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Perturbing the edge magnetic field of a tokamak produces a counterintuitive response: particles entering the confined region rather than escaping it.
A tokamak uses magnetic fields to confine plasma and reach the high temperatures necessary to produce nuclear fusion reactions. In some tokamak plasmas, confinement in a particular part of the
Implications for Future Fusion Plants
These experiments and the associated modeling demonstrate that magnetic perturbations can be applied to a tokamak plasma without reducing fusion performance. This is important for a future fusion pilot plant. Researchers expect these plants to operate at power levels where the power flux expended by large ELMs can damage the device. This will make applied magnetic perturbations important for these devices.
The recent experiments suggest that ELMs can be mitigated without surrendering fusion performance if the plasma is rotating in a direction opposite to its current. Building on this work, the design of a fusion pilot plant will benefit from further insights into pedestal confinement and development of maximally efficient ways to use a magnetic perturbation system.
Managing Instabilities and Enhancing Performance
Under conditions of strong confinement, the pedestal of a tokamak can produce instabilities that expel particles into the device wall. Researchers use applied magnetic perturbations to disturb the pedestal such that these instabilities (ELMs) are either reduced in magnitude or eliminated entirely, although these perturbations do disturb the tokamak magnetic field in such a way that some particles escape confinement.
When an ELM occurs, pedestal pressure decreases, but only until the high level of confinement within the plasma allows the pressure to build up again, eventually leading to another ELM event. However, perturbing the pedestal and reducing confinement adversely affect plasma density and fusion performance. Thus, the key has been to allow just enough particle loss to keep the pedestal pressure below the level that results in an ELM.
In experiments at the DIII-D National Fusion Facility, researchers provided the first demonstration of applying magnetic perturbations and experiencing increased plasma density across the pedestal. In this new regime, magnetic perturbations caused the pedestal confinement to improve. Total particle flux on the tokamak wall caused by ELMs was unchanged during the applied perturbations, while the increased plasma density corresponded to an improvement in fusion performance.
The researchers attribute the improved confinement to the applied magnetic perturbations reducing density turbulence in the pedestal and causing an inward particle flux. As these dynamics become better understood, it may help researchers design a perturbed tokamak scenario that mitigates ELMs while simultaneously increasing fusion performance.
Reference: “Improved Particle Confinement with Resonant Magnetic Perturbations in DIII-D Tokamak H-Mode Plasmas” by N. C. Logan, Q. Hu, C. Paz-Soldan, R. Nazikian, T. Rhodes, T. Wilks, S. Munaretto, A. Bortolon, F. Laggner, F. Scotti, R. Hong and H. Wang, 10 November 2022, DOI: 10.1103/PhysRevLett.129.205001
This work was supported by the Department of Energy (DOE) Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility.