic plasma experiment varies depending on the problem being addressed. Researchers must weigh the merits of a particular experi- mental effort against the required costs to carry out this research. While much of this chapter focuses on small-scale and single-investigator projects, it is important to emphasize that some important problems cannot be addressed by this mode of investigation—the nature of the science sets the scale. For example, study of the physics of burning plasmas must be done in what are now the state-of-the-art magnetic fusion devices. There is much forefront, fundamental plasma science research that requires intermediate-scale facilities—experimental facilities larger than can be easily fielded by a single investigator but smaller than those at the larger national research installations. A recent and successful example of such an intermediate-scale experimental re- search effort is the creation of a national facility to study basic plasma problems that require large volumes of magnetized plasma. By cooperative agreement in 2001, the NSF and DOE OFES initiated support for the operation of a device of this type as a national facility. The research program, highlights of which are discussed in the subsection on plasma waves, structure, and flows, studies Alfvén-wave physics and associated phenomena, including electron acceleration mechanisms, electron heat

OCR for page 184
Plasma science 214 transport, and the formation of localized structures. This program allows teams of researchers nationwide to come together to study important phenomena that require very large volumes of magnetized plasma and a suite of state-of-the-art diagnostics. This project can be regarded as a model for addressing basic plasma science problems that require facilities larger than required by the typical effort of a single principal investigator. During the course of the committee’s work, the plasma community indicated that other scientific problems would benefit from intermediate-scale facilities of this type. One example is a facility to study HED phenomena intermediate in scale between the tabletop laser scale and the largest facilities such as that at the Univer- sity of Rochester and at the National Ignition Facility. The limited access and shot rate and the program-oriented focus of the large HED facilities makes difficult their use for basic HED plasma science. The forefront of basic high-intensity laser research now rests with petawatt-class lasers. These systems, while smaller than that at NIF, are large enough to make it difficult to maintain outside a national lab or a large university-based center. To remain a leader in this field and to exploit fully the new opportunities presented by ultrabright lasers, the United States should support and operate, either separately or jointly with other programs, mid-scale laser user facilities (including petawatt-class lasers) for unclassified research. A second example of the need for a mid-scale facility, and also one with wide- spread community support, is the need for a new experiment to study magnetic reconnection in three dimensions. As has been discussed, there has been dramatic progress in the last decade in studying reconnection through a new generation of computer simulations and laboratory experiments. These successes provide a roadmap for further progress toward a more complete and general understand- ing of this fundamental and important class of phenomena that are relevant to magnetic confinement fusion as well as to space and astrophysics. As discussed above, present magnetic reconnection experiments do not have sufficient separa- tion of spatial scales to isolate the physics of the reconnection process from plasma boundaries. This inhibits the study of many important phenomena, such as plasma flows and the associated slow shock waves predicted to originate in the reconnec- tion region. Conclusion: There are important basic plasma problems at intermediate scale that cannot be addressed effectively either by the present national facilities or by single-investigator research. Several areas of basic plasma science would benefit from new intermediate- scale facilities. For instance, at the present time, there is a clear need for a national facility for the exploration of reconnection phenomena. Similarly, there is also a need for intermediate-scale user facilities, including petawatt-class lasers, to study HED plasma phenomena. Constructing and operating such facilities may require

OCR for page 184
Basic Plasma science 21 additional resources. The DOE Office of Science should serve as a framework for soliciting, evaluating, and prioritizing such proposals and resources. Recommendation: The plasma community and the relevant federal gov- ernment agencies should initiate a periodic evaluation and consultation process to assess the need for, and prioritization of, new facilities to address problems in basic plasma science at the intermediate scale.

OCR for page 184