## 2 Experimental Setup and Control  
In general, good experimental conditions in laser-plasma interaction experiments require an optimal energy transfer between the incoming laser beam and the used target. Such a laser-plasma coupling efficiency is a key parameter to enhance the performance of a laser-plasma accelerator. This can be satisfied if both the laser beam focusing element, typically an Off-Axis Parabola (OAP), and the target positioner are handled carefully in order to place the target precisely into the position of the previously optimized focal spot (high encircled energy). The full procedure usually consists of three consecutive steps: (i) Definition and referencing of the interaction point position; (ii) OAP focal spot optimization and its positioning at the interaction point; (iii) Target alignment in 3D and its positioning at the interaction point. Alignment procedures are typically available and well described in technical manuals at high power laser facilities. Low repetition rate laser beams, with a time interval between shots of several minutes, are typically focused and aligned on targets before each shot using a low power mode (unamplified version) of the laser beam. On the other hand, high repetition rate facilities require overall alignment of the target frames prior to the experimental run, due to the fact that the focus position must be optimized. Subsequently, all the targets must be pre-aligned before each irradiation sequence (at least few hundreds of shots) with high accuracy. Therefore, standard focal spot alignment procedure needs to be combined with a reliable target positioning system, which is capable of precise and repeatable operation, as described below. Naturally, overall pointing stability and vibration-free transport of the beam up to the focusing optics was ensured by careful engineering of both the mirror mounts and the vacuum systems, typically using decoupling between vacuum vessels and optical breadboard. Furthermore, additional potential unwanted effects, such as thermal lensing of optical components and slow laser beam drift, must be compensated. The interaction point in the experimental chamber must be precisely defined since ion/plasma diagnostics and other experimental equipment (e.g., magnetic systems) are pointed at it, and it is considered a reference point for ideal observation of the laser-target interaction and further utilization of the accelerated particles. The interaction point definition relies on the mechanical precision of the used components and is usually realized by placing a reference tip in the desired position. The tip is usually a conically sharpened (stainless steel) wire reaching a diameter of a few tens of micrometers. The alignment of different detectors with the interaction point itself is usually carried out using a small alignment laser beam (laser pointer perpendicularly attached to the detectors case front surface) propagating from the detectors towards the expected interaction point. This simple check ensures that the detector is oriented to the interaction point, and it has a clear view of the interaction. The intersection of the different detector alignment beams marks the interaction point where a reference tip is placed. Since the laser beam path in the experimental chamber was typically aligned horizontally (the beam axis is at the same height above the breadboard), the interaction point also remained at this defined height. Subsequently, the laser focal spot must be positioned in this point. As soon as the interaction point is precisely defined and marked with a reference object, the monitoring system, consisting of a microscope objective with a given magnification, can be pre-aligned with the laser optical axis (the direction is given by the off-axis angle of the OAP) and pointed at the reference object placed at the objective's working distance. The microscopic objective is then fixed, and the OAP focal spot is optimized and positioned onto the reference tip.