Exploring a Novel Way to Mitigate Surface Damage in Tokamaks

Project Overview

Advancing tokamak magnetic fusion energy requires furthering the understanding of the microscopic "sheath" region between the hot plasma and solid material boundaries of the machine, along with improving techniques for mitigating the surface damage. Conventional schemes to mitigate damage involve injecting cold gas, an impurity species that contaminates the core fusion plasma and reduces the fusion rate. This project considered an innovative approach to the problem that relies on surfaces that emit cold electrons. The project team developed a new sheath simulation model that includes three crucial features: an oblique magnetic field, collisions, and electron emission. We showed for the first time in particle simulations that (ion-repelling) inverse sheaths can exist under strong thermionic emission in tokamak conditions, which could reduce ion-impact sputtering erosion and prevent arc damage. We also found that a common assumption in the magnetic fusion community, that emitted electrons gyrate back to the surface before they reach the plasma, can break down if self-collisions among the high density, low temperature emitted electron species displace gyro-orbits further from the surface. Emitted electrons can therefore cool the plasma significantly, and may help facilitate "detachment" where the surface is protected from high heat flux. The team was awarded run time at a plasma science user facility to carry out experiments intending to demonstrate phenomena observed in emissive sheath simulations.

Mission Impact

Research progress in tokamak divertor physics and fundamental plasma science falls within the missions of the U.S. Department of Energy (DOE) Office of Science, Fusion Energy Sciences. The fusion plasma physics scope of the work enhances the Lab's Core Competencies in High-Energy-Density Science. The code development aspects of the project fall within the high-performance computing core competency. The research addresses the long term mission focus area of energy and resource security. 

Publications, Presentations, and Patents

Johnson, Grant R. and Michael D. Campanell. 2021., "Effects of emitting surfaces and trapped ions on the sheath physics and current flow in multidimensional plasma systems," Plasma Sources Sci. Technol. 30, 015003.

Michael D. Campanell, "Electron-Emitting Surface Phenomena in Low Temperature Plasma Devices and Tokamaks" (Invited Presentation, Gordon Research Conference in Plasma Processing Science., Andover, NH, July 2022).

M. D. Campanell and G. R. Johnson, "Effects of Inverse Sheath Formation at Divertor Plates, Dust Grains, and Negative Ion Sources, on Tokamaks" (Presentation, American Physical Society Division of Plasma Physics Annual Meeting, virtual meeting, Nov 10, 2020). LLNL-PRES-816511

M. D. Campanell and G. R. Johnson, "Innovative Operating Modes of Many Plasma Devices Enabled By Thermionic Surfaces with Inverse Sheaths" (Presentation, International Conference on Plasma Science, virtual meeting, Dec 9 2020). LLNL-PRES-817384

M. D. Campanell, "Effects of Inverse Sheaths on the Plasma-Surface Interactions in Magnetic Fusion Research Areas" (Presentation, International Conference on Plasma-Surface Interactions in Controlled Fusion Devices, virtual meeting, Jan 26, 2021). LLNL-PRES-818580

M. D. Campanell, "Thermionic Emission from Divertor Plates: Friend or Foe?" Plasma-Facing Materials Conference" (Presentation, 18th Conference on Plasma-Facing Materials and Components for Fusion Applications, virtual meeting, April 29, 2021). LLNL-PRES-822045

M. D. Campanell, A. Friedman and D. P. Grote, "Simulations of the plasma-wall transition with oblique magnetic field, collisions, and electron emission" (Presentation, American Physical Society Division of Plasma Physics Annual Meeting, virtual meeting, Nov 9 2021). LLNL-PRES-828744

M. D. Campanell, "Coupled Oscillations of the Cathode Temperature and the Sheath in Self-Sustained Arcs" (Presentation, Gaseous Electronics Conference, Sendai, Japan, Oct 4 2022). LLNL-PRES-840460

M. D. Campanell and G. R. Johnson, "Effects of Inverse Sheath Formation at Divertor Plates, Dust Grains, and Negative Ion Sources, on Tokamaks" (Presentation, American Physical Society Division of Plasma Physics Annual Meeting, virtual meeting, Nov 10, 2020). LLNL-PRES-816511

M. D. Campanell and G. R. Johnson, "Innovative Operating Modes of Many Plasma Devices Enabled By Thermionic Surfaces with Inverse Sheaths" (Presentation, International Conference on Plasma Science, virtual meeting, Dec 9 2020). LLNL-PRES-817384

M. D. Campanell, "Effects of Inverse Sheaths on the Plasma-Surface Interactions in Magnetic Fusion Research Areas" (Presentation, International Conference on Plasma-Surface Interactions in Controlled Fusion Devices, virtual meeting, Jan 26, 2021). LLNL-PRES-818580

M. D. Campanell, "Thermionic Emission from Divertor Plates: Friend or Foe?" Plasma-Facing Materials Conference" (Presentation,18th Conference on Plasma-Facing Materials and Components for Fusion Applications, virtual meeting, April 29, 2021). LLNL-PRES-822045

M. D. Campanell, A. Friedman and D. P. Grote, "Simulations of the plasma-wall transition with oblique magnetic field, collisions, and electron emission" (Presentation, American Physical Society Division of Plasma Physics Annual Meeting, virtual meeting, Nov 9 2021). LLNL-PRES-828744

M. D. Campanell, "Coupled Oscillations of the Cathode Temperature and the Sheath in Self-Sustained Arcs" (Presentation, Gaseous Electronics Conference, Sendai, Japan, Oct 4 2022). LLNL-PRES-840460