2025 – Investigation of fusion-produced alpha driven wave-particle instabilities and validation of theory

Investigation of fusion-produced alpha driven wave-particle instabilities and validation of theory

2025 Research Campaign, Torkil Jensen Award

Purpose of Experiment

This experiment will deliver measurements and validation of theory for drive of fast-ion instabilities including ion cyclotron emission (ICE) in plasmas with a fast-ion population dominated by 3.6 MeV α particles from the D-3He reactions in 3He plasmas heated by D neutral beams. The α particles from D-3He reactions are very similar to the 3.5 MeV α particles expected to dominate the fast-ion population in a burning D-T plasma.

Experimental Approach

These studies will take place in hot (Ti ~ 15 keV), high current (IP = 2 MA) discharges. At this high value of Ti, the D-3He reaction rate will be comparable to the D-D reaction rate and the higher concentration of 3He compared to beam ions will lead to more products from D-3He reactions than D-D reactions. The high current will maximize α confinement by minimizing the radial size of α particle drift orbits, while the 14.7 Mev protons from D-3He reactions will be very poorly confined due their much large orbit size, leaving α particles the dominant fusion product. Beam modulation will be used to create periods largely free of the beam ions, which thermalize much more quickly than α particles, so that α particles are the dominant source of drive for fast-ion driven instabilities. A population dominated by fusion-produced α particles will differ from the beam ion populations common to many fusion research tokamaks in several keys ways. First, the fusion produced α population is expected to be highly super-Alfvénic, in contrast with beam ions, which are sub-Alfvénic in DIII-D and many other tokamaks. Second, the fusion-produced α population is also much more isotropic at birth. Both factors are expected to play a significant role in the drive for these instabilities. The experiment may potentially provide a similar opportunity for Alfvén eigenmodes.

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