The mechanism by which intense X rays ablate a material surface and generate recoil momentum provides the physical foundation for critical engineering applications, including asteroid defense and X ray driven propulsion. While this process is well established in plasma physics under typical laboratory conditions, a systematic understanding of momentum generation and its evolution under low specific energy, long duration, and long distance conditions-relevant to asteroid defense and ablation propulsion-remains lacking. In this study, we employ the FLASH radiation hydrodynamics code to construct a one dimensional irradiation driven model specifically designed for such regimes. The model incorporates detailed radiation transport, material equation of state, and energy deposition physics to capture the coupled evolution of temperature, pressure, and density fields in aluminum targets under intense X ray irradiation.
Based on this model, we divide the momentum generation process induced by intense X ray irradiation into four distinct stages according to their dominant physical characteristics: (I) radiation ablation, (II) gas breakout, (III) shock impedance matching, and (IV) spallation expansion.
For each stage, we quantitatively analyze its contribution to the total momentum increase and elucidate the underlying physical phenomena, including the "temperature-pressure peak misalignment" induced by radiative preheating and the "separation between the shock front and the radiative precursor wave" caused by solid-liquid phase transition.
To overcome the systematic deviation of "spontaneous expansion" introduced by the neglect of material strength constitutive models in radiation hydrodynamic codes, we propose a correction method for the impulse coupling coefficient based on Riemann invariants. This method provides a self-consistent way to extract the asymptotic expansion velocity and the effective ablation pressure, thereby improving the accuracy of the impulse coupling coefficient calculation. Our numerical results show that for intense X-ray irradiation with a spectrum concentrated within 1 keV and a fluence range of 50-200 J/cm², the impulse coupling coefficient for aluminum lies between 0.3 and 0.7 Pa·s·cm²/J. These values are in good agreement with existing experimental measurements and the theoretical predictions of the vaporization impulse theory.
This paper establishes a clear stage-resolved physical picture of momentum generation under intense X-ray irradiation, clarifying the respective contributions of ablation, gas breakout, impedance matching, and spallation. A correction method for the spontaneous expansion effect in radiation hydrodynamic simulations is proposed—a numerical artifact that has been previously overlooked and unresolved in radiation hydrodynamic simulations of recoil momentum. This work provides a unified and physically transparent analytical approach for understanding the dynamic response of materials under extreme radiation loading. This framework is not only applicable to laboratory Z-pinch experiments but can also be directly applied to real-world asteroid deflection missions, where accurate momentum prediction is essential for impact efficiency assessment and trajectory optimization.