Spark extinction time distributions as a function of assumed free calcium diffusion coefficient in the dyadic cleft. In these simulations, couplons consisted of 7 × 7 RyRs, with k_om = 0.117 ms⁻¹, Ca50 = 10 µM, JSR volume of 0.35% of cell volume divided among 20,000 couplons, and refilling time constant of 90 ms in the presence of 30 mM Casq. Only detected sparks that terminated within 300 ms are included in the histograms.