(color online) Plots of nonlinear coefficient A versus population of nonthermal ion α for δ₁ and δ₂. Here orange (themost left) curve represents δ₁ = 3, δ₂ = 10, dash line represents δ₁ = 3/2, δ₂ = 4, and blue line (the most right) represents δ₁ = δ₂ = 2.