Effect of internal magnetic structure on energetic ion confinement in tokamaks

For the tokamak magnetic confinement concept, theory has predicted a distinct relationship between the plasma shape, the internal magnetic structure, and the presence or absence of fast ion losses in the presence of plasma instabilities. We have, for the first time, carried out measurements of the magnetic safety factor profile, q(r), in plasmas unstable to a specific instability, the so-called ''fishbone'' mode. The experimental equilibria reconstructed from these data have been used to demonstrate that when the plasma is unstable to fishbones, the fast ion confinement properties depend strongly on the radius of the magnetic surface where q(r)=1.