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High-energy ions reduce plasma turbulence

​Based on experiments conducted in the European tokamak JET, physicists from the IRFM and their partners demonstrated that the increased presence of highly energetic deuterium ions improves the stability of the fusion plasma. ITER's plasma should benefit from a similar effect, thanks to the helium nuclei produced by fusion.

Published on 21 July 2022

​In a tokamak, the fusion reaction of hydrogen's two isotopes, deuterium and tritium, produces highly energetic alpha particles (helium nucleus: 4He) at 3.5 MeV and neutrons at 14 MeV.

This reaction is only possible if the plasma is brought to a very high temperature through two main techniques.

  • Deuterium ions are accelerated outside of the tokamak and then their electric charge is neutralized; the neutrals (atoms) thus obtained can then penetrate into the plasma core, to which they transfer part of their energy via collisions (Neutral Beam Injection, NBI).
  • Specific electromagnetic waves – whose frequency is tuned to an ionic cyclotron resonance – accelerate the movements of charged particles present in the plasma and increase their temperature (Ion Cyclotron Resonance Heating, ICRH).

Previous experiments showed that deuterium ions of 100 keV to 1 MeV likely interact with magnetic field oscillations (called Alfvén waves); in the event of resonance, they transfer a significant amount of energy to these oscillations by degrading the plasma confinement.

 To delve deeper, IRFM researchers analyzed plasmas produced in the European tokamak JET with a significantly increased population of energetic deuterium ions (by a factor of 10), in the presence of intense Alfvén waves. These ions, injected by NBI, are selectively "over-accelerated" to the MeV range by ICRH heating waves, which are finely tuned to their frequency. The scientists then compared the resulting confinement to that of plasmas heated exclusively by injection of neutrals (NBI) with energies around 100 keV.

Surprisingly, the energy confinement in the presence of ions in the MeV range was improved by 40%! A reflectometry diagnostic was used to attribute this increase to a strong reduction in plasma density fluctuations.

To interpret their results, the physicists developed multi-scale simulations that describe both the small-scale turbulence and the large-scale fluctuations of the Alfvén waves. These simulations accurately replicate the experimental data and make it possible to identify the mechanism underlying the improved confinement.

These results suggest that the plasma confinement of ITER and other future tokamaks could benefit from the presence of high-energy alpha particles.

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