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Hole in one: Technicians smoothly install the center stack in the NSTX-U vacuum vessel

With near-surgical precision, PPPL technicians hoisted the 29,000-pound center stack for the National Spherical Torus Experiment-Upgrade (NSTX-U) over a 20-foot wall and lowered it into the vacuum vessel of the fusion facility. The smooth operation on Oct. 24 capped more than two years of construction of the center stack, which houses the bundle of magnetic coils that form the heart of the $94 million upgrade.

“This was really a watershed moment,” said Mike Williams, the head of engineering and infrastructure at PPPL and associate director of the Laboratory. “The critical path [or key sequence of steps for the upgrade] was fabrication of the magnets, and that has now been done.”

The lift team conducted the final steps largely in silence, attaching the bundled coils in their casing to an overhead crane and guiding the 21 foot-long center stack into place. The clearances were tiny: The bottom of the casing passed just inches over the shielding wall and the top of the vacuum vessel. Inserting the center stack into the vessel was like threading a needle, since the clearance at the opening was only about an inch. Guidance came chiefly from hand signals, with some radio communication at the end.

Key features

The installation merged three key features of the upgrade that had been developed separately. These included the casing, the bundled coils and the work to ready the vacuum vessel for the center stack. Slipping the casing over the bundle was a highly precise task, with the space between them less than an inch. “The key word is ‘fit-up,’” said Ron Strykowsky, who heads the upgrade project. “We had a robust-enough design to handle all the very fine tolerances.”

Installation of the center stack completed a key portion of the upgrade and opened another chapter. “For me, the burden is off our shoulders,” said Jim Chrzanowski, who led the coil project and retired on Oct. 31 after 39 years at PPPL. “We’ve delivered the center stack and are happy,” added Steve Raftopoulos, who worked alongside Chrzanowski and succeeds him as head of coil building. “This is my baby now,” said Raftopoulos, noting that he will be called on to resolve any problems that occur once the center stack is in operation.

Praise for technicians

The two leaders praised the many technicians who made the center stack possible. They ranged from a core of roughly a dozen workers who had been with the project from the beginning to technicians throughout the Laboratory who were called on to pitch in. “We drafted everyone,” Chrzanowski said.

Their tasks included sanding, welding and applying insulation tape to each of the 36 copper conductors that went into the center stack, and sealing them all together through multiple applications of vacuum pressure impregnation — a potentially volatile process. Next came fabrication and winding of the ohmic heating coil that wraps around the conductors to put current into the hot, charged plasma that fuels fusion reactions.

“Everyone who worked on this feels a lot of pride and ownership,” Raftopoulos said. “Steve and I were the conductors, but the technicians were the orchestra,” Chrzanowski said. “We’ve got to give credit to the guys who actually build the machines. They take our problems and make them go away.”

Completion of the upgrade now rests with technicians working under engineer Erik Perry.  Their jobs include connecting the center stack to the facility’s outer coils to complete a donut-shape magnetic field that will be used to confine the plasma. The work calls for installation of layers of custom-made electrical equipment plus hoses for water to cool the coils, all of which must fit around diagnostic and other equipment. Also ahead lies the task of connecting a second neutral beam injector for heating the plasma to the vacuum vessel. “It’s like a big puzzle,” said Perry. “Everything must fit together, and that’s what we excel at.”




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Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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