Plasma Physics: The State of the Universe
By David Schaffner
Everyone knows the three phases of matter: solid, liquid, and gas. A smaller subset knows about the so-called “fourth state of matter”: plasma. However, for all the attention and buzz that solids, liquids and gases get—from grade school science to college chemistry—it’s the plasma state that the universe reveals to be most abundant. Upwards of 99.9% of baryonic matter exists as a plasma: from the bellies of solar furnaces, to the vast emptiness of interstellar space, to the raging death spirals of accretion disks around black holes. Closer to home, on the other hand, plasmas are a rarer occurrence. Naturally, they can occur in the Earth’s upper atmosphere in the ionosphere as well as the auroras Borealis and Australis (Northern and Southern Lights). Within the last century, plasma has been explored and utilized using human-made technology such as in discharge lighting (florescent lights), but also in industrial processing (plasma processing, chemical vapor deposition) and inside tokamaks and other fusion-relevant plasma experiments which aim to recreate the very nature of the solar furnace itself. Despite its growing importance, the fourth state of matter remains very much in fourth place in the hierarchy of terrestrial matter.
What is it about plasma that makes it such a rare occurrence on Earth? In short, our terrestrial world is just too crowded and too cold. A plasma is created by taking matter in the gaseous state and ionizing the constituent atoms or molecules into electrons and ions. This ionized gas, as plasma is sometimes called, behaves like a gas—it retains statistical properties such as pressure, temperature, and density—but consists of charged particles rather than neutral particles, and so attains additional features such as the ability to carry current, or be affected by electromagnetic fields. For a plasma to avoid returning to a neutral gas state, enough members of the system must remain distinct charged particles and exhibit collective behavior. (The requirement for collective behavior is what differentiates a beam of charged particles, such as from an electron gun, from that of a plasma). Given enough energy per particle, clouds of ions and electrons can coexist interspersed within one another. When a plasma gets too crowded or too cold, enough recombination collisions occur to snuff out the long-range effectiveness of charged electrons or ions. This occurs approximately when the mean free path for 90° collisions between particles becomes smaller than the effective extent of long-range electromagnetic field interactions—a distance called the Debye length.
On Earth, such a condition is rarely met and any plasma generated very quickly recombines into a gaseous state. In vacuum conditions, either naturally created at high altitudes or hot temperatures, or artificially created in the laboratory, the mean free path can be made quite larger than the Debye length so plasma can persist. In space, there is just that much more, well, space. In regions of the solar system, mean free paths between particle collisions can be on the order of A.U.
When a plasma is allowed to persist, either naturally or artificially, it is a system that can exhibit an immense amount of complexity, driving interested plasma physicists to study it both for the sake of its inherently fascinating characteristics and for its potential utility as tool for technology.
For the physicists focused on basic physical principles, plasma systems offer an extremely wide range of phenomena to explore. Due to the long-range nature of electromagnetic interactions, the particle constituents of plasma can easily affect one another non-locally, as well as interact directly with static electric or magnetic fields, and electromagnetic radiation. This leads to a myriad of fluctuations that can propagate through the system as waves. Some waves are pressure-based in nature, akin to standard sound waves in a gas, while others can propagate due to electrostatic interactions, or full electromagnetic interactions. Moreover, when the kinetic properties of particles and the frequency variation of electromagnetic fluctuations are taken into consideration, some waves can morph into other waves types as parameters are varied or changed. To this day, a major component of plasma physics research remains the exploration and categorization of wave and instability phenomena in numerous variations of plasma systems, whether experimental, theoretical, or simulated. While particle physics holds the claim of being the “zoo of particles”, plasma physics clearly earns the title of “zoo of waves”.
Additionally, plasma researchers study these systems to better understand the global nature of complicated systems of particles. In particular, the study of plasma turbulence stands at the forefront of interest in the community. Like a fluid, a plasma can exhibit highly non-linear interactions between sections of the plasma or parcels. Given the right conditions, these non-linear interactions can lead to the formation of flow vortices and turbulence. However, the charged nature of plasma adds to the complexity of behavior—one can study simultaneous the turbulent structure of flows and fields in a plasma.
Beyond the basic plasma research aims, the plasma research community falls very roughly into three camps: astrophysical plasma, fusion energy plasma, and industrial or applied plasmas. Astrophysical plasma research seeks to understand the nature of the most common, naturally occurring plasma—that found in interplanetary, interstellar, and intergalactic space. Some focus on heliospheric plasma, ranging from the thermonuclear burning plasma at the sun’s core, to the hot, wispy corona, to the interplanetary solar wind, and to the violent collision of solar wind plasma with planetary magnetospheres. Some seek to better understand basic plasma behavior in these massive systems, such as the massive transfer of energy that occurs in magnetic reconnection at the Earth’s magnetopause or magnetotail, or the nature of magnetic turbulence in the pristine, expanding solar wind. Others have a more practical goal in mind—understanding and predicting solar weather. Characteristics and behavior of major solar events such as solar flares or coronal mass ejections are studied closely with an eye toward prediction and projection.
For both goals, the primary tools have been in-situ spacecraft and remote observation. Satellites and space craft such as ACE, WIND, and Ulysses, have been measuring the properties of solar plasma for decades, accumulating reams of magnetic, temperature, density, and velocity data. Other craft, either in orbit, such as SOHO, or launched on sounding rockets, have taken numerous photos of the sun in action. Holding most researchers’ attention, however, have been the two most recent major spacecraft launches, the Magnetospheric Multiscale (MMS) mission and the Parker Solar Probe (PSP) mission. MMS consists of four satellites orbiting Earth in a tetrahedral pattern. Its primary goal is to better understand magnetic reconnection mechanisms in the magnetosphere while also exploring wave and turbulence observations with unprecedent resolution. Launched just this summer, PSP is the mission “to touch the sun.” Its planned seven-year journey around the sun will take it to within 9 solar radii of the sun itself, exploring the inner corona in an effort to unearth the mechanisms that generate the solar wind in the first place.
Astrophysical plasma research has a terrestrial component as well called laboratory astrophysics. In many Earth based experiments, astrophysical phenomena are modeled or recreated in a more controlled laboratory setting. Research at locations such as UCLA, University of Wisconsin, Madison, Naval Research Laboratory, Princeton Plasma Physics Laboratories, and Swarthmore College have developed and pursued techniques studying systems relevant to space physics, including the study of magnetic reconnection, solar dynamo, Alfvenic turbulence, and Whistler waves.
Plasma physics has maintained a vigorous applied focus since the field’s conception, particularly in its dogged pursuit of fusion power. Since the understanding that fusing two smaller nuclei into a larger nucleus can result in a tremendous conversion of mass energy into kinetic energy, plasma physics has been a natural home for the development of a controlled method of this fusion. Since scientists on Earth do not have the luxury of a solar-mass amount of gravitation pull to bring nuclei together, other means have been sought. For a sustained fusion reaction to occur, and for more energy to be released than input, the system in question must attain a certain temperature at a certain density for a long enough time—a set of conditions called the Lawson criterion. A plasma is an ideal system to achieve such conditions as its properties lend to a multitude of ways of heating and confinement. Current fusion research is pursued along three avenues:
- confinement in a magnetic bottle such as a tokamak (DIII-D and NSTX in the US, JET, MAST, and of course ITER in Europe, KSTAR and EAST in Asia), a stellerator (W7 in Germany), or a magnetic mirror (Gamma 10 in Japan and gas dynamic traps in Russia),
- confinement using inertially imploding plasmas driven by lasers (NIF at Livermore National Labs),
- confinement using a combination of inertia and magnetic fields called magnetoinertial fusion (currently pursued by venture-capital startup companies such as TAE Technologies, General Fusion, Helion).
While the approaches can be very different, the problems are all too similar. Confinement of the plasma is limited by instabilities that grow, leading to turbulence or violent disruptions of equilibria and stability. Energy must be injected into the plasma to increase heat and density but inevitably leads to steep unstable gradients. Hot plasma can be damaging to materials designed to hold it. Most current research in fusion energy falls into solving these three issues. Attempts to mitigate turbulence and eliminate disruptions are pursued through computer modeling and experiments exploring plasma flows and magnetic structures. Methods for heating plasma through neutral beam injections and electromagnetic radiation are continuously developed and improved. Plasma-material interactions are studied closely for improving confinement and lifetime of both plasma and container.
Finally, plasmas have played and are increasingly playing a crucial role in industrial processes and technologies including the semiconductor industry and propulsion. Solid state and nanomaterials are often constructed using plasma-based techniques such as chemical vapor deposition (CVD). Semiconducting material can be doped or probed using ion beam plasmas. Ion and plasma-based thrusters for rockets or spacecraft have been developed and employed for fine control of current spacecraft and potentially primary thrust for space exploration. Relatively recently, medical applications of plasmas have also been developed. These atmospheric plasmas are typically very weakly ionized but retain enough plasma properties to be utilized. In particular, plasmas can be used for anti-microbial applications or for modifying cellular behavior (such as cancer cells).
While plasma may be the least widely known of the four phases of matter, it is an ever-growing field with a mix of ambitious projects, expanding applications, and a forum for study of complicated systems. For the early career scientist, plasma physics offers a wide-open expanse of research opportunities along experimental, theoretical, and computer modeling avenues. Unlike many other mature fields where massive, collaborative aims dominate, fundamental questions about this fascinating medium can still be explored in small- to intermediate-size groups and settings. So grab some energy, ionize some gas, and start playing with plasma.
David Schaffner received his bachelors degree in physics (minor mathematics) from University of California, Los Angeles (UCLA) in 2006, and his master's and PhD degrees in physics, also from UCLA in 2007 and 2013, respectively. After a postdoctoral fellowship in Department of Physics and Astronomy at Swarthmore College, he joined the faculty at Bryn Mawr college in 2015. David's research interests include: Study of magnetohydrodynamic (MHD) turbulence in a laboratory plasma device.
Parker Solar Probe, named after Eugene Parker who first theorized the existence of the solar wind, will reach as close as 9.86 solar radii. Its first close pass of the sun will occur in November of 2018. The spacecraft has a massive heatshield behind which all the instruments are placed to protect them from the brutal onslaught of solar wind particles. (Images from Johns Hopkins University Applied Physics Laboratory)
Construction of ITER, to be the world’s largest tokamak and first burning plasma device, is underway in Provence in southern France. First plasma experiments are set to begin in 2025. Startup companies are racing to beat ITER to the fusion punch, however, including the recently announced SPARC, a joint venture between MIT researchers and venture capitalists. (image from iter.org)