Interevent Time Correlations for Avalanches on a Conical Bead Pile.

A conical bead pile subject to slow driving is used as a model critical system to experimentally investigate the distributions of avalanche sizes and time between events. The pile is composed of roughly 20 000 steel beads, 3 mm in diameter; we drive the pile by adding one bead at a time to the apex of the pile. We record the changes in pile mass over the course of tens of thousands of bead drops to characterize the distribution of avalanche sizes. An external magnetic field may be applied, inducing cohesion among the beads. We are investigating how the avalanche statistics (number, size, and timing of avalanches) scale due to the addition of cohesion to the system. Previous work [Lehman et al., Gran Matt 24:35 (2022)] shows that with increased cohesion, large avalanches become more frequent and potentially quasiperiodic. Here we use the Bi test to assess the time correlations between avalanches and to determine whether avalanches cluster together in time or whether they are more regularly distributed. Initial findings indicate that small avalanches exhibit clustering in time, while in contrast, large avalanches appear to follow a quasiperiodic pattern. The trends with cohesion are also discussed.