Metrology

When assembling a puzzle, the pieces should only fit together as they were cut. What if this puzzle had another variable that determined its assembly? The National Spherical Torus Experiment - Upgrade (NSTX-U) is in many ways, like a puzzle. It too, is intended to be assembled in accordance with how the parts were built. However, it has additional variables in its assembly, one of those being symmetry, with respect to its magnetic fields. To aid in achieving this symmetry, and overall improved performance, metrology is used to improve the alignment between key NSTX-U components: The Toroidal Field/ Ohmic Heating (TF/OH) bundle (also known as the Centerstack), the Centerstack (CS) casing, the Vacuum Vessel (VV) and the Poloidal Field (PF) Coils. 

The TF/OH Bundle

If NSTX-U were the Earth, the axis of the Centerstack would be the axis about which it spins, and the center of the Earth would be at the center of that axis. The axis of the TF/OH was defined with the use of metrology software and equipment. From this axis, its center was derived. This will serve as the axis for the entire experiment, and the centerpoint will serve as the coordinate system for referencing the location of other NSTX-U components. This metrology was performed with the Centerstack outside of NSTX-U.

The CS Casing

The CS Casing is a protective armor for the Centerstack, and thus resides within it when NSTX-U is assembled. This armor is already built, but it is important to ensure proper fit between these components. To do this using metrology, an axis, a centerpoint, and a best fit inner diameter were also established for the CS Casing. When the axis and centerpoint of the CS Casing are aligned with the axis and centerpoint of the Centerstack, it is possible to determine whether the best fit outer diameter of the Centerstack fits within the inner diameter of the CS Casing, while also allowing for enough space for diagnostics and insulation.

The Trial Fit

Through the use of metrology, it was shown that the Centerstack and CS Casing could be assembled together. But it had to be proven experimentally, in a Trial Fit of these NSTX-U components. The experimental results matched the analytical expectations.

PF Coils

If NSTX-U were the Earth, then the PF Coils would be its lines of Latitude (the PF 4/5 Coils being closer to the Equator), and the Vacuum Vessel would be the Earth’s atmosphere. Metrology on the Centerstack and CS Casing were done by removing both components from the “atmosphere.” Metrology on the upper and lower PF 4/5 Coils are done while they are still connected to the vacuum vessel. The challenges with measuring them in-place include: recreating the Centerstack axis and centerpoint in the vacuum vessel to reference the coils; acquiring measurements of the coils in a congested Test Cell; and creating a network of permanent measurement points from which to continuously reenter NSTX-U’s coordinate system. 

In this analogy, the PF1A Coils are closer to the North and South poles. Their designs were modified following the last NSTX-U run. In addition to the coil shop at PPPL, the prototype designs were provided to three external shops. The completed prototypes were then inspected, using metrology software and equipment, for adherence to dimensions and tolerances in the prototype fabrication drawings.