Fusion Dreamsby Cyrus Ance
Fusion Dreamsby Cyrus Ance
I want to do a few columns based on US Department of Energy's 20 year plan for science facilities. The DOE is a major source of funding for basic research in the US and this plan gives insight on what major science facilities will be built in the near future. Number one on the list is ITER.
There are many ways to get energy out of the stuff we find about us in the universe. Chemical energy is the most familiar and through the burning of fossil fuels is the primary way we use energy on earth. The average chemical bond in a hydrocarbon holds about 10 eV, using particle physicists units, of energy. Nuclear energy holds the prospect of a factor of 100,000 increase in the amount of energy that can be extracted from a certain amount of stuff. The average binding energy in a nucleus is a few million eV.
There are two ways to get at this nuclear binding energy. This plot shows the binding energy stored in nuclei as a function of their atomic mass. One can get energy by splitting large nuclei into smaller ones in a process called fission. This has proved less than successful as a source of power for two reasons. Safety concerns brought on by two accidents at fission generating stations ( Three Mile Island and Chernobyl) and the continuing concern about what to do with the waste products have made the cost of building new fission power stations prohibitive.
The other way is to get a clue from the ultimate source of energy on earth, the sun, and fuse two or more small nuclei together in a process called fusion. This takes advantage of the universe's most abundant elements, hydrogen and its relatives, and has the potential to release much more energy than fission as the plot shows. There are some practical difficulties that continue to prevent fusion from being realized in practice. ITER is a major effort to get around these difficulties and achieve power generation via fusion.
The first difficulty should be obvious after visiting the ITER home page. See that little person down there in the lower right? ITER is giant. There is a reason for this. To achieve fusion hydrogen atoms have to be warmed to very high temperatures so that the nuclei can have enough heat energy to get close enough to fuse before being pushed away by electrical repulsion from the similarly positively charged nucleus. This is done by pumping electricity into a cloud of hydrogen. Heat energy is lost at the edges of the cloud thus one wants to maximize the size of the center of the cloud to minimize heat loss. A big volume is better.
Also the nuclei have to be dense enough that the chances of one nucleus encountering another before the heat energy is lost are high. This is made easier because the hydrogen gas is broken into electrons and nuclei when the electrical energy is pumped in, forming a plasma, which is a gas made of electrically charged constituents. Electrically charged things can be pushed around with magnetic fields, and intense magnetic fields generated by superconducting magnets are used to confine the plasma in ITER. Thus right next to the superheated plasma are supercold magnet coils.
Finally once fusion reactions start to take place the plasma has to be confined, again by magnetic fields, and the heat energy of the fusion reaction extracted. This is all done in a Tokamak which is a big donut and a convenient shape around which to have a confining magnetic field. In its final phase ITER is projected to be able to produce 500 MegaWatts of power in a steady state. This is small on the scale of power stations, which are typically thousands of MegaWatts, but ITER is meant as the prototype for a reliable, commercial power generating station.
There are some unpleasant details. The fuel will not be cheap, abundant, safe hydrogen but rather expensive, rare, radioactive isotopes of hydrogen, deuterium and tritium. These are used because they fuse at lower temperatures than pure hydrogen. Tritium is also the primary component in hydrogen bombs. ITER was conceived in the 1980's as a worldwide cooperative science venture. Wrangling among the international partners over design, cost, and the site have been a feature from day one. The US pulled out of ITER from 1999 to 2003 due to dissatisfaction with how the project was going. The wrangling continues as a site has yet to be agreed on; the choice is between France and Japan. Already some $650 million have been spent to produce a design, and the project is expected to cost $10 billion over its life. Plasma will not appear in ITER until ten years after a site is chosen and construction begins. Let us say this will happen in 2015. Three to five years after that ITER will be filled with deuterium and tritium and begin producing rather than consuming power. It would be at least another decade after that before fusion would be used to produce power for consumers in a commercial fusion reactor. Generously say this will be in 2030.
Not to be pessimistic but I feel compelled to point out that twenty years ago when I was a graduate student in physics at an institution that had a major plasma physics program I was told that the time scale for the commercial generation of fusion power was 20-30 years in the future. Today, twenty years later, ITER's proponents claim exactly the same time scale, 20-30 years in the future, for the commercial generation of fusion power.