The world’s energy sources are limited and in four or five decades they will be in short supply. However, the world’s increasing energy demands have led scientists to investigate alternative energy sources. One alternative, discovered during the twentieth century, was that there are nuclear fusion reactions in the Sun and the stars.
The sun radiates an enormous amount of energy-at a rate of 3.9x1026 Joule per second. This is roughly equivalent to the energy of a 10 billion megaton TNT bomb every second. This huge amount of energy has been maintained for several billion years and will continue for several more. The fusion reaction of the Sun is a process in which hydrogen burns, transforming into helium, which is then followed by thermonuclear explosions. Isotopes of hydrogen, such as deuterium and tritium, are fused to form heavier helium. During this process the released energy can be as high as 17.6 MeV. The energy released from a 17 lbs deuterium fusion is equal to 1,000 kilotons of TNT. Every second the Sun fuses 675,000,000 tons of hydrogen into 653,000,000 tons of helium.
Scientists have attempted to make fusion work on the earth to make larger amounts of energy, thus solving our energy problems for the future. The first nuclear fusion trials were carried out for nuclear weapons. The released energy from the fusion trials was 500 times higher than that from the fission reactions of nuclear weapons1. The energy released was equal to that of approximately 12 million tons of TNT. The civilian applications for energy production began in the early 1950s, and we are still trying to solve how to control this amount of energy in reactors.
In nuclear fusion, the negative and positive ions of hydrogen, called plasma, reach temperatures of 100 million degrees. To achieve the plasma parameters of the Sun, for example, the same temperature and density, the plasma must be heated to 100 million degrees Celsius and be kept dense and confined for at least 1 second.
Plasmas are mostly heated by Ohmic (resistive) heating, beam injection, or radio frequency heating. Ohmic heating is the result of an induced current being passed through the plasmas. This mechanism is also used to make electric bulbs and heaters work. Ohmic heating cannot attain plasma temperatures; such heating does not rise above 20-30 million degrees Celsius. When the temperature increases, the resistivity of the plasma decreases. Natural beam injection is one of the mechanisms used to obtain higher energy temperatures. Injecting a high-energy beam of neutral atoms into the plasma causes more collisions and increases the plasma temperature by transferring the atoms’ energy to the plasma. Radio frequency heating is another collision mechanism that increases the plasma temperature. Radio waves generated by oscillators transfer their energy at appropriate frequencies to ions or electrons, thus increasing the plasma temperature. Scientists have managed to get to high enough temperatures; however, these plasmas cannot be contained by the reactor walls easily and the reactions cannot be sustained. To prevent a loss of reaction control and to make the plasmas denser, magnetic confinement mechanisms have been developed such as TOKAMAK, Z-PINCH and ICF.
The TOKAMAK (Toroidal Chamber) device was invented in the late 1950s by the Russian physicists Igor Tam and Andrei Sakharov. In this system, mixtures of deuterium and tritium plasmas, confined by doughnut-shaped magnetic fields, are produced by the toroidal coils, which are then heated to very high temperatures. The temperature achieved by the Princeton Labs is 510 million degrees-almost 30 times greater than the temperature of the Sun. One of the major problems in TOKAMAK is that superconducting magnetic coils are needed for the electricity demand, but the superconducting magnets only operate at cold temperatures. So, a space between the plasma and coils must be maintained to avoid the plasma reaching the coils and damaging them. This mechanism is still assumed to be the best for the confinement of plasmas2.
Another confinement system is the Z-pinch (Zeta-Pinch) pulse power device. The current flow of experimental devices is in the Z-axis, so the device was called the Z-pinch by the British scientists in the late 1950s. In this mechanism, very tiny wires, thinner than a human hair, are positioned in different configurations, such as cylindrical or nested geometries, and are then placed in an anode cathode gap.
Applying high voltage on the system causes the energetic plasmas to compress and heat the deuterium or tritium fuel in small pellets. The current flows through these wires axially, generating magnetic fields that confine the plasma. The temperature achieved is about 1.6 billion degrees; this result, reported by the Sandia National Labs, is almost 250 times higher than the interior of the Sun. Z-pinches produce the most powerful plasmas, but the generated plasmas are very unstable3.
Lasers were invented in 1962, and have been applied in many areas. Lasers were used in infusion research to confine the plasma in the late 1960s by scientists at Lawrence Livermore. This laser-based process is called ICF (Inertial Confinement Fusion). In this mechanism, laser light is used to compress and heat the pellet. The temperature achieved is about 100 million degrees Celsius and the plasma is compressed almost 1,000 times its liquid density. However, this confinement occurs in less than in a microsecond, which is not enough time to allow the ions to build on the energy of their own inertia.
Today, many countries have invested millions of dollars in confinement and ignition systems to create fusion power. ITER is an International TOKAMAK fusion project that will be built in France (for more information: http://www.iter.org/). Its participants have agreed to provide funding of $13.1 billion. When it is completed, the ITER will be one of the most expensive scientific projects in the world. However, despite the high cost, there are good reasons why scientists insist on the use of fusion. One of these is that no CO2 is produced during the process. Everyone is aware that CO2 has negative effects; for example, it leads to increased pollution and global warming. Another reason is the abundance of hydrogen available for fusion in seawater and on the earth’s crust. Another important reason is that fusion is safer than fission or other energy sources: There are no nuclear accidents, and in case of malfunction, the plasma is absorbed and cooled by the reactor walls. Also, the generated amount of radioactive particles is fewer than those generated by fission.
If everything goes well, scientists expect that fusion will be used as a source of energy in a couple of decades. If fusion is successful, it can provide clean, safe, reliable, sustainable, and widely applicable energy.
M. Fatih Yilmaz is a graduate researcher at Physics Department, University of Nevada.
1. Frisch O. R.: “The Discovery of Fission – How It All Began.” Physics Today 20 (1967), 11, pp. 43-48; http://en.wikipedia.org/wiki/Nuclear_fission.
2. http://en.wikipedia.org/wiki/Tokamak; http://www.ppdl.gov.
3. James Glanz, Science 18 July 1997:Vol. 277. no. 5324, p. 306 DOI: 10.1126/science.277.5324.306.