Each molecule that makes up a solid (or liquid) mass of metal is close enough to its neighbors to produce an interesting effect. While the negatively-charged electrons that have the lowest energies remain closely and tightly bound to the positively-charged nucleus of one particular atom in one particular molecule, the electrons with the highest energies are so weakly attracted to each nucleus that they can move quite freely among the atoms throughout the entire piece of metal. Being so free, these higher-energy electrons flow readily as electric currents in response to any external electromagnetic forces that the metal is exposed to. Such currents aren’t produced in nonmetallic liquids or solids because few if any electrons in those materials are energetic enough to not be bound to some particular nucleus’ positive charge; bound electrons don’t flow freely in response to external forces.
The molecules that make up a gas are in constant motion, hardly constrained except for when they collide with each other or with the atoms of a solid or liquid. An ordinary gas is, in at least one respect, like a nonmetallic liquid or solid: practically all the electrons are bound within particular molecules, and the molecules all contain enough electrons to make them electrically neutral; thus the gas lacks free negative or positive charges that can flow in response to an external electromagnetic field. But if the gas has a significant number of positive ions or free negative electrons in it, the ions or electrons will respond to external fields much as the free electrons in metals do. Gas with such quasimetallic properties was named plasma[Wikipedia] by one of its early investigators, Irving Langmuir, because it carries positive ions and electrons in a way that reminded him of how blood plasma carries red and white corpuscles.[Reference, p. 7]
Plasma is interesting from numerous technical and scientific perspectives, some much more widely known than others. Several of these are the subjects of research sponsored by the Department of Energy.
Two recent reports describe different uses of a plasma’s positive ions to produce chemical reactions in solid materials.
A homogeneous solid made of just one material, like sodium chloride (table salt)[Wikipedia], can have its atoms arranged in a completely regular repeating structure, with the spacing between atoms depending on the atoms’ sizes. On the other hand, solids made of layers of different materials tend to have some irregularities in their atoms’ arrangement, because where the different materials meet, the atoms in each material generally have different sizes, so the natural spacings of the different materials’ atoms won’t mesh. Where such spacing irregularities occur among the atoms of electronic devices, electron flow may be disrupted and the devices’ effectiveness may be reduced.
One approach to addressing this problem in solar-cell materials is described in the preprint “Hydrogenation of Dislocation-Limited Heteroepitaxial Silicon Solar Cells”[SciTech Connect] by researchers from the National Renewable Energy Laboratory and Ampulse Corporation. The authors show that, although silicon layers can develop defects in their own atomic arrangement as they’re grown on a substrate whose atoms have different sizes and spacing, exposing the silicon to a plasma made from a hydrogen-argon gas mixture results in the hydrogen’s reacting with the silicon at the defects. As a result, the defects’ disruption of electron flow was reduced while the silicon’s conversion of light energy into electrical energy became, under one treatment, about four times as efficient.
A different use of plasma surface treatment involves the CEBAF[Wikipedia] accelerator at Thomas Jefferson National Accelerator Facility. CEBAF produces intense beams of high-energy electrons for use in examining atomic nuclei. The electrons are accelerated to their high energies by radio waves produced in superconducting cavities. Contaminants on these cavities’ inner surfaces degrade the radio waves they produce, thus degrading the electron beams and the observations of nuclei. Preliminary findings on contaminant removal by exposing the surfaces to argon and argon-oxygen plasmas are reported by researchers at Jefferson Lab and Old Dominion University in “Large-Volume Resonant Microwave Discharge for Plasma Cleaning of a CEBAF 5-Cell SRF Cavity”[SciTech Connect]. They describe success in removing organic and some residual metallic impurities, as well as a need for additional work to make the process reliable.
Plasmas can not only clean particle accelerators, they can accelerate particles themselves. Just as waves of pressure variations—sound waves—can travel through a gas made of electrically neutral molecules, waves of varying positive and negative charge density can travel through a plasma. Wherever the charge density is high, large electric fields will be produced, and these fields can accelerate beams of charged particles that pass through them.
Since a plasma wave’s electric fields can be much stronger than the fields produced by a radiofrequency cavity, a particle accelerator based on plasma waves could be much smaller than a cavity-based accelerator that accelerates the same kinds of particles to the same energies. Because of this potential, as well as the variety of uses to which high-energy particle beams are already put, overcoming technical obstacles to plasma-based particle accelerators is a matter of considerable interest and effort. Some of the efforts are described in recent reports from SLAC National Accelerator Laboratory.
SLAC’s new Facility for Advanced Accelerator Experimental Tests, or FACET, in which plasma-acceleration experiments (among others) are conducted, is described in “Plasmas, Dielectrics and the Ultrafast: First Science and Operational Experience at FACET”[SciTech Connect], by researchers from SLAC and UCLA. After describing the facility itself and initial operating experience with it, the report discusses the results of various experiments, some of which involved particle acceleration by plasma. Two of the plasma-acceleration experiments are described in more detail in separate reports: “Betatron Radiation from a Beam Driven Plasma Source”[SciTech Connect] and “Head Erosion with Emittance Growth in PWFA”[SciTech Connect] (in which “PWFA” stands for “Plasma Wakefield Acceleration”).
The latter report, by researchers at SLAC, UCLA, the Max Planck Institute for Physics in Munich, and the University of Oslo, deals with a limitation of one plasma acceleration method. As the report describes, one way to make a plasma that can accelerate charged particles is to use the charged particles themselves to ionize the gas they travel through and thus transform the gas into a plasma. This can automatically produce a plasma wave that travels aligned and in sync with the charged-particle beam, and has strong electric fields to accelerate the beam. If the particle beam is made of electrons, those electrons at the head of the beam will repel plasma electrons, leaving positive ions behind them that will attract the beam electrons behind the head. This focuses the beam by countering its electrons’ natural tendency to repel each other as they move forward. However, this focusing of the beam only happens behind the beam head. The head still tends to diffuse, which eventually decreases its electric field strength below what’s needed to ionize the gas it runs through, which means that ionization of the gas and focusing by the positive ions gradually occur further and further back in the beam as more and more of the beam diffuses. Continued long enough, the whole beam would erode enough to cease generating an accelerating plasma at all.
Thus the amount of energy that particles gain when they ionize their own accelerating plasma has an upper limit based on the beam-head erosion rate. Calculations indicate that the erosion rate decreases with increasing beam current, and increases with increasing particle energy and what’s known as the beam’s emittance[Wikipedia], which describes the variation in beam particles’ positions and momenta. The report “Head Erosion with Emittance Growth in PWFA”[SciTech Connect] describes an initial experimental test of the calculations that confirmed the increase of erosion rate with larger beam emittance.
Accelerated charged particles can lose small amounts of their energy in the form of electromagnetic waves whose frequency and intensity depend on how fast the particles are moving and what accelerations they undergo. The frequencies can vary from radio-wave through visible-light to gamma-ray frequencies. The waves emitted by the charged particles can also be used in experiments just as the charged particles can. The waves can also be observed to measure how stable the beam is.
The latter function was investigated and reported in the SLAC report “Betatron Radiation from a Beam Driven Plasma Source”[SciTech Connect]. In the experiment described there, the gamma rays emitted by the charged particle beam while the plasma focused it showed that the beam size was not well matched to the plasma density, so the beam wasn’t as stable as it might have been. One way to reduce such instability is indicated by researchers at SLAC, UCLA, and the University of Southern California in their report “Beam Matching to a Plasma Wakefield Accelerator Using a Ramped Density Profile at the Plasma Boundary”[SciTech Connect], which shows how an appropriate increase in the plasma density where the beam encounters it can ensure a beam size/plasma density match.
Efforts toward the other use of electromagnetic waves (in this case, x-rays) from a charged-particle beam, as the main product instead of as a beam diagnostic, are described in the recent report “Compact X-ray Free Electron Laser from a Laser-plasma Accelerator using a Transverse Gradient Undulator”[SciTech Connect] from SLAC and Lawrence Berkeley National Laboratory. Much as an accelerator has to be optimized in one way to produce high-energy charged-particle beams, it has to be optimized a different way to function as an x-ray laser, in which an x-ray spontaneously emitted by any one charged particle can stimulate the emission of identical x-rays by other charged particles so all the x-rays’ vibrations will be in sync. For this to happen, the x-rays need to have identical energies, which means the charged particles that emit them need to each lose identical amounts of energy, which is most easily done if the charged particles have equal energies initially. The report describes a structure within a plasma-based particle accelerator that will reduce energy differences between the accelerated charged particles and thus increase the emitted x-rays’ coherence.
Plasmas are not only capable of accelerating small bunches of individual elementary particles, but larger objects, as described in the slide presentation “A Microwave Thruster for Spacecraft Propulsion”[SciTech Connect] from Los Alamos National Laboratory. Rockets whose thrust derives from burning fuels can push heavy payloads hard enough to lift their weight from ground level into outer space, but the amount of momentum such rockets add to each unit of their payload’s mass is small compared to what thrusters based on heated plasma can add. Present-day plasma rockets can’t push with the large forces that chemical-fuel rockets can to launch payloads from the earth’s surface into space, but they can push for much longer times, which is an advantage for payloads that are already orbiting the earth or on their way elsewhere. The slide presentation illustrates the use of plasma-based rockets in two recent space probes as well as a potential use for raising a space-station module to a higher orbit.
A different kind of plasma turns up in the experiments discussed in the Ohio State University report “Final Report for Project ‘Theory of ultra-relativistic heavy-ion collisions’”[SciTech Connect], which deals with efforts to understand the behavior of atomic nuclei that collide near the speed of light at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC). Whereas high-temperature gases form plasmas when a significant fraction of their atoms or molecules have one or more electrons loosened from individual atoms or molecules, highly energized colliding nuclei exhibit a similar loosening of quarks’ and gluons’ usual binding into individual protons and neutrons. The binding and loosening of quarks and gluons involves a different kind of interaction from the electromagnetic ones that can bind electrons and ions. Unlike electromagnetism, the strong nuclear interaction between particles gets stronger instead of weaker as the particles affected by it move further apart. Also, where electromagnetism involves only one kind of charge, either positive or negative, strong nuclear interactions involve three kinds of positive and negative charge, arbitrarily named red-antired, blue-antiblue, and green-antigreen. From experiments at RHIC, quark-gluon plasma has been found to behave like a liquid with a very low viscosity whose magnitude has been precisely determined. These experiments have provided much information about how the strong nuclear interaction works.
The plasmas considered in the reports mentioned above are laboratory products, but not all plasmas are artificial. In fact, plasma makes up the bulk of ordinary matter in the universe[Reference, p. 6]; solids, liquids, and gases of neutral particles predominate only in exceptional places like planets. The slide presentation “Fe Atomic Data for Non-equilibrium Ionization Plasmas”[SciTech Connect] by researchers at Los Alamos National Laboratory, Rutgers University, the Smithsonian Astrophysical Observatory at Harvard, and the University of Pittsburgh describes examinations of the plasmal remnants of the supernova observed in A.D. 1572[Wikipedia], and notes what they can tell us about particular species of ionized atoms. A different, more extensive plasma, the intracluster medium[Wikipedia] at the centers of galaxy clusters, is the focus of another report from researchers at SLAC and National Taiwan University: “Analysing the Effect on CMB in a Parity and Charge Parity Violating Varying Alpha Theory”[SciTech Connect]. The “CMB” of the title is the cosmic microwave background[Wikipedia], the low-temperature electromagnetic waves that come from every part of the sky. According to the theory of elementary particle interactions that the report describes, the cosmic microwave background should be affected by its passage through the intracluster medium in particular ways. Observations of cosmic background microwaves that have traversed intracluster plasma would provide a test of the theory and inform us about the existence and relative strength of the interactions the theory assumes.
The form of plasma pervading the universe that is most visible to us is that of the sun and the other stars. The plasmas they’re made of are so hot, with temperatures ranging from thousands of kelvins at their surfaces to millions of kelvins in their cores, that their positive ions collide hard enough and often enough every second for enormous numbers of them—in the sun’s core, on the order of 1038 per second—to overcome their mutual like-charge repulsion and fuse together. As the small nuclei of the original ions fuse, their constituent protons and neutrons’ strong nuclear attraction pulls them together into a lower-energy arrangement, with the energy lost being emitted in the form of gamma rays. The gamma rays pass through the outer layers of the stellar plasma, heating them up as they collide with its ions and electrons, until they pass beyond the star’s outer surface. Some, with little or no energy loss to the outer plasma, will still be gamma rays; most will have lost more energy to emerge as x-rays, ultraviolet light, visible light, infrared light, microwaves, or radio waves. From the sun, this huge output of electromagnetic waves provides the chief source of heat and light at the earth’s surface; the same kind of waves from the other stars make them visible to the unaided eye or through telescopes from as far away as a few light years to billions.
A plasma hot enough for many of its ions to fuse together would rapidly expand, disperse, and soon quit fusing unless something kept it from dispersing. In stars, that something is gravity. But gravity is strong enough to confine a hot fusing plasma only in large, heavy objects like stars. Achieving continual fusion in a smaller plasma requires some substitute for gravity. One candidate for a substitute is magnetism. Much of the effort worldwide to use plasma fusion as an energy source has focused on plasma’s metal-like magnetic properties to confine plasmas by magnetic fields in toroidal (torus- or doughnut-shaped) devices known as tokamaks.[Wikipedia] The technical challenges are great and numerous but so are the potential benefits. Much of the difficulty involves understanding the behavior of magnetized plasmas, with the complexity of their charged and neutral particles’ motions.
Two of the recent reports on this subject, though they constitute a very small sample, indicate something of the range of problems being addressed. “Numerical Verification of Bounce Harmonic Resonances in Neoclassical Toroidal Viscosity for Tokamaks”[SciTech Connect], by researchers with the Princeton Plasma Physics Laboratory and Columbia University, reports a computer calculation that confirms the earlier-suspected effect of one type of charged-particle motion on the plasma’s confinement. “Comparison of Gas Puff Imaging Data in NSTX with the DEGAS 2 Simulation”[SciTech Connect], by researchers with PPPL and the Chinese Academy of Sciences’ Institute of Plasma Physics, describes how data from PPPL’s National Spherical Torus eXperiment was used to verify a computer code that simulates the motion of neutral particles in the torus. This is particularly important since neutral deuterium atoms typically provide the fuel for discharges in tokamaks, and can also affect the plasma’s energy balance. Accurate measuring and calculation of neutral atoms’ behavior would allow assessment of the deuterium’s effects and the efficiency of fueling sources.
While it’s easy to describe the key difference between plasmas and the more familiar gases, liquids, and solids of our ordinary experience—namely, plasmas’ separated positive and negative charges—this difference has profound implications for plasmas’ physical properties and technological usefulness, as the reports described here suggest.
Prepared by Dr. William N. Watson, Physicist
DoE Office of Scientific and Technical Information