Vertical stability is one of the central fundamental challenges for tokamak plasmas. While a toroidal plasma with a perfectly circular cross-section can be prevented from moving up or down by the inclusion of a small radial magnetic field (self-generated by the plasma current!), it is often desirable to stretch (or “elongate”) tokamak plasmas by pulling on the top and bottom with additional magnetic field coils. Unfortunately, this introduces a destabilizing force, which must either be compensated for with active control algorithms or counteracted with passive conducting plates.

A cartoon of vertical stability control in tokamaks. Left to right: A plasma with a circular cross-section requires a radial magnetic field to prevent vertical drift. An elongated plasma is subject to an additional destabilizing force introduced by external field coils. Active control, or passive stabilization plates, can counter this destabilizing force.

Getting all of this physics and control correct is vital for successful operation of tokamak plasmas. If a plasma goes vertically unstable, plasma confinement be lost, potentially triggering a disastrous “disruption” event that could have the requisite energy needed to severely damage large machines. As such, it is vital that we understand the physics leading to this phenomenon so that we may develop robust engineering solutions.

In addition to developing robust routines to safely run plasmas in existing devices, I am currently working on understanding the implications of vertical stability control for several future machines, including the SPARC tokamak at Commonwealth Fusion Systems.

Selected publications on this subject:

Implications of Vertical Stability Control on the SPARC Tokamak
Nelson, A. O., Garnier, D. T., Battaglia, D. J., Paz-Soldan, C., Stewart, I., Reinke, M., Creely, A., & Wai, J., Nuclear Fusion 64, 086040 (2024).

Assessment of vertical stability for negative triangularity pilot plants
Guizzo, S., Nelson, A. O., Hansen, C., Logak, F., & Paz-Soldan C., Plasma Physics and Controlled Fusion, 66 065018 (2024).

Vertical Control of DIII-D Discharges with Strong Negative Triangularity
Nelson, A. O., Hyatt, A., Wehner, W., Welander, A., Paz-Soldan, C. & Anand, H., Plasma Physics and Controlled Fusion 65, 044002 (2023).