Laser-driven X-ray sources are essential for probing ultrafast dynamics in high-energy-density matter. Nano/micro-structured targets are widely used to enhance laser energy absorption, yet the response of characteristic Kα emission remains inconsistent—often showing only modest enhancement or even reduction despite significantly increased absorption [1-3]. This work investigates the underlying mechanisms through a combination of experimental campaigns at the PHELIX and TRIDENT laser facilities and advanced particle-in-cell (PIC) simulations.  Experiments were conducted using high-contrast, relativistic laser pulses (≥10²⁰ W/cm²) on flat and structured titanium and copper targets. While bremsstrahlung and thermal line emission (Heα, Lyα) were substantially enhanced (up to an order of magnitude) in structured targets, Kα emission exhibited only a modest increase (∼1.6× for microstructured Ti) or even suppression (for Cu nanowire foams). PIC simulations revealed that microstructured targets indeed enhance hot electron generation—especially in the MeV range—through increased interaction surface, vacuum heating, and geometric trapping. However, the simulations also showed that structured targets undergo much faster bulk heating, causing the bulk electron temperature to exceed the critical threshold for cold Kα emission (∼100 eV for Ti) earlier in the laser pulse. This premature shutdown of Kα production counteracts the benefits of increased hot electron generation, explaining the observed modest enhancement.  These results challenge the conventional view that improved laser coupling directly translates to higher Kα yields. Instead, they highlight the critical role of rapid target heating dynamics and the temperature sensitivity of Kα emission. The findings underscore the need for careful interpretation when using Kα radiation as a diagnostic for hot electron generation or relaxation processes, and provide guidance for designing structured targets optimized for bright, narrow-band X-ray sources. 

Laser-driven X-ray sources,Structured targets,Laser-solid interaction,K-alpha emission,Ultrafast dynamics,High-energy-density plasma