In the high-stakes world of nuclear fusion, managing runaway electrons (REs) is a critical challenge that could significantly impact the future of high-performance tokamaks like ITER. A recent study published in the journal *Nuclear Fusion* (translated from the original title) delves into the intricate dynamics of REs during plasma disruptions, offering insights that could enhance the operational safety and longevity of fusion reactors.
The research, led by Yuxiang Sun of Beihang University in Beijing, focuses on the stochastic transport and deposition of seed runaway electrons during disruption mitigation. Runaway electrons, which can reach nearly the speed of light, pose a substantial risk to the first wall surfaces and structures within tokamaks. If left uncontrolled, these electrons can replace bulk electrons as the main current carrier, leading to localized, uncontrolled deposition and potential damage.
“Localized, uncontrolled REs deposition can result in serious damage to first wall surfaces and structures in the devices,” Sun explained. “One way to avoid such current replacement is to deplete the seed REs within the plasma through stochastic trajectory loss before they have time to avalanche.”
To investigate this phenomenon, Sun and his team conducted guiding center simulations of seed REs using the PTC code, based on fluid fields produced by JOREK simulations. The study focused on an ITER plasma scenario following Shattered Pellet Injection, a technique used to mitigate disruptions by breaking up and healing flux surfaces. The researchers examined the transport properties of REs as the stochasticity of the magnetic field evolved.
The findings revealed self-similar density profiles and exponential decay of seed REs in cases with sufficiently stochastic magnetic fields. The team also investigated the diffusion of seed REs with varying momentum, pitch angle, and initial location, obtaining transport coefficients statistically through simulations. These coefficients were compared with the effective RE radial flux to estimate the efficiency of stochastic RE depletion during the mitigation process.
“Self-similar density profiles and exponential decay of seed REs are found for cases with sufficiently stochastic magnetic field,” Sun noted. “We also examine their timescale of loss and compare it with that of the RE avalanche to estimate the efficiency of stochastic RE depletion during the mitigation process.”
Using a realistic 2D wall model, the researchers presented the deposition pattern of REs on the first wall, providing estimates of its asymmetry. This detailed analysis offers valuable insights into the behavior of runaway electrons and the effectiveness of mitigation strategies.
The implications of this research are significant for the energy sector, particularly in the development of fusion power. Understanding and controlling runaway electrons is crucial for ensuring the safety and efficiency of future fusion reactors. By optimizing disruption mitigation techniques, researchers can enhance the operational limits and component lifetime of tokamaks, paving the way for more reliable and sustainable energy solutions.
As the field of nuclear fusion continues to evolve, studies like this one will play a pivotal role in shaping the future of energy production. The work of Yuxiang Sun and his team at Beihang University represents a crucial step forward in the quest for clean, sustainable, and efficient energy.