  In confined plasmas, a localized fluctuation in a marginal or weakly damped
region will propagate and generate an avalanche if it exceeds a threshold. In
this letter, a new model for turbulence spreading based on subcritical
instability in the turbulence intensity is introduced. We derive a quantitative
threshold for spreading from a seed in a stable region, based on a competition
between diffusion and nonlinear growth of the turbulence intensity. The model
resolves issues with the established Fisher equation model for turbulence
spreading, which is supercritical and cannot support the stationary coexistence
of multiple turbulence levels. Implications for turbulence spreading are
discussed, including the dynamics of ballistic penetration of turbulence into
the stable zone. Tests of the theory are suggested.
