An LDX Exhibit at the Boston Museum of Science opened on 7/1/05.
A touchscreen text accompanying the exhibit is reproduced below.
Will nuclear fusion power the future?
We depend on the Sun for our very existence: the chain of life on Earth works only because plants turn sunlight into chemical energy. At present, however, our reliance on the Sun as a source of energy has limits. Although scientists have harnessed a tiny amount of the Sun’s power directly as solar-generated heat and electricity, most of our electricity comes from power plants that burn fossil fuels such as coal and oil. Those plants have three disadvantages. Not only do they generate large quantities of waste and contribute to global warming, but their fuel supplies are limited and will run out.
But what if we could make energy in just the same way as the Sun? That process is clean ¾ it doesn’t generate harmful byproducts ¾ and it has practically limitless supplies of fuel.
The Sun’s energy stems from an extremely high-pressure and high-temperature process called nuclear fusion, which occurs spontaneously in the Sun because it is so massive and hot. On our own planet, scientists inspired by solar fusion have spent decades performing experiments with extremely high temperatures and magnetic pressure to see whether they can produce similar energy on Earth. That research shows increasing promise.
What is fusion energy?
The Sun and other stars are giant balls of gas, made mostly of hydrogen and helium. Hydrogen, the lightest gas in our universe has an atomic nucleus that consists of a single proton a particle with a positive electric charge while the nucleus of helium, the second lightest gas, has two protons. Under the appropriate conditions, hydrogen nuclei can fuse together to form helium nuclei and give off a large quantity of energy in the process. However, it takes an enormous amount of pressure and extremely hot temperatures for that to happen. The reason is because the positively-charged protons of two hydrogen atoms repel each other with powerful electromagnetic force. In stars, massive gravitational pressures and million-degree temperatures provide the protons with enough force and speed to overcome this repulsion, permitting the nuclei to stick together, or “fuse.”
Introducing the LDX
Researchers have long tried to produce fusion energy here on Earth. They include a group at MIT’s Plasma Science and Fusion Center working with what they have named the Levitated Dipole Experiment (LDX). The team wants to know whether gigantic magnets can control electrically-charged gases called plasma that exist at temperatures and pressures so high that their atoms have split into their positive and negatively-charged components. Using appropriate magnetic pressure, the LDX team hopes to force the nuclei of the gas molecules to fuse and give off energy. When controlled in such a way that the reaction emits energy slowly, the process can generate electricity.
The inspiration for such an experiment comes from observing objects in our solar system. Planets such as Earth, Jupiter, Saturn, Uranus and Neptune, are surrounded by magnetic fields, or zones of magnetic forces, that trap and hold plasma. The LDX scientists, from MIT and Columbia University, figure that if they can establish a similar environment in their laboratory 3⁄4 on a much smaller scale than a planet 3⁄4 they should be able to create and control the super-hot plasma and produce fusion energy.
The LDX is unique because it uses a levitated superconducting magnet. At very low temperatures, certain substances become superconductors: negatively-charged electrons flow freely without bumping into other atoms. The LDX team, led by Dr. Jay Kesner of MIT and Dr. Michael Mauel of Columbia, uses this unique characteristic to sustain a large current in the levitated magnet for hours at a time. Floating the magnet without any leads or connectors further minimizes potential energy losses, because nothing physically interferes with the magnetic fields that contain the plasma created within the LDX.
Structure of the LDX
At first glance, the 16-foot wide, 10-foot tall outer shell of the LDX looks like a giant, three-legged metal canister. The shell, called a vacuum vessel, weighs 11.5 tons -- almost as much as two male African elephants. The vacuum vessel contains a huge metal doughnut called the “floating” coil. As large as a freight truck tire, this weighs more than half a ton (1300 pounds to be exact). The coil contains a superconducting magnet that, at extremely cold temperatures, can keep an electrical current of more than 750,000 amperes flowing through without needing further electrical input to boost itvessel contains, orNE.]]]n the ey supercondcutors:RE, TO SET UP THE RST OF TEH m -- the . Several additional layers, including a lead radiation shield and almost 100 thin sheets of insulation, cover the doughnut-shaped magnet before the final stainless steel outer encasement is laid on. The LDX team designed the floating coil and each of its components to ensure that the outer edge of the ring can withstand temperatures of millions of degrees while the inside of the ring maintains a temperature of just 4°K ¾ that’s -450°F, or 4°C above absolute zero.
Below the floating coil, the vacuum vessel contains a thick metal cylinder called the charging coil. This is another superconducting magnet that can magnetically transfer current into or out of the floating coil when it rests inside the charging coil. Above the floating coil, just under the vacuum vessel’s ceiling, a third superconducting magnet called the levitating coil magnetically supports the floating coil in the center of the vessel. Optical lasers monitor the floating coil to guard against tilting or any other undesirable changes in the magnet’s position.
One additional structure in the LDX serves to levitate the floating coil. Called the launcher, it lifts the doughnut-shaped magnet up from the charging coil to the middle of the vacuum vessel, after which it can be lowered out of the way. In an emergency -- if the floating coil falls, for example ¾ the launcher can catch the superconducting ring and prevent it from damage.
To cool the floating coil to less than 12°K ¾ the temperature of -438°F at which the magnet develops superconducting properties ¾ scientists pump liquid helium through the metal “doughnut.” At the same time they energize the charging coil to 4.4 mega-amps to create a powerful magnetic field, and then cool it down so that it becomes a superconductor and maintains that field. Electrical currents in the charging coil create magnetic forces that induce currents in the super-cold floating coil. This process generates a large current in the floating coil without requiring a physical electrical connection. The multiple layers of insulation in the floating coil allow the outside of the ring to heat up to extremely hot temperatures while the inside remains cold enough to act as a superconductor.
Once the floating coil is energized, the launcher lifts the magnet into the middle of the vacuum vessel. Meanwhile, the levitating coil is charged up to enable it to support the weight of the floating coil magnetically. When required, the launcher is lowered away. Once all the magnets are operational and in place, the LDX team releases a small amount of gas into the vacuum chamber and heats it with pulses of powerful microwaves. At the end of the experiment, the launcher lowers the floating coil back into the charging coil. The scientists remove the energy in the floating coil by transferring its current to the charging coil. Heating with warm helium gas then annuls its superconducting properties.
Instead of regular hydrogen, the LDX experiments use a variation called deuterium. Just like the hydrogen nucleus, a deuterium nucleus has only a single proton; but it also contains a neutron, a particle similar in size to a proton that has no electric charge. When two deuterium nuclei fuse together, the two protons and two neutrons combine to form a helium nucleus and give off a large quantity of energy. But whereas hydrogen nuclei combine only at billion-degree temperatures, deuterium nuclei fuse at a relatively cool 10 million degrees. The fusion reaction creates no waste or radioactive by-products. And because deuterium occurs naturally in seawater, there is a practically limitless supply available.
Creating and controlling the plasma
The LDX team first created plasma in August and September 2004. Those initial tests lasted six hours apiece. During each test the scientists lifted the superconducting floating coil into place, supported by both the levitating coil and the launcher. Ultimately, the levitating coil alone will magnetically support the floating coil. But last year’s experiments had the less ambitious goal of demonstrating that the LDX works and can create plasma during its operation. In each of the tests, the scientific team used pulses of high-powered microwaves to heat up small amounts of deuterium gas. The heating resulted in almost 50 different plasma discharges that each lasted 4-8 seconds. There are numerous aspects of this initial plasma creation that the LDX researchers are currently investigating ¾ such as temperature, shape and compressibility ¾ but eventually the team will determine conditions for creating the longer-lived plasma that is needed to stimulate a fusion reaction.
Because it provides scientists with the conditions in which they can create super-hot gases and control them with magnetic forces, the LDX allows researchers in a laboratory here on Earth to study the elements of the fusion that powers the Sun and other stars. As their next step, Dr. Kesner and his colleagues want to establish conditions where they can create and maintain plasma in the LDX without much loss of energy. Then they hope to show that the LDX can gain net energy ¾ reliably producing large quantities of fusion energy that exceed the amount of conventional energy used to power up the equipment. Ultimately, the scientists hope to apply the knowledge they gain from the LDX tests to create a nuclear fusion reactor that produces enough energy to satisfy community needs such as lighting sidewalks, heating homes, and powering computers.
Garnier, D. “Status of the LDX project” February 24, 2000 presentation at Innovative Confinement Concepts Workshop 2000, Berkeley, CA.
Garnier, D.T., A.K.Hansen, M.E. Mauel, E.E. Ortiz, A. Boxer, J. Ellsworth, O.Grulke, I. Karim, J. Kesner, J. Minervini, P. Michael, A. Zhukovsky. “Overview and Experimental Program of the Levitated Dipole Experiment” October 27, 2003 presentation at American Physical Society 45th Annual meeting, Albuquerque, NM.
Elizabeth A. Thomson, “MIT, Columbia begin new 'hot' fusion experiment “, MIT Tech Talk, Vol.49, Number 12 (December 8, 2004).