# A Versatile 100 Hz Laser System with Few-Cycle and TeraWatt Pulses for Applications
## 6 Applications
### Development of a Liquid Sheet Target System  
The intended maximum repetition rate for neutron generation is 1 kHz. To support this repetition rate, a liquid-based special target system was developed. Here, two microjets with round nozzles collide and create an ultrathin layer of liquid sheet. A metal cone with a hole in the middle, heated in vacuum and cooled in air, collects the liquid beneath the jets and ensures the liquid recirculation. The shape of the liquid sheet is maintained by a  
Figure 12: The beam size (**a**) and the size of the beam focused by a 2000 mm focal length achromatic lens (**b**), measured on the compressed beam reflected from a 1% sampler.  
pulsation damping system. The length and width of the liquid sheet, depending on the liquid's flow rate, is less than 1.5 mm and 0.4 mm, respectively. The position of the thinnest point of the liquid sheet slightly depends on the position from the top of the sheet as well as on the flow rate, so that the minimum thickness of 200 nm can be reached, in principle, at any flow rate [27].  
Due to the tight focusing requirements with f/2 optics, the target needs to be positioned with a 10-micrometer accuracy to keep the focused peak intensity as high as possible. Therefore, the position stability of the liquid sheet is critical for providing the same accelerated ion yield shot by shot. In continuous operation, the ultrathin, consequently very light, liquid sheet may behave as a swinging pendulum when hit by laser pulses with a given frequency. To reveal this interaction dynamism between our custom-made liquid jet system and the laser pulses having a certain repetition rate, a series of experiments were carried out using the TW laser branch of the system, varying the repetition rate from 1 Hz up to 100 Hz. The pendulum-like behavior depends on the momentum of the pulse, hence on its energy, but not on the intensity. Therefore, the uncompressed laser pulses with an energy of 50 mJ and a duration of few hundred ps were used. In this way, we avoided the generation of particles.  
In this experiment, the liquid sheet formed by two colliding round jets from glass nozzles (10 to 18 um diameter and 0.48 to 1.28 mL/min flowrate) was shot under 45\({}^{\circ}\) incidence angle by the pulses of the laser system mimicking the particle generation arrangement (insert of Figure 13). The displacement of this sheet was measured perpendicular to the liquid sheet surface by a high precision laser ranger (ILD2300-20 by Micro-Epsilon, Ortenburg, Germany). It worked with a 1.5 kHz readout frequency and provided distance data with an accuracy of 0.24 um. The measuring spot size was well below 100 um diameter at the center of the liquid sheet surface. Scattered light of the exciting laser from the liquid sheet was eliminated with a colored bandpass filter for the wavelength of the laser of the ranger.  
The results of the measurement series provided a detailed picture of the liquid target stability behavior during the laser's interaction with liquid. Without any impact of the laser pulses, the stochastic motion ranged a few microns. However, the laser pulses caused a displacement of dozens of micrometers at 33 Hz repetition rate (resonant case), while any further increase in the repetition rate lowered the amplitude (Figure 13).