Events at Physics |
given insufficient attention. This is particularly true in comprehensive numerical
simulations, like gyrokinetics, where beyond ascertaining that there is a physically
meaningful saturated state, most of the analysis is best described as characterization of
fluctuations and the transport they produce. As a result, certain widely held ideas about
saturation do not hold up under closer scrutiny. In this talk we examine new
developments in the understanding of how ion temperature gradient (ITG) turbulence
saturates. We present evidence from gyrokinetic simulations that ITG turbulence
saturates primarily by energy transfer to damped modes within the wavenumber range of
the instability, rather than by a cascade to small scale. Zonal flows catalyze this transfer,
providing an efficient energy transfer channel while absorbing very little of the
transferred energy. Zonal flow shearing is not the primary saturation mechanism, but is a
secondary (weaker) effect at best. We also show that zonal-flow-catalyzed transfer
excites stable modes with tearing parity. These give rise to a stochastic magnetic field at
very low beta values, and electron thermal transport. While small at low beta, this flutterinduced
transport becomes significant at beta approaching 1%. Magnetic fluctuations
created by transfer to damped modes affect zonal flows. Above a critical beta, they
induce irreversible charge loss from rational surfaces, shorting out zonal flow potentials
and leading to a state of very high transport. This phenomenon, called the non zonal
transition (NZT), has long been observed in gyrokinetic simulations, and in experiment
could limit gradients in stiff discharges.