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Fast ions carry ~1/3 of the plasma kinetic energy. Magnetically confined fusion plasmas contain highly non-thermal populations of fast ions resulting from fusion reactions and plasma heating. With energies in the MeV range, two to three orders of magnitude above the bulk ion and electron energies, the fast ions typically carry 1/3 of the plasma kinetic energy. A considerable fraction of the free energy in the plasma is thus associated with the fast ion population. It is essential that this energy is channelled into heating the thermal bulk plasma. It is, however, also available for doing mischief.
Fast ions drive turbulence. The fast ions can drive waves and turbulence through wave-particle interaction. This in turn can act back on the fast ions, redistributing them or even ejecting them prematurely from the plasma. The turbulence is of course also likely to affect bulk confinement.
Sawteeth redistribute fast ions. Fast ions are known to modify the sawtooth instability, lengthening its period and increasing its amplitude. The sawteeth in turn appears to redistribute parts of the fast ion population. To ameliorate the effect of this non-linear process, or even tailor it to the benefit of fusion performance, we need to understand the processes and confirm the effects of any active control of the sawteeth.
Ion Cyclotron Resonance Heating. Wave particle interaction is also the basis of Ion Cyclotron Resonance Heating (ICRH); one of the main plasma heating schemes relying on the absorption of radio waves.
Phase space distribution, cause and effect. In all cases the wave particle interaction depends critically on the phase space distribution of the energetic ions. The effects of waves, turbulence and instabilities on the fast ions manifests itself in their phase space distribution.
Collective Thomson Scattering. Understanding the plasma dynamics involving fast ions is essential for optimising fusion performance. Experimental data on the phase space distribution of energetic ions are particularly valuable in challenging and improving our understanding here, but fast ions are notoriously elusive to experimental observation. Fast ions do, however, draw a wake in the electron distribution, detectable by Collective Thomson Scattering (CTS).
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