A collaboration between the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) and the UK Atomic Energy Authority (UKAEA) has yielded new insights into managing plasma exhaust. In a recent experimental campaign at the Culham Centre for Fusion Energy, physicists remotely operated the MAST Upgrade spherical tokamak from a control station in New Jersey. The experiments focused on developing stable plasma scenarios that mitigate the intense heat flux striking the divertor, a critical challenge for future fusion power plants. The team successfully sustained H-mode plasmas while actively controlling divertor conditions, a key step in integrating high-performance core plasmas with manageable edge physics. Source: PPPL

The primary achievement involved inducing and controlling a state of divertor detachment, where a cushion of cool, dense gas and plasma neutralizes incoming heat and particles before they impact material surfaces. Using a Super-X divertor configuration, which lengthens the magnetic field lines in the exhaust region, the team injected deuterium and nitrogen gas to dissipate energy. Diagnostics confirmed a significant reduction in heat flux at the divertor target plates, with peak loads dropping by over an order of magnitude compared to attached conditions. This was achieved without causing a deleterious effect on the core plasma's energy confinement time, maintaining a normalized beta above 2.0. Source: PPPL

Using a Super-X divertor configuration, which lengthens the magnetic field lines in the exhaust region, the team injected deuterium and nitrogen gas to dissipate energy.

These results are directly relevant to the design of compact fusion pilot plants, such as the one envisioned by the U.S. national strategy. Spherical tokamaks offer a potential path to higher power density and lower capital cost, but their compact geometry concentrates heat loads on smaller divertor surfaces, making effective exhaust handling paramount. The successful demonstration on MAST Upgrade provides an integrated validation of the Super-X divertor concept under high-power, long-pulse conditions. The data will inform predictive modeling and engineering designs for next-generation devices, including the STEP (Spherical Tokamak for Energy Production) program in the UK. Source: PPPL

The international nature of the collaboration underscores a growing trend in fusion research, combining hardware from one institution with the operational expertise and theoretical teams of another. The remote participation from PPPL allowed for round-the-clock analysis and experiment planning, accelerating the research campaign. Future experiments are planned to test the detachment scenarios with increased neutral beam injection power, pushing the limits of the divertor system and exploring the operational space for steady-state, high-performance scenarios. The results from this campaign will be submitted for peer-reviewed publication later this year. Source: PPPL