DOE Pulse
  • Number 399  |
  • October 14, 2013

Together at last: PPPL binds the heart of the NSTX-U into a complete whole

PPPL technicians lower the fiberglass-wrapped bundle of 36 magnetic field conductors into the VPI mold.

PPPL technicians lower the fiberglass-wrapped
bundle of 36 magnetic field conductors into the
VPI mold.

After some 18 months of painstaking preparation, a team of engineers and technicians has bound all 36 magnetic field conductors for the center stack of the National Spherical Torus Experiment Upgrade (NSTX-U) into a unified whole. Team members lifted the lid of the mold for the latest crucial stage of the process and found that the task of combining all four quadrants of 20-foot-long conductors into one nearly 20,000-pound cylindrical bundle had gone smoothly as planned.

The bundle lies at the heart of the upgrade, which will double the power of the NSTX-U when the overhaul is completed in fall, 2014. The increased power will enable PPPL scientists to investigate the behavior of plasma, the hot, electrically charged gas that fuels fusion reactions, under higher temperature and magnetic field conditions, and for longer periods of time, than previously had been possible.

Final step

The final step of the bundle’s construction provoked some anxiety as workers prepared to open the mold used to join the quadrants of fiberglass-wrapped copper conductors through what could be a volatile process known as vacuum pressure impregnation (VPI). The procedure called for workers led by technician Buddy Kearns to inject liquid epoxy into the mold and gradually heat it to 170 degrees centigrade — or 330 degrees Fahrenheit — to seal and insulate the conductors. 

The results could be unpredictable. “There are two main causes of concern in this process,” said engineer Steve Raftopolous, who co-directed the overall construction of the bundle with Jim Chrzanowski, the center stack lead engineer. “You want the mold to be fully filled and you want the epoxy to make it through the VPI without leaking or overheating.”

The VPI took a week as technicians raised the temperature in a carefully controlled sequence designed to ensure that the epoxy cured to full strength while minimizing the risk of mishaps. “Patience is a virtue in this type of work,” said Chrzanowski. “There’s been such an investment in the bundle up to this point that you want to make sure that nothing goes wrong.”

Plenty of practice

His team of engineers and some dozen technicians has had plenty of practice. Each of the four quadrants that make up the bundle underwent its own VPI process. “You can never take these VPIs for granted,” Chrzanowski said. “Each one has its own personality.”

Completing these tasks has been a major achievement. “One reason this went so well is that we benefitted from lessons learned from the last four VPIs,” Ron Strykowsky, head of the overall NSTX-U project, said of the bundle operation. “This was a true team effort."

Strykowsky pointed with pride to an outside evaluation of PPPL’s fabrication of the magnetic conductors. The report from the National High Magnet Field Laboratory in Florida found that PPPL “is using the latest best business, technical and quality practices, and that the team is eager and capable of finishing the project within the present schedule predictions.”

“Of course, we knew this all the time,” Strykowsky said, “but it’s nice to hear our peers recognize the talents and dedication of our people.”

Construction of the NSTX-U center stack now heads into the home stretch. Still another VPI process will take place early next year when technicians wind an ohmic heating coil — a part that will add heat and put an electric current into the plasma — around the bundle of conductors. 

Technicians working under the supervision of NSTX-U construction manager Erik Perry will finish assembly of the center stack by installing the bundle into a case made of inconel — a nickel-chromium-based alloy more heat-resistant than stainless steel.  They will then set the stack inside the NSTX-U and connect the unit to water lines, vacuum seals and external magnetic coils in preparation for the start of plasma experiments late next year.

[John Greenwald, 609.243.2672,]