Date: Thu, 16 Jan 1997 11:41:12 -0600 (CST) From: "Peter H. Brenton" Subject: Science of Fusion Power Plants (long, gearhead oriented) Some things I learned at the MIT Plasma Sciences & Fusion Center's tour yestarday. Tokamak (toroidal confinement; i.e. doughnut shaped) Fusion Reactors are expected to be the shape which will finally produce a sustained fusion reaction. The inside chamber is lined (at TL8-9) with Molybdenum tiles (just like the underside of the space shuttle) or, sometimes, Graphite tiles. Temperature inside the reactor must reach 100 million degrees centirade to trigger fusion. Pressure is (only!) about 10 atmospheres. (this confuses me since the confinement chamber is stressed to such high levels) The "easiest" reaction uses Deuterium and Tritium in a 50-50 mix. Deuterium is easily obtained, but Tritium must be made and has a 12 year half life. The "dream" of fusion is to make a reaction with only deuterium, which is theoretically possible. Fuel may be supplied either by "puffing" (deuterium) into the chamber in the center, or by "firing" frozen deuterium pellets into the plasma from the outside. Plasma is simply matter which has had the electrons stripped away from the nuclei of the atoms. This usually requires high temperatures (or generates high temperatures) and subjects the matter involved to control by magnetic forces. Magnetohydrodynamics (MHD) is the theory of how that matter will behave when subjected to magnetic and electrical forces of various types. In addition to three different magnetic polarizing fields, a tokamak also sets up an electrical current within the plasma, such that the plasma acts as a "secondary" for a transformer. I think this is identical to the way an electric motor works; the outer windings of coil have a current supplied to them which sets up an induction to the windings around the inner coil to cause the inner coil to rotate. In the Tokamak, the plasma acts as the inner coil. So there is a "flow" of plasma (quite fast too) going around the center of the tokamak. you might say that there is a current (in the river sense) caused by a current (in the electrical sense). When fusing, Two type of particles are emitted; Alpha particles and electrons. The electrons will remain confined and circulate back into the plasma (along with most of their energy) the alpha particles are what we need to capture energy from. Most current designs put a steam turbine in circuit where the water is heated by the alpha particles to create electricity. The alphas are then recycled by passing through a lithium "blanket" which is wrapped around the outside of the reactor (inside the magnets).[I am unclear what the lithium does with the alphas at this point] The net energy of the power plant is released in the form of the kinetic energy associated with the electrons and alphas. This can be captured by harnessing the heat in these particles, or by some other means. To take a science fiction tack, it would be useful for the alphas to somehow be converted directly to electricity, without a messy steam turbine in the way. Perhaps an "alpha absorbtion layer" which uses damper technology to "resist" the alphas and convert their kinetic energy into electriicity (negatively charged particles, in other words) [help me, I'm handwaving and can't get up!] The plasma cross section is not perfectly circular. Rather the circle is elongated in the up-down direction and flatter on the inside of the tokamak (towards the center) than on the outside. The shape of the plasma ring is a major theoretical area of study; future tokamaks are even a bit more elongated than those running now. A cross section of plasma as it currently exists can be found on the PSFC Web Site. Heating the plasma is accomplished in a number of ways. The electrical current that is set up by induction causes a geat deal of the initial heating (20 million degrees C I think he said) . To boost the temp up to 100 million requires the use of other methods; RF heating uses radio frequency antennas ("at just below the frequency used by WGBH in Boston") at about 80 Mhz to energize tha particles. Some newer reactors use laser or particle beams to energize the plasma, thereby heating it (CT method is using lasers). Divertor physics is another major field of exploration. in the newest reactors a "divertor" chamber is at the bottom of the torus all the way around. The magnetic field is shaped to "catch" all the escaped particles (and therefore heat) and divert them into this chamber to be cooled harmlessly, or receyled back. The shape of the divertor field, and the physical shape of the divertor is a matter of intense study. Turbulence in the plasma is one of the major stopping points of the fusion reactor. Chaos theory arose, in part, to study this phenomenon. Princeton has (I think) a good picture on their web site of turbulent plasma (simulated by computer and colored). The speaker (my boss) equated making fusion to a balancing act, where forces are kept carefully balanced and active feedback systems constantly monitor the adjustments that need to be made many times per second. An unstable plasma will not only collapse, it will do so in a matter of 1-5 milliseconds, at which point you'll have a jug full of hot gasses. Plasma does not emit light. Pictures and films of the plasma showed only the edge of the plasma where cooler gasses were escaping. The plasma itself is invisible. some components of a Fusion Reactor; Vacuum Chamber Vacuum Pump RF antennas Pulse Flywheel (or fast discharge electric capacitor). Molybdenum Tiles (or Lanthanum (according to CT) at higher TL, Graphite at lower). High Power Electromagnets (several winding the chamber in different directions). Deuterium (or just hydrogen) pellet injectors. Numerous POwer Supplies Large Electrical transformer Special Diagnostic Probes, Sampling Probes (all made of Molybdenum or Lanthanum) Lithium Blanket Computers, Computers, and more computers Alpha particle absorbtion device (that makes electricity) Cooling tubes, coolant recirculators (we use Liquid Nitrogen btw) Coolant storage (Ignore this if you use Liquid Hydrogen as both fuel and coolant) Neutron Radiation Shielding Electric Transformers for storing, absorbing, and distributing electricity Heavy lift equipment for disassembly and maintanence of the reactor Numerous monitors, readouts, gauges, idiot lights, flashing operations lights etc. One thing that came to me is that a standard Fusion Reactor as installed should probably be considered two or three reactors. The reactor here needs to be taken apart and reassembled on a regular basis; this could be considered inconvenient (to say the least) if we depended on it for propulsion, life support, etc. Another issue is damage. If the shape of the plasma varies by a very small amount, it will collapse. So any hit to the Power Plant on a starship will shut down that power plant immediately. If you have three PPs that means you're available power is cut by 1/3, but if you have only one... I hope this proves useful to some of you out there. Pete ------------------------------