In the relentless pursuit of sustainable energy, the international fusion community is inching closer to a future where the sun’s power is harnessed here on Earth. At the heart of this endeavor lies ITER, the world’s largest tokamak, designed to demonstrate the feasibility of fusion power. But before ITER can light up the grid, scientists must navigate the complex physics of plasma behavior, particularly during the crucial start-up phase. Recent research led by Y. Zhang from the Key Laboratory of Materials Modification by Laser, Ion and Electron Beams at Dalian University of Technology in China, sheds new light on this challenge, with significant implications for the energy sector.
The ITER tokamak, currently under construction in France, will use tungsten (W) as the primary material for its first wall armor, a shift from the initially planned beryllium. This change, driven by safety and operational considerations, brings with it a unique set of challenges. Tungsten, while robust and resistant to high temperatures, can significantly alter plasma behavior due to its high radiated power.
Zhang and their team employed the SOLPS-ITER code to simulate hydrogen plasmas interacting with a tungsten limiter, a component that protects the tokamak’s walls from direct plasma contact. Their findings, published in the journal ‘Nuclear Fusion’ (translated from Chinese as ‘核聚变’), reveal a fascinating self-regulating property of the plasma-limiter system. “The plasma exhibits strong self-regulating properties due to the strong dependence of tungsten self-sputtering on the electron temperature at the last closed flux surface,” Zhang explains. This self-regulation limits the power crossing into the scrape-off layer, reducing heat loads on the first wall but increasing the core radiated fraction.
The team’s simulations also uncovered a simple scaling relationship between plasma temperature, density, and heating power, a crucial finding for constructing boundary conditions in time-dependent scenario simulations. Moreover, they explored the impact of prompt redeposition of eroded tungsten particles, finding that a significant portion resides in the plasma for less than a gyration time. This insight underscores the need for self-consistent simulations with appropriate redeposition models to improve ITER’s predictive capabilities.
The research also ventured into the realm of real-world impurities, performing preliminary simulations with nitrogen seeding. The results suggest that even moderate nitrogen content can saturate the power crossing the last closed flux surface, redistributing radiation and preferentially cooling the edge. This finding could inform strategies for managing impurities in future tokamaks, a critical aspect of commercial fusion power.
So, what does this mean for the energy sector? As ITER paves the way for commercial fusion power, understanding and mitigating these plasma-wall interaction challenges will be paramount. Zhang’s work offers valuable insights into the complex dance of particles and power in a tokamak, bringing us one step closer to a future where fusion energy is a reality. The implications for the energy sector are profound, promising a nearly limitless, clean, and safe power source. As the world watches ITER’s progress, research like Zhang’s will continue to shape the future of fusion power, driving innovation and inspiring the next generation of energy solutions.
