Direct observation of phase-space dynamics during energetic-particle-driven MHD bursts

The phase space (velocity and real space) dynamics of bulk ions is a critical issue for the heating scenario in magnetically confined plasma. Bulk ions accelerate parallel to the magnetic field, and the velocity distribution function can depart from a Maxwellian due to wave-particle interactions such as Landau and transit-time damping. Here we show flattening in velocity space (a decrease in the velocity gradient) and a rapid increase in ion energy parallel to the magnetic field within 0.1 ms of the onset of the MHD burst, by 30% at the location of the MHD burst in the Large Helical Device. In velocity space, the anisotropy of ion-velocity distribution due to the acceleration of ions parallel to the magnetic field disappears within the ion-ion collision time scale (2 ~ 3 ms). The transient increase in ion energy decays within 6 ms at the plasma periphery but persists much longer in the plasma core, contributing to the rise in core-ion energy.