## 1 Introduction  
THz science and technology [1] have been continuously growing since the early report on THz wave emission [2]. Applications have been demonstrated in imaging [3] with optical component developments [4], spectroscopy for mesoscopic structures [5], coherent measurements of intense THz wave [6], gas-detection in Fabry-Perot cavity [7], tunable/compact laser [8], sensing for medical uses [9], and next generation 6G wireless communication [10]. Recent progress includes studies under scanning tunneling microscopy [11; 12; 13] with nanoscale spatial resolution. For further developments in the field, innovations in THz wave sources are expected. One of the experimental methods for THz wave emission, apart from the well-developed techniques with semiconductors such as ZnTe and others [14], is through femtosecond laser-based plasma formation [15] since much higher incident laser intensity can be put into the plasma formation. While solid samples are irreversibly damaged under such intense laser irradiation conditions, air [16; 17] and noble gases (He, Ne, Ar, Kr, and Xe) [18; 19] have been used as targets for plasma-based THz emission, thus circumventing the issue. Laser excitation with chirped pulses [20] or two-colors [16; 21; 22; 23; 20] has been carried out. The mechanisms of the THz wave emission have been discussed mainly in terms of four-wave mixing and ponderomotive force. Furthermore, a feedback method of laser wave-front manipulation can control THz wave emission [24]. One disadvantage of gaseous targets is low absorption cross section at the incident laser wavelength. Considering the damage issue in solid samples, liquids can be an ideal target for plasma-based THz wave emission since it can be refreshed continuously during experiments. Indeed, experiments with a water film [25; 26; 27; 28; 29] or a line [30] and other liquids such as deionized-water, alcohols, acetone, dichloroethane, or carbon disulfide [31] have been carried out. It is also reported that THz wave emission can be induced in association with X-ray emission from thin water flow and its intensity is enhanced under double-pulse excitation [32]. Water, among various liquids, is considered to be one of the best targets for intense laser irradiation for THz wave emission without serious degradation. Even at high laser repetition rates, liquid target can be used. Indeed, experiments with distilled water and other solutions were reported recently as described, discussions started on the emission mechanisms and characteristics of THz source. Similar to THz wave emission at meV-photon energies, X-ray emission at keV from water and aqueous solutions with solutes or additives such as electrolytes [33; 34] and gold nano-particles [35; 36] has been also reported. Water as a universal solvent is a good target for investigation of mechanisms and for development of THz wave sources for bright emission. Double-pulse excitation is another method for the THz wave emission enhancements [37; 38; 39; 32]. Under double-pulse excitation of Ar gas at delay times of 500 ps, THz emission increases 10-times as compared with single-pulse excitation [37]. In the case of flat water flow, when the thickness of jet was 100-240 \(\upmu\)m at the delay time of \(\sim\)6 ps, THz emission increases 4-times [39], while in the case of water jet of 210 \(\upmu\)m diameter at the delay time of 50 ps, THz pulse energy increases more than 8-times [38]. For thinner flat water flow with 20 \(\upmu\)m thickness, the enhancement was about 100-times in \(\mid\!\! with the wavelength of the incident laser pulses to micro-scale comparable with the wavelength of resultant THz wave emission. In this paper, we report simultaneous measurements for THz wave and X-ray emission from thin water flow under double-pulse excitation with delay times up to 15 ns. Finely synchronized THz wave and X-ray radiation is required for their complimentary usage, for instance, for imaging [42].