In the quest to harness the power of nuclear fusion, scientists are continually refining their understanding of the complex processes that occur within tokamaks, the doughnut-shaped devices designed to confine and control plasma. A recent study published in the journal Nuclear Fusion, led by P.C. de Vries of the ITER Organization, sheds new light on the formation of runaway electrons during the start-up phase of tokamak discharges, a critical period that can significantly impact the efficiency and safety of fusion reactors.
Runaway electrons are high-energy particles that can escape the magnetic confinement of the plasma, potentially damaging the tokamak’s walls and disrupting the fusion process. Understanding and controlling these electrons is crucial for the development of sustainable fusion power, a technology that promises nearly limitless, clean energy.
De Vries and his team used the SCENPLINT code, updated to include runaway electron physics, to simulate the generation and loss of these electrons during the start-up phase. The simulations were compared with experimental observations from the Joint European Torus (JET), one of the world’s largest operational tokamaks. This comparison proved challenging due to the multitude of parameters involved and the explosive growth potential of runaway electrons.
One of the key findings was the importance of the ratio of the electric field to the critical electric field (E/E_c) in the plasma. “Using the secondary generation model proposed by Aleynikov and Breizman ensures a better match to the experimental observations,” de Vries explained. This model predicts reduced secondary generation when E/E_c is less than 5, a condition often met during the start-up phase of tokamak discharges.
The study also highlighted the significant impact of assumed runaway electron loss rates on the simulation results. The researchers found that assuming the runaway electron confinement time scales with the discharge current provided a better match to the experimental data. However, determining which runaway electron loss model—one based on drift-orbit losses or another based on diffusion due to magnetic turbulence—provided a better match remained elusive.
So, what does this mean for the future of fusion energy? As de Vries puts it, “These findings will be crucial to improve predictions of start-up runaway electron formation in future devices such as ITER.” ITER, currently under construction in France, is designed to demonstrate the feasibility of fusion power on a commercial scale. The insights gained from this study could help engineers design more robust and efficient tokamaks, bringing us one step closer to a future powered by fusion energy.
The energy sector is watching closely. Fusion power, with its potential for abundant, clean energy, could revolutionize the way we power our world. By improving our understanding and control of runaway electrons, scientists are paving the way for a future where fusion power is a viable and sustainable part of our energy mix. The research published in Nuclear Fusion, which translates to Nuclear Fusion in English, is a significant step in that direction, offering valuable insights that could shape the future of fusion energy.
