SPECIAL PLASMA SEMINAR: Mechanisms of energetic-particle transport in magnetically confined plasmas [W.W. Heidbrink and R.B. White]

Super-thermal energetic particles (EP) occur in both space and fusion plasmas. Because guiding-center drifts are proportional to energy, EP orbits necessarily deviate substantially from magnetic surfaces. Orbits are described by conserved constants of motion that depend on topological boundaries for different orbit types. Electric and magnetic field perturbations produced by instabilities can disrupt particle orbits, causing the constants of motion to change.  The statistics of the "kicks" associated with these perturbations determines the resulting cross-field transport. A unifying theme of this tutorial is the importance of phase in wave-particle interaction. A distinction is made between field perturbations that resonate with an aspect of the orbital motion and those that do not.  Resonance occurs when the wave phase returns to its initial value after an integer number of orbits. Resonant instabilities with unvarying wave-particle phase cause convective transport. Alternatively, multiple wave-particle resonances usually decorrelate the phase, resulting in diffusive transport. Large orbits increase the number of important resonances and can cause stochasticity even for relatively small amplitude waves. In contrast, in the case of non-resonant perturbations, orbital phase averaging reduces transport.

 

Large field perturbations introduce additional effects, including nonlinear resonances at fractional values of the orbital motion. In summary, large orbits are a blessing and a curse: For nonresonant modes, orbit-averaging reduces transport but, for resonant transport, large orbits facilitate jumps across topological boundaries and enhance the number of resonances.