Layered targets of aluminum and fused silica have been irradiated by 80 fs prepulse free laser pulses at 1017W/cm2laser irradiance. Time‐integrated x‐ray spectra obtained in the 0.7–0.9 nm wavelength range show both resonance lines from Al11+ions and K&agr; lines from Al0+‐Al5+ions. Conversion efficiency measurements give x‐ray intensities of a few TW/cm2. A detailed analysis of K&agr; emission points out the role of energetic electrons in the interaction physics. Based on a detailed comparison of experiments with hydrodynamic simulations, it is concluded that K&agr; emission is not longer than the laser pulse duration.