An ancient Greek myth relates that Prometheus, son of gods, stole fire from the heavens and gave it to the people on Earth. This gift and all its many applications catapulted humankind from a harsh, primitive existence toward civilization. Today, scientists in many parts of the world seek to perform a Prometheus-like act, granting human civilization a new energy source that will propel it forward once again. They are looking for ways to harness nuclear fusion, the process by which the sun and other stars generate their abundant energy. Formerly this process was used for destructive purposes: to fuel the explosion of the hydrogen bomb. Today’s researchers, however, pursue peaceful applications of nuclear fusion for its greatest potential benefit: a safe and abundant energy source for all humankind.
The existence of nuclear fusion was first hypothesized in the 1920s, and in the late 1930s it was described in quantitative terms by Hans Bethe and others. They discovered that light chemical elements appear to act in an “anti-holistic” manner: the whole is somehow smaller than the sum of its parts. For example, the overall mass of the nucleus of helium (which can be created by fusing two hydrogen atoms) is smaller than the joint mass of its components. What happens to the small mass missing from the helium nucleus? It turns out that when the helium nucleus is created by the fusion of its parts, this mass is converted into energy in accordance with Einstein’s famous equation, E=mc2 (energy = mass x the speed of light squared).
From this equation, one can see that a very small amount of matter can yield enormous amounts of energy. Indeed, calculations show that the energy released by fusing 25 grams of hydrogen into helium could provide all the energy needs of a person living in the Western world throughout his or her entire lifetime. And hydrogen is the most abundant element in the universe. Fusion for energy production is environmentally friendly, safe and clean: there is substantially less radioactive waste than with nuclear fission and no harmful byproducts such as the carbon dioxide associated with fossil fuels – a significant contributor to global warming. The catch is that building an installation to convert the energy of nuclear fusion into electricity is like trying to harness a raging tiger to a supermarket shopping cart. Nuclear fusion is a tumultuous event with huge, destructive potential; safely controlling the process is the biggest challenge facing fusion researchers today.
Nuclear fusion is the result of a sort of atomic train wreck: two stripped-down hydrogen isotopes speed toward each other, accelerating as they go, until they slam into each other with so much force they stick together, releasing “surplus” energy as heat. Because the isotopes – essentially positively charged nuclei – repel each other, their speed must be quite high to overcome the strong forces of electrical repulsion between them.
Cooking with plasma
For any kind of energy production to be feasible, one must get more energy out of the system than one puts in, over time. Though this may seem self-evident, fusion until now has demanded a colossal amount of energy to achieve the conditions required to get it up and running, and these still fall short of what’s needed to sustain it in a controlled manner: The hydrogen atoms must reach a temperature of at least 100 million degrees centigrade. (In comparison, the temperature at the center of the sun is “only” 15 million degrees centigrade.) This intense heat transforms the hydrogen gas into a plasma of hot, electrically charged ions. (Plasma is the fourth state of matter after solids, liquids and gases.)
Unfortunately, hot and dense plasma, the favored medium for fusion, seeks to expand, thereby losing its heat and reducing the chances for atom collision. The energy invested in containing the plasma and creating the right conditions for nuclear fusion has until now been much greater than the energy obtained. Today, intense research efforts are aimed at finding ways to increase the efficiency of the process and enable it to become “profitable.”
Several methods have been proposed to this end. For instance, giant, hollow donuts, the size of football fields, employ strong magnetic fields to keep plasma going for about one second. That’s an eternity compared to the life span of the plasma created in a second approach, where much hotter and denser plasma is formed in a mini-scule area, just a few millimeters in size, for only ten billionths of a second. In this method, there’s no need to contain the plasma – it’s so hot and the atoms are so close together, fusion takes place within that tiny instant.
One way to create the extremely hot and dense plasma needed for the second method involves compressing the plasma with powerful pulses of magnetic fields produced by strong electrical currents. In the past decade this approach has led to several breakthroughs, most of them achieved at the Sandia National Laboratories in the United States, which has close ties and an open exchange of ideas with the Weizmann Institute. Still, numerous basic questions need to be answered, calling for additional research. That’s where Prof. Yitzhak Maron and his plasma laboratory in the Particle Physics Department enter the picture.
Modest facility, international scale research
The Weizmann Institute of Science is one of only two research institutions outside the United States involved in the University Excellence Center – a prestigious initiative sponsored by the United States Department of Energy that promotes research into hot and dense plasma. Maron and the scientists who work with him in the relatively small system on the Weizmann Institute campus study the behavior of plasma created using the leading approach in this area, known as Z-pinch. (Z-pinch refers to the vertical, or Z, axis on a three-dimensional grid; in this method, magnetic pulses compress plasma in a vertical tube along the Z axis.) By investigating the behavior of Z-pinched plasma and the different processes by which this plasma’s kinetic energy is converted into heat and radiation, the scientists hope to identify those in which energy will undergo the most efficient conversion. In other words, they are looking for ways to get the most usable radiation energy output for the least input.
Capturing processes that take place within nanoseconds at blistering temperatures requires sophisticated methods. Maron’s group uses spectroscopy, the technique scientists employ to study the most remote and violent areas of the universe. Their equipment captures the spectra of radiation emitted from the plasma, translating light intensities and wavelengths into information about plasma components and their properties: temperature, density, electric and magnetic fields, and atomic velocities.
Expanding magnetic field in plasma
Researchers in Maron’s lab, in collaboration with scientists from the University of Jena and the GSI Laboratory in Germany, have recently come up with innovations in spectroscopy, making it possible to obtain better information about dynamic plasma processes. With these innovations, Dr. Eyal Kroupp and his team in the Institute’s plasma labmeasured and tracked very rapid changes in the energies of the plasma ions. A series of spectral images captured at increments of one per nanosecond (one billionth of a second), at ultrahigh spectral resolution, is what allowed them to see more clearly than ever what happens in plasma. The images revealed cases in which most of the compressed plasma energy is converted to radiation. The innovative methods developed by Maron’s team are now being employed in large laboratories in the United States and Europe.
In other experiments, Dr. Ramy Doron and a team of researchers in Maron’s laboratory were able to observe plasma phenomena never before seen in a lab, such as ions separating and electrons accelerating at the advancing edge of magnetic fields. Similar phenomena have recently been recorded by research satellites monitoring the plasma and magnetic activity of the sun, and the scientists are currently discovering what laboratory measurements can tell us about events observed from space. Thus out of one relatively modest laboratory comes research that contributes insights into the universe in which we live, as well as those that bring us closer to the use of controlled nuclear fusion for making our own world more livable.
Prof. Yitzhak Maron’s research is supported by the Monroe and Marjorie Burk Fund for Alternative Energy Studies; Sandia National Laboratories; and Dr. and Mrs. Robert Zaitlin, Los Angeles, CA. Prof. Maron is the incumbent of the Stephen and Mary Meadow Professorial Chair of Laser Photochemistry.
Time series (t=25nanoseconds to t=65ns) of an expanding magnetic field in plasma. Red denotes a strong field, blue a weak one