In the high-stakes world of fusion energy, where the promise of near-limitless, clean power is tantalizingly close, researchers are grappling with a critical challenge: the risk of magnet quenches. These catastrophic events occur when superconducting magnets, essential for confining the hot plasma in fusion reactors, suddenly lose their superconducting properties and heat up rapidly. The consequences can be devastating, potentially halting the entire fusion process and causing significant damage to the reactor.
A recent study led by Jacob M. John of the UK Atomic Energy Authority (UKAEA) in Abingdon, UK, sheds new light on this issue. The research, published in the journal ‘Nuclear Fusion’, focuses on the impact of irradiation damage on the stability of superconducting magnets. The findings are both alarming and instructive, offering a pathway to mitigate the risks associated with magnet quenches.
Superconducting magnets are the backbone of fusion reactor designs, enabling the high current densities needed to generate the powerful magnetic fields that confine the plasma. However, these magnets are not immune to the harsh conditions inside a fusion reactor. Irradiation from high-energy neutrons can cause defects in the crystalline lattice of the copper stabilizer material, which carries the magnet current during a quench. These defects store energy, known as Wigner energy, which can be released suddenly, causing local heating and increasing the risk of a quench.
John and his team found that as the fluence—the total number of neutrons passing through a given area—increases, the risk of a quench also rises. “The presence of defects reduces thermal conductivity, electrical conductivity, and specific heat capacity,” John explains. “This thermally insulates the superconductor, increases the temperature ramp rate during a quench, and increases thermodynamic instability.” In other words, the magnet becomes more prone to overheating and failure.
The researchers measured an energy release of 0.023 J g−1 from copper when the temperature was increased from 10 K to 18 K following irradiation. This energy release was sufficient to cause the same temperature increase spontaneously. Extrapolating from these data, the team estimated critical fluence values between 1.74×10^18 n cm−2 and 2.85×10^19 n cm−2 for neutron irradiation of copper at 20 K. These values represent the point at which spontaneous heating due to energy release from irradiation-induced defects could occur.
The implications for the energy sector are significant. As fusion reactors move closer to commercial viability, understanding and managing the risk of magnet quenches will be crucial. The study highlights the need for in-situ cryogenic calorimetry experiments to provide more certainty for fusion magnet system designers. Additionally, periodic annealing of defects through controlled temperature cycling will be essential to manage the increasing risk of quench as the superconducting magnets accumulate dose.
John emphasizes the importance of further experimental examination to establish the ideal frequency and dynamics of these maintenance temperature cycles. “Periodic annealing of defects through controlled temperature cycling will be essential in fusion power plants to manage the increasing risk of quench as the superconducting magnets accumulate dose,” he states. This ongoing research will shape the future of fusion energy, ensuring that the technology can deliver on its promise of clean, abundant power.
As the energy sector looks to the future, the insights from John’s research will be invaluable. By addressing the challenges posed by irradiation damage and magnet quenches, researchers can pave the way for more stable and reliable fusion reactors, bringing the dream of fusion power one step closer to reality.
