US-based Holtec International has completed the fabrication and delivery of a critical component for the Princeton Plasma Physics Laboratory’s (PPPL’s) fusion research. The Centre Stack Casing (CSC) is a key component of PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U).

The US Department of Energy (DOE) notes that a fusion reactor has to produce high performance plasma with low-cost magnetic fields. As a result, the CSC must be designed and fabricated with exceedingly tight tolerances to meet exacting metrology and magnetic permeability requirements. The CSC will provide the inner vacuum wall of the NSTX and structural support for plasma-facing components and coils. These will have to withstand a 10-million-degree Celsius thermal environment.

The challenging design meant that Holtec’s weld engineers needed to develop and qualify many unique welding procedures and tools to meet PPPL’s requirements. These were rigorously implemented by Holtec’s machinists and welders qualified and certified under DOE’s Quality Assurance Programme.

Professor Steve Cowley, Lab Director at PPPL, noted: “This beautiful piece of inconel metal – which I fondly call ‘the hedgehog’ due to its stud-covered design – will hold the centre magnet of our flagship experiment, NSTX-U.” He added: “The casing is not only magnificent, but well-travelled: It was forged in Italy, machined in New Jersey, and assembled in Pittsburgh. Its final arrival to us at the Lab was a significant project milestone and will allow us to begin important recovery work.”

This work depended on the close partnership between PPPL and Oak Ridge Technologies, a joint venture between Holtec International and Gilmartin Engineering Works. This enabled the final design of this component to be matured iteratively. “To fabricate something of this calibre required expertise and weldments of the highest quality,” said Brian Farnsworth, Holtec Director of Weld Technology. “I am immensely proud of our team’s dedication and diligence to produce this critical component which in our initial review appeared to be almost unachievable.”

The NSTX-U is the flagship fusion facility at PPPL. The spherical device is shaped more like a cored apple than the doughnut-like shape of conventional tokamaks. It can produce high-pressure plasmas with relatively low and cost-effective magnetic fields. PPPL said this capability makes its compact spherical design a candidate to serve as the model for a fusion pilot plant followed by a commercial fusion reactor. NSTX-U has three main objectives:

  • To explore the capability of the spherical facility to produce stable, high-performance plasmas with low-cost magnetic fields.
  • To develop the understanding and tools required to start-up and sustain such plasmas non-inductively, meaning without what is known as a “solenoid” magnet to start the process.
  • To develop techniques to handle and control the waste heat from fusion reactions.

Image: The Centre Stack Casing of the NSTX-U (courtesy of Holtec)