## 3 Target Delivery System
Accurate alignment of each individual target before the laser shot sequence is one of the most important parts of the experimental run preparation since the accurate positioning of the target into the laser focal plane heavily affects the laser-target interaction conditions and, consequently, the laser-plasma acceleration process. The alignment procedure consists of several key steps to be followed: (i) defining the interaction point by placing a reference object (sharp pin) in the desired position; (ii) aligning a monitoring system that monitors the interaction point; (iii) fixing the monitoring system to use it as a reference for the next steps of target alignment; and (iv) removing the tip and placing the PTDS to allow its large metallic frame to be at the same position of the tip. Once the frame is set according to the imaging system focal plane (typically with an accuracy of 10-20 \(\upmu\)m), the alignment of each individual target includes recording of its  
Figure 1: A picture of (**a**) the planar target delivery system and (**b**) the graphical user interface for control of the target system. unique coordinates (x, y, and z, when the target is in focus). The other two coordinates (pitch and tip) are usually common for the whole target frame. Damaged or broken targets can be marked and automatically bypassed during the high repetition-rate laser shot sequence. Alignment of one frame with 100 targets generally takes about 10 min and usually can be done in advance. Ultimately, the software provides a recorded table of target coordinates that is used for automatic target switch to the subsequent position by means of an external trigger sent by the laser system during the experimental run. It is crucial to prevent any hit of the target holder frame by the laser beam during the shot sequence. For this purpose, the control unit of the PTDS was equipped with a trigger input. The target swapping starts right after the incoming trigger is delivered with the laser shot. An output feedback signal indicates the completion of the target tower motion necessary to set the fresh target at the interaction point. A stable performance of the system at the repetition rate of 1 Hz is demonstrated in Figure 2. The PTDS starts motion right after the trigger (blue signal) associated with the laser shot. The finalization of alignment for the new target is indicated by the falling edge of the output feedback (orange signal) before the next laser pulse arrives. The feedback output provides an estimated value of the potentially reachable target repetition rate during a laser shot sequence operation of about 1.25 Hz. Optical snapshots of the target positions were taken by means of the monitoring system during target renewal (see Figure 3). Monitoring system is supplied with LED placed on monitoring system to improve image quality. In this case, the target frame moves vertically from the upper to the lower target. The image becomes sharp after 800 ms since the target starts its motion, hence clearly indicating that a new target is set to the pre-aligned position and ready for the following shot. This observation is consistent with the feedback signal from the target tower. In fact, the monitoring system can hardly be used for the detection of a new target in the preset position, since image transfer from the camera and online analysis are demanding, with respect to data transfer infrastructure and computing power. Instead, analog feedback can be a reference for the data acquisition system. As a part of the control system, such a signal prevents potential damage of equipment by the laser beam during the target frame motion. Another important property of the PTDS to be verified is the target position reproducibility when the targets are switching automatically. An accuracy test, with the aim of evaluating potential misalignment of different targets during a shot sequence, was performed using the target tower coupled with the imaging system.