Currents Always Find the Fastest Detour

The Science

In fusion experiments, disruptions of the superhot material, or plasma, can induce large currents in the vacuum vessel wall and surrounding support structures. These currents can transfer between the plasma and vessel wherever plasma comes into contact with the wall. Researchers have mapped these shared current paths using hundreds of magnetic field sensors while avoiding ambiguities encountered in other experiments. For the first time, scientists have directly measured the relationship between helical plasma displacements and toroidal currents flowing through the wall. Wall currents bypass electrical breaks in the wall by transiting through the plasma edge.

The Impact

Currents conducting through the walls of a tokamak, a bagel-shaped design that confines superheated plasmas with magnetic fields, interact with local magnetic fields. The interaction produces forces that may be large enough to damage the walls of fusion experiments and future fusion reactors. In a reactor, these forces will be the same magnitude as the thrust from a Saturn V rocket. Accurately predicting these currents and their 3-D paths is vital for designing reactor walls, developing plasma control strategies, and ensuring safe shutdown procedures. With these direct measurements, scientists now have additional information to validate physics models used in the design of fusion reactors.

Summary

The walls of fusion energy experiments, such as ITER, must be carefully designed to handle large electromagnetic forces. During the abrupt termination of discharges (called "disruptions"), plasma motion and dissipation generate large currents at the plasma boundary, provide paths for current to flow from the plasma to the wall, and cause large electromagnetic forces. The current path at the interface between the plasma and the first wall is under debate among plasma physicists, and experiments are needed to distinguish between various theories and models.

Columbia University scientists and students are now taking a detailed look at how 3-D geometries of the wall, plasma, and magnetic fields affect the currents. Insulating quartz breaks in a unique vacuum vessel have allowed scientists to make or break major electrical connections around the device, revealing how induced currents can thwart attempts at redirecting them during disruptions. When an electrical connection is broken to prevent a current path, the plasma allows current to flow anyway by making a short circuit around the break. Hundreds of magnetic sensors close to the plasma surface now provide comprehensive measurement of the relationship between helical plasma distortions and plasma-wall currents, and show that leading competing theoretical treatments must be combined to explain the full disruption evolution.

Funding

This work was supported by U.S. Department of Energy, Office of Science, Office of Fusion Energy Science, under grant DE-FG02-86ER53222.

Publications

J.P. Levesque, J.W. Brooks, M.C. Abler, J. Bialek, P.J. Byrne, C.J. Hansen, P.E. Hughes, M.E. Mauel, G.A. Navratil, and D.J. Rhodes, “Measurement of scrape-off-layer current dynamics during MHD activity and disruptions in HBT-EP.” Nuclear Fusion 57, 086035 (2017). [DOI: 10.1088/1741-4326/aa75ea]