|MadSci Network: Physics|
Alison: I hope the following helps you better understand how the high temperatures needed for fusion are generated. It is a bit lengthy, but your question does not have a simple answer. Since you also mentioned that what you have found about Fusion has so far been a bit confusing, I wanted to (hopefully) give you some background about Nuclear Fusion so that my discussion of high temperatures would make a bit more sense. First, however, I want to clear up what may be a misunderstanding based on your question. Hydrogen is used in two very different ways to generate energy. The first method is by using a device called a Fuel Cell (or sometimes a Hydrogen Fuel Cell), where Hydrogen atoms breaks away from nearby atoms (but leave behind their electrons), passes through a specially made wall (called a semi-permeable membrane), and chemically reacts with Oxygen to form water. The electrons cannot pass through this semi-permeable membrane, so the result is a container with positive charges on one side of the semi-permeable membrane, and negative charges on the other. The Fuel Cell is now just like any other battery, and connecting a wire between two halves of the cell will result in the flow of electric charge, which can run a motor, light up a room, or do any other kind of job that a battery can do. The second method of energy generation which uses Hydrogen is known as Fusion (it can also be called Hydrogen Fusion or Nuclear Fusion). Unlike the Fuel Cell, which uses chemical reactions to generate electricity, Hydrogen Fusion relies on nuclear reactions to generate energy: the nuclei of two atoms actually stick together (in a chemical reaction, the nuclei never get anywhere near each other). After a Hydrogen Fusion reaction, there is no Hydrogen left! It's actually turned into another element: Helium. To be clear, then, neither of the two methods of energy generation actually produce Hydrogen. We have to use the Hydrogen that is already available to us. In fact, most Fuel Cells actually run on fossil fuels like natural gas or even the gasoline that cars use. Some Fuel Cells can run on pure Hydrogen, which can be extracted from water (H2O). Unfortunately, it takes energy to break apart water, and the Laws of Thermodynamics say that we will always lose some of the energy when we break apart water, and then reform it while trying to do something useful with the energy we've liberated. The main interest in Hydrogen Fuel Cells is that they do not pollute at all since only water is produce in the reaction. Now, to return to your original question - how are the high temperatures needed to create Hydrogen Fusion generated? The nucleus of most Hydrogen atoms consist of just a single proton. One out of every 100,000 atoms of Hydrogen has a neutron attached to the proton. Scientists call nuclei with the same number of protons but different numbers of neutrons "isotopes," and Hydrogen has three of them. Normal Hydrogen has only the single proton. Deuterium is the name we give to Hydrogen when it has one neutron stuck onto the proton, and Tritium is Hydrogen with two neutrons stuck onto the single proton (Tritium is also radioactive,and turns into Deuterium when it loses one of its neutrons ). Nuclear Fusion occurs when two nuclei come close enough for a nuclear reaction to occur. The result of a Fusion nuclear reaction is usually one or two new nuclei and energy. Most experimental Hydrogen Fusion reactions combine Deuterium and Tritium together. The result of this nuclear reaction is the formation of a Helium nucleus (2 protons and 2 neutrons) and one extra stray neutron. If you were to weigh the Deuterium and Tritium nuclei before they react, then weight the Helium nucleus and the neutron after the reaction, you would find that some mass is "missing" (less than 1% of the total mass of the original two nuclei is "missing"). It is this "missing" mass which is turned into the energy we are interested in, and the amount of energy can be calculated from Einstein's famous formula, E=mc^2 (where m is the "missing" mass). Note that although only a very small amount of matter is converted into energy, the amount of energy we get is very large since we are multiplying a small number by a much larger number (the speed of light squared). The biggest problem with Hydrogen Fusion is that the two nuclei have to get very close to each other in order to for a nuclear reaction to occur. However, both nuclei are positively charged, and we know that like charges repel (and the closer the two nuclei get, the more they repel each other). There are two ways to get the two nuclei close to each other - squeeze them really hard or throw them at each other very fast (or, of course, a combination of the two). This is where the temperature becomes important, since the temperature of an atom is related to how fast the atom is moving (there are a number of different ways to define temperature, this one is known as the kinetic temperature and is the most important one for Hydrogen Fusion). Unlike Hydrogen Fuel Cells, which are now used in a variety of applications (NASA's Space Shuttle uses them), Hydrogen Fusion reactors are still in the experimental stage, and there are two competing methods now being investigated. The first is called Plasma Fusion and the second is called Inertial Confinement Fusion (since these methods use Hydrogen Nuclear Fusion, the words Hydrogen and Nuclear are usually left off so that the names don't get too cumbersome). A Plasma is a gas made up of atoms which have had one or more of their electrons removed (this is called an ion). Since every isotope of Hydrogen has only a single electron, a Hydrogen Plasma is a gas made up of free electrons and the positively charged nucleus (proton, proton+neutron, or proton+2 neutrons depending on whether the isotope is Hydrogen, Deuterium, or Tritium). Some sort of container is needed to keep the Plasma from escaping, but the cool container walls make it very difficult to keep a Plasma hot (place your hand near a window on a cold morning, and you'll notice the air temperature is much cooler near the window than in the center of the room). Scientists use a neat trick to keep the Plasma away from the cold metal walls: magnets! Charged particles do not travel in straight lines when they encounter a magnetic field. Instead, they spiral around the magnetic field lines. One way of picturing this motion is to partially fill a bathroom sink with water and float some tiny scraps of paper on the surface. Then unplug the sink and hold a pencil right above the drain so it points straight out. This pencil represents a line of magnetic force and the paper represents the Plasma. As the sink drains, a funnel forms, and as the paper gets closer to the funnel it moves faster and faster around the pencil while at the same time getting closer to the bottom of the sink. In a Plasma, the ions and electrons are trying to move "down" the drain, but travel around in circles at the same time. And as the field gets stronger (closer to the drain), the Plasma moves faster. However, unlike the water and paper, most of the Plasma actually gets reflected back (think of the bubbles created at the throat of the drain which float back out of the drain into the sink). If we are clever with our magnets, we can create a "magnetic bottle" out of which the Plasma cannot escape. But unlike water in a water bottle, the Plasma in a magnetic bottle is constantly bouncing back and forth inside the bottle. Occasionally the ions hit each other, and if they hit hard enough, the two colliding nuclei will get close enough for a nuclear reaction to occur. How do we get the ions to move fast enough? This is your original question about how we generate high temperatures, but re-stated in terms of high speed instead of high temperature. For Plasma Fusion, we can shoot very fast electrons into the magnetic bottle (your television tube is shooting somewhat fast electrons onto a chemical painted onto the glass which emits light when hit by an electron, so it's pretty easy to create a device to create fast electrons). These fast electrons hit the Deuterium and Tritium ions, causing them to speed up. Radio waves are a form of electromagnetic radiation (a blend of electric and magnetic fields which oscillate over time). Since the ions are charged particles, they can interact with the oscillating electric waves from a strong radio wave transmitter. The ions pick up speed by "surfing" the electric field waves. In either case, the speed of the ions is increased enough so that a nuclear reaction occurs when a Deuterium ion hits a Tritium ion (D-D and T-T collisions need much higher temperatures (or speeds) before nuclear reactions occur). This is a very simplified description of Plasma Fusion, and the details I've left out are the ones which make bottling and heating the Plasma very difficult in practice. Nature likes to tempt us with possibilities, then tease us by hiding the important details. Scientists always love a challenge, and one of our greatest joys is when we uncover one of Nature's hidden secrets. The second method of Hydrogen Fusion, Inertial Confinement Fusion, uses a very different method to get the Deuterium and Tritium nuclei close enough to fuse. It starts not with a hot ionized Plasma, but with a small sphere of solid Deuterium and Tritium encased in a thin metal shell. This small fuel pellet has a diameter of about 5 mm and has a starting temperature of only a few degrees above absolute zero! It seems odd, but in order to create the very high temperatures needed for Hydrogen Fusion we must first start off the the coldest Hydrogen we can create. Is it necessary for the Hydrogen to be cold? Not really. However, it is necessary for the Hydrogen to be solid, and on the surface of the Earth, solid Hydrogen can exist only at temperatures slightly above absolute zero. So how does this work? We take this very cold fuel pellet and throw it into the center of a vacuum chamber. This central point is also the focal point of many high energy lasers. As soon as the pellet reaches the center of the vacuum chamber the lasers turn on, and the outer shell of metal on the pellet absorbs the energy almost instantaneously. This causes the metal surface to literally explode. The shock wave from the explosion travels both outwards and, more importantly, inwards. This inward travelling shock wave compresses and heats the solid mixture of Deuterium and Tritium, which causes the nuclei to get close enough to each other for nuclear reactions to occur (as I mentioned before, a nuclear reaction can occur even at low temperatures if we squeeze the two nuclei together hard enough). The trick here is that the shock wave compresses and heats the atoms before they have a chance to move away from each other. Inertia is the property of matter which resists changes in motion (Newton's First Law: objects resist acceleration). Since the lasers are hitting the pellet from all angles, the pellet is uniformly compressed before the atoms have a chance to react to their newly acquired temperature - the atoms are confined by their inertia, hence the name "Inertial Confinement" Fusion. I hope that my discussion has helped you better understand Hydrogen Fusion. It's a fascinating subject which has the potential of solving most of the world's energy supply problems, although it will probably be at least twenty years before the first commercial Fusion reactors are built. Scientists have made great strides in developing the science and technology behind both Plasma Fusion and Inertia Confinement Fusion, but much more experimental and theoretical work is needed. For example, although Hydrogen Fusion releases a tremendous amount of energy, the current methods for capturing this energy and converting it into a more useful form are very inefficient (for example, using the energy to turn water into steam, which then can be used to power an electric generator). It will likely be the next generation of scientists (that's you, by the way) who will ultimately solve these problems. Links: http://www.pppl.gov/fusion_basics/pages/fusion_basics.html Plasma fusion basics, Princeton University http://ippex.pppl.gov/fusion/default.htm Interactive Plasma Physics Education Experience (plasma fusion info) http://fusion.gat.com/icf/ General Atomics http://www.nuc.berkeley.edu/thyd/icf/IFE.html National Ignition Facility, University of Californa, Berkeley http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/finert.html Links to different ways to heat ICF pellets
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