The U.S. Department of Energy's Princeton Plasma Physics Laboratory, which is managed by Princeton University, has completed a major first step in the upgrade of its primary research facility, the National Spherical Torus Experiment (NSTX).
“If we had a script, I couldn’t think of a better outcome.” That’s how Ron Strykowsky, head of the NSTX Upgrade, described recent results for a critical stage of the project’s construction. Riding on the outcome were months of work on the first quadrant of magnetic field conductors for the tokamak’s new center stack, which forms the heart of the $94 million upgrade.
The crucial stage called for sealing and insulating the first quadrant through a volatile process known as vacuum pressure impregnation (VPI). Preparing the nine 20 foot-long, 350-pound copper conductors for this step required the coordinated efforts of engineers and some dozen skilled technicians. The multiple tasks included soldering cooling tubes into the conductors under the direction of Steve Jurczynski, and sandblasting, priming and wrapping the units with fiberglass tape in operations led by Mike Anderson.
All these efforts were on the line during the VPI procedure. “When everything’s said and done, you must join the conductors and provide mechanical and electrical strength,” said Jim Chrzanowski, who leads the center-stack construction team. “VPI does both those things and it’s a very delicate process.”
Technicians led by Buddy Kearns began the process by injecting liquid epoxy into a carbon steel, vacuum-sealed mold that engineer Bruce Paul designed. The first quadrant lay inside. Workers gradually raised the temperature to combine the epoxy with the fiberglass tape to create a composite insulating material.
The critical moment came when the process neared 100 degrees centigrade—the temperature at which water boils and the epoxy generates heat and turns solid in what is called an exothermic reaction. The danger was that a sudden runaway reaction could cause the epoxy to burn itself up and destroy the project. Adding uncertainty was the fact that PPPL had never before used this particular epoxy. “We held our breath and were on pins and needles,” recalled engineer Steve Raftopolous.
These anxious minutes passed without incident. The now-solid epoxy cured at 100 degrees for 10-to-12 hours, and at 170 degrees for a similar period. Workers then opened the mold and examined the quadrant. “As soon as we lifted the lids off the mold everyone felt better,” said Mike Anderson.
What the team found was gratifying. The process produced uniform insulation throughout the quadrant with just a few cosmetic blemishes. And the insulation easily passed electrical tests. While the composite material needed to show a resistance of at least 1 billion ohms to keep the current in each conductor separate, the insulation measured 20 billion ohms and did so between all nine conductors. “That’s as good as it gets,” Strykowsky noted.
The near-perfect results boded well for the three remaining center-stack quadrants. “The fact that the first one came out so well is an incentive to do the other three the same way,” said Buddy Kearns. “It gives us a more comfortable feeling that we can get this done successfully.”