Anomalous shock scaling in ICF implosions dominated by high-energy X-ray preheat

The design of inertial confinement fusion implosions relies on the predictable control of shock dynamics, where the first-shock strength is conventionally governed by the initial laser picket intensity. Here, using shock timing experiments at the SG 100 kJ laser facility, we report a distinct deviation from this conventional scaling: when a high-power main pulse precedes shock breakout, the first-shock velocity in liquid deuterium decreases with increasing picket drive. We attribute this anomaly to preheating of the ablator by high-energy X rays from the early main pulse. Integrated simulations and theory reveal that this preheating induces a rapid pressure rise at the ablator fuel interface. The resulting enhanced pressure overrides the picket driven dynamics during shock impedance matching, accelerating the shock into the fuel and thus inverting the expected scaling. Our findings establish a pressure-dominated mechanism that revises the framework for shock tuning in preheat-affected implosions and provides direct guidance for pulse shaping in future high-energy-drive designs.