## 1 Introduction  
Plasma-based ultrafast X-ray sources are of great importance for many intriguing applications [1]. They provide well-defined line and continuous emission that can be used for time-resolved X-ray spectroscopy of high-density matter. Such ultrashort duration (femtosecond to picosecond time scale) sources of X-ray emission can be designed using an interaction of intense ultrashort laser pulses with solids. At relativistic intensities (>1.3 \(\times\) 10\({}^{18}\) W\(\cdot\upmu\)m\({}^{2}\)/cm\({}^{2}\)), the laser pulse can strongly ionize solid or liquid targets and accelerate electrons beyond 1 MeV [2]. The resulting plasmas act as bright sources of particles (electrons, protons, and neutrons) and X-rays. Corresponding experiments have been carried out at large, joule-energy class, multi-terawatt peak power laser systems with sub-picosecond and, nowadays, femtosecond pulse duration in a single-shot regime [3; 4; 5; 6; 7]. However, from a practical point of view of applications in X-ray spectroscopy, high-repetition rate sources of X-rays are of great importance. Therefore, small scale, mJ-level energy, and kHz repetition rate laser systems are successfully used as drivers for X-ray sources [8; 9; 10]. Such portable sources with high brilliance have already been proven to be a powerful tool, e.g., for time-resolved X-ray diffraction [11; 12].  
Bremsstrahlung emission is often considered as a parasitic background radiation that has to be suppressed. Therefore, insignificant attention has been paid to the investigation into the possibility of effectively generating X-ray photons with energies above 20 keV at a high repetition rate. However, hard X-ray continuous emission has also been used in high-energy density (HED) experiments, such as inertial confinement fusion, in which bremsstrahlung sources are widely used as a \"back-lighter\" in inverse-bremsstrahlung absorption for time-resolved measurements of electron density [13; 14]. Meanwhile, softer bremsstrahlung emission is suitable for X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) [15; 16].  
The bremsstrahlung yield can be enhanced by designing the target's geometry. Nanoparticle coating and specially synthesized nanostructures can greatly increase the X-ray flux [17; 18; 19]. It has been shown theoretically that nanoscale structures with optimum parameters, depending on laser pulse duration and intensity, can significantly increase the absorption of laser energy [20; 21]. In particular, the nanowire morphology of the target enables deep (much deeper than skin depth) penetration and highly efficient absorption of the laser energy, leading to so-called volumetric heating of the plasma [21]. Consequently, this leads to more efficient generation of hot electrons and, thus, X-ray photons.  
In this paper, we report on the experimental investigation of characteristic line and hard X-ray emission generated in ZnO semiconductor nanowire targets of different morphologies, irradiated by ultrashort laser pulses at intensities below 10\({}^{17}\) W/cm\({}^{2}\). Such intensities at 800 nm wavelength correspond to a non-relativistic interaction, implying that the electron dynamics are not relativistic (the quiver velocity of the electrons in the laser field is smaller than the speed of light). Significant enhancement of the photon flux with energies above several hundred keV (gamma range) and extension of photon energies up to 0.8 MeV is demonstrated. This enhancement is related to the observed dramatic increase in the number and energies of hot electrons generated from nanostructured solid targets.