Investigation on x-ray radiation from microstructured targets heated by short-pulse relativistic laser pulses*

The enhancement effect of microstructured surfaces on laser absorption and characteristic x-ray emission has been investigated by measuring K-shell emission from titanium (Ti) targets irradiated with high-intensity (∼ 10²⁰ W/cm²), sub-picosecond (500 fs) laser pulses. The experimental results indicate a modest enhancement (1.6×) of Kα emission from microstructured targets compared to flat foils, but with similar intensity and profile of Heα and Li-like satellites. Particle-in-cell (PIC) simulations are implemented to further understand the underlying physical processes in the laser interaction with both targets, interpreting the mechanisms for the Kα enhancement. The reasons for the lower-than-expected enhancement of Kα emission are discussed. The rapid heating of the bulk plasma might result in the premature shutdown of Kα emission before the thermalization of hot electrons or even the end of laser pulses, suggesting that the use of Kα emission as a diagnostic of the hot-electron yield or relaxation could lead to a misinterpretation. This work reveals that an optimized microstructured target shows promise to produce high-brightness, quasi-monochromatic laser-driven x-ray sources for many probing applications.