Mechanism of radial redistribution of energetic trapped ions due to m=2/n=1 kink instability in plasmas with an internal transport barrier in the Joint European Torus

Abstract

Internal radial redistribution of MeV energy ion cyclotron resonance frequency driven hydrogen minority ions was inferred from neutral particle analyzer measurements during large amplitude magnetohydrodynamic activity in plasmas with an internal transport barrier in the Joint European Torus. A theory is developed for energetic ion redistribution during a m=2/n=1 kink mode instability. Plasma motion during the instability or during subsequent magnetic reconnection generates an electric field which can change the energy and radial position of the energetic ions. The magnitude of ion energy change depends on the value of the safety factor at the plasma core, q(0) from which the energetic ions are redistributed. A relation is found for the corresponding change in canonical momentum Pφ, which leads to radial displacement of the ions. The model yields distinctive new features of energetic ion redistribution such as more vertical particle displacement as q(0) increases from 1 to 2. Predicted characteristics of ion redistribution are compared with the measurements, and good correlation is found. Sometimes the energetic ions were further transported to the plasma edge due to interaction with a long-lived magnetic fluctuation (often in the form of a magnetic island) with chirping frequency in the laboratory frame which developed after the m=2/n=1 kink instability. Convection of resonant ions trapped in a radially moving phase-space island is modeled to understand the physics of such events.