Abstract
We study the phenomenon of self-organized criticality (SOC) as a transport problem for electric charges. A model for SOC based on the idea of a dynamic polarization response with random walks of the charge carriers gives critical exponents consistent with the results of numerical simulations of ordinary sand-pile models, and stability properties, associated with the scaling of the control parameter vs. distance to criticality. Overdriving of the system is shown to have a destabilizing effect on SOC. The instability is qualitatively similar to the internal kink mode in tokamaks with high-power beam injection and has serious implications for the functioning of complex systems.