Fusion joins atomic nuclei (while fission splits the nucleus) to release huge amounts of energy. The recent design depends on coils wound with a precision that requires frequent checking by coordinate-measuring machines (CMMs) during production.
The Princeton Plasma Physics Laboratory (PPPL), Princeton, N.J. (www.pppl.gov), is working with the Oak Ridge National Laboratory, Oak Ridge, Tenn., to build the National Compact Stellarator Experiment (NCSX) and will have a fully assembled machine ready for testing by July 2009. Team members are using two portable CMMs from Romer CimCore and PowerInspect software from Delcam Inc. to perform the measurements required to place several thousand pounds of strandedcopper wire that make up NCSX's magnet conductors.
Nuclear fusion occurs in hot, ionized gas called plasma (also called the fourth state of matter). Most fusion devices to date have used toroidal (doughnut-shaped) vacuum vessels and flat magnetic coils. But these simple magnet geometries yielded plasmas that were stable for only fractions of a second. In contrast, plasma in the NCSX will be magnetically confined within a helically twisted toroidal-vacuum vessel and controlled with cryogenically cooled magnets. NCSX conductors will follow complex paths to generate and maintain more-stable plasma.
"The CMMs must make thousands of dimensional measurements of the flexible copper conductor, while 10 or 11 layers are being wound, depending on the coil, on perhaps the most strangely shaped mandrel ever made. The portable probes can reach into the mandrel's severely restricted spaces," says Steve Raftopoulos, project engineer in charge of metrology.
The 18 individual mandrels on which the copper conductor is wound are frequently characterizedas twisted and distorted raceways. Conductors are placed in side-by-side raceways around the inside of each mandrel. With all the mandrel's twists and turns, each raceway is nearly 22-ft long. When energized, the magnet system will generate enough force to lift a railroad locomotive.
The 18 machined castings, made from an alloy with properties similar to AISI Grade 316 stainless steel, make up the NCSX outer casing and provide structural support for the entire machine. The NCSX will be more than 30 ft in diameter and stand almost that high. Each shell-andraceway casting weighs about 7,500 lb.
"Because much of this is still experimental, it calls for a lot of dimensional measurements," explains Raftopoulos. "In building components so sensitive to deviations from the ideal, measurement and verification is both crucial and almost endless."
After coils are wound, PPPL uses the CMMs and PowerInspect software for feedback in final positioning and adjustments of the conductor.
"We can push the conductor bundles as much as 3/16 of an inch, though it's rare we have to do that," says Raftopoulos. "For analysis, the software report is dumped to a spreadsheet. This data guides the coil-winding team in loosening the clamps and tweaking the alignment of the conductor layers, helping us get the coil in the right place with minimal manipulation."
Images courtesy of Princeton Plasma Physics Laboratory.