Higher implosion efficiency is of great significance in direct-drive fusion research. We demonstrated the critical role played by the intensity gradient of the main drive laser pulse in improving efficiency of direct-drive implosions, using a double-gradient nanosecond pulse. Compared with a square pulse, the burn-through depth was increased by over 200%, and the shell velocity was increased by $\sim 2.1$ times with an optimized double-gradient pulse. As the result, the implosion efficiency was enhanced by $\sim$ six times. It was found that by limiting the intensity gradient of the main drive pulse to no more than $\sim 2.5\times {10}^{15}\mathrm{W}/\left(\mathrm{c}{\mathrm{m}}^2\mathrm{ns}\right)$, heat flux inhibition by nonlocal electron thermal transport effects could be eliminated, and ultimately an efficient mass ablation process was achieved. These results have relevance for pulse designs in ignition-scale direct-drive implosions.

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• Direct drive
• Inertial confinement fusion
• Plasma fusion