Transition to Spiral-Defect Chaos in Low Prandtl Number Convection

We report a distinct transition at $\epsilon \approx 0.55$ from a state with dislocation defects to spiral-defect chaos in a Rayleigh-Bénard system of aspect ratio $\Gamma =40\mathrm{}$. The transition occurred over a narrow range in $\epsilon$ and was seen in angle-time representations of azimuthal power spectra, in the spatially averaged roll curvature, in the skewness of the structure function, and in the roll-nucleation frequency of sidewall foci. Comparison with work by others shows that the onset of spiral-defect chaos moves to smaller $\epsilon$ as $\Gamma$ increases.