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Surface and globally averaged heat fluxes can be highly time variable in gyrokinetic simulations. During a burst of turbulence, instantaneous surface averaged fluxes may mo- mentarily reach levels several times higher than their typical (median) levels: the result is that time-averaged flux levels are often considerably larger than typical (median) values. The bursts can in this sense be said to drive a substantial fraction of the transport. Burst- driven fluxes and flows often have long radial correlation lengths, and frequently take the form of radially propagating fronts, or avalanches. Given the large radial scale lengths of the bursts, they could play a significant role in non-local transport, and possibly in depar- tures from gyro-Bohm scaling. We explore the characteristics of the bursts and examine how levels of fluctuation in the system depend on parameters like plasma size and gradi- ent scale lengths. One interesting result is that there is a strong relationship between the avalanche properties and time-averaged global flow structures in the simulation. We find that the propagation direction of the avalanches depends on the sign of the shearing rate. We test an explanation for avalanche propagation based on the linear dispersion of waves which have been tilted by sheared flow. Global gyrokinetic simulations of CYCLONE plasmas using the ORB5 code(1) show bursts in the global flux level; flux plots versus radius and time show inward and outward propagating structures, as do other fluid (2) and gyrokinetic(3) simulations. We present relatively long sim- ulations with ρ ∗ = 1/140 and ρ ∗ = 1/280: figures 1(a) and 1(b) show the flux versus time and radial position for these simulations.

Avalanchelike bursts in global gyrokinetic simulations