In the quest for cleaner and more efficient energy sources, scientists are continually exploring innovative fusion reactions that could potentially revolutionize the energy sector. One such promising avenue is the deuterium-helium-3 (D– $^3\mathrm{He}$ ) fusion reaction, which produces no neutrons and thus is considered “aneutronic.” However, the challenge lies in measuring the fusion reactivity directly, as traditional neutron-based diagnostics are ineffective in these plasmas. A recent study published in the journal “Nuclear Fusion,” translated to English from its original publication, offers a potential solution through the use of ion cyclotron emission (ICE) diagnostics.
The research, led by T.W. Slade-Harajda from the Centre for Fusion Space and Astrophysics at the University of Warwick, focuses on the distinctive 14.68 MeV protons generated in D– $^3\mathrm{He}$ fusion reactions. These highly energetic protons, when collectively relaxing under the magnetoacoustic cyclotron instability (MCI), emit cyclotron radiation that can be detected and analyzed. This emission provides a valuable diagnostic tool for understanding the fusion process and the behavior of energetic ion populations in the plasma.
Slade-Harajda and his team conducted kinetic simulations of ICE spectra using the particle-in-cell (PIC) code EPOCH. This sophisticated code self-consistently solves the Maxwell and Lorentz equations for millions of interacting simulation particles, providing a detailed picture of the plasma dynamics. The simulations were motivated by recent observations of ICE from trace fusion-born 3 MeV proton populations in the KSTAR and LHD experiments.
“We adopted scenarios where the 14.68 MeV protons are distributed in velocity space as a drifting ring-beam with perpendicular velocities similar to the Alfvén velocity, and the rest of their kinetic energy is directed parallel to the magnetic field,” explained Slade-Harajda. “Plasma and magnetic field parameters were similar to those at the outer mid-plane of the Joint European Torus (JET).”
The simulations revealed strong tilting of successive cyclotron harmonic features in frequency-wavenumber space, due to the Doppler shift arising from the large parallel velocities of the protons. To correct for these Doppler shifts, the researchers performed slanted integrations along Doppler-shifted isofrequency lines, yielding simulated ICE power spectra with strong spectral peaks at Doppler-shifted proton cyclotron harmonics.
The findings suggest that future experimental studies of ICE from D– $^3\mathrm{He}$ plasmas would greatly benefit from higher wavenumber resolution. Moreover, ICE from aneutronic D– $^3\mathrm{He}$ plasmas could play a crucial role as a diagnostic of fusion reactivity and of fusion-born ion populations.
The implications of this research are significant for the energy sector. As the world seeks to transition to cleaner energy sources, the development of efficient and reliable fusion reactors is paramount. The ability to accurately diagnose and understand the behavior of fusion-born ions in aneutronic plasmas could pave the way for more advanced and effective fusion energy technologies.
“This research opens up new possibilities for diagnosing and optimizing fusion reactions in aneutronic plasmas,” said Slade-Harajda. “By improving our understanding of the underlying physics, we can make significant strides towards achieving sustainable and efficient fusion energy.”
As the field of fusion energy continues to evolve, the insights gained from this study could shape the development of next-generation fusion reactors, bringing us closer to a future powered by clean, abundant, and sustainable energy.