A new bottle for a sun
Fusion researchers at the College of Engineering are celebrating the achievement of a long-sought milestone in the construction of a unique "magnetic bottle" called a stellarator, capable of holding the same ingredients and reactions that power the stars. At 5:15 p.m. on August 31 1999, the crew of the HSX Plasma Laboratory made first plasma in their new stellarator. After eight years, more than 100,000 man-hours, $7.5 million, and an incredibly complex design and fabrication process, the 14-member team has reason to celebrate.
"It's the only helically symmetric stellarator in the word," says David T. Anderson, the project's principal investigator. "It's nice when you can make a plasma on the first try."
Stellarators are a type of toroidal or donut shaped device used to contain plasmas. HSX has a unique screw shape that is expected to better confine plasma and bridge the gap between other types of experimental magnetic-confinement fusion devices. The device is also unique among other stellarators in that it was designed completely on computer. Anderson says HSX was designed from the inside out. The optimal shape of the plasma was determined first. The stellarator was built to fit, resulting in the spiral-shaped plasma chamber.
Plasma is the fourth state of matter, hot, ionized gas. Fusion plasma research has the goal of making a miniature star by heating the same naturally occurring heavy isotopes of hydrogen found in water to ignition.
Fusion occurs when the nuclei of lighter elements such as hydrogen, are fused together at extremely high temperatures and pressures to form heavier elements such as helium. Fusion plasmas are 100,000 times less dense than air, and cool quickly if they touch the wall of the vacuum chamber. Thus, fusion plasmas must be confined in devices that do not use material walls.
Since plasma particles are charged, conduct electricity, and can be constrained magnetically, doughnut-shaped (toroidal) "magnetic-bottles" are used for this purpose. In the presence of magnetic fields, charged particles orbit around the magnetic field lines. They can generally travel only parallel to those lines. With the proper orientation of the magnetic lines of force, the particles can be prevented from touching material walls.
Stellarators confine plasma by means of currents flowing in several types of magnet coils that wrap around the torus. Each stellarator weighs about three tons and is about seven feet in diameter and six feet high. The magnets are powered by 12 locomotive generators, each with a 1.5 ton flywheel. Over a period of about five minutes, power is pulled from the utility grid spinning the flywheels up to 2,000 r.p.m. Then in under three seconds, the flywheels come to a stop as they transfer their energy through the generators to the magnets. At this point, plasma is injected, heated, usually with electromagnetic radiation, and its density, temperature, and confinement time are measured. In doing this researchers study how the plasma acts under the influence of the confining and heating fields and determine what must be done to advance towards the goal of fusion.