Transport inferred from consideration of particle orbits in drift turbulence

Abstract

Particle orbits in the 2D potential hills/valley ('blobs') of low-frequency drift turbulence are analysed. For most of a given blob's lifetime a particle initially on the blob circulates on an orbit locked to that blob even though the blob shape and amplitude are changing. This locking corresponds to an adiabatic invariant being preserved. However, when the blob amplitude decreases to a critical value, the adiabatic invariant is destroyed, the locking ceases, and the particle detaches from the dying blob and attaches to a newly forming blob. This gives a detailed physical picture of the well known gyro-Bohm transport scaling D approximately gamma /k2 where gamma is the turbulence autocorrelation time (corresponding to blob lifetime) and k is the perpendicular wavenumber (corresponding to blob diameter). It is further shown that gyro-Bohm transport is consistent with global measurements in a wide variety of experiments, in addition to the large tokamaks to which it has traditionally been applied. Finally, it is shown that ion orbit behavior becomes qualitatively different if the blobs become sufficiently sharp and steep.