In a hypothetical nuclear fusion reactor, the fuel is deuterium gas at a temperature of $7 \times 10^{8} \mathrm{~K}$. If this gas could be used to operate a Carnot engine with $T_{\mathrm{L}}=100^{\circ} \mathrm{C},$ what would be the engine's efficiency? Take both temperatures to be exact and report your answer to seven significant figures.