## 3 Target Delivery System  
Modern systems, which are capable of delivering laser energy on target with several tens of Joules at high repetition rates (1-10 Hz) require at least several thousands of targets per hour since the target is destroyed after each laser shot. Moreover, there are strict technical requirements for the target system, such as fast target refreshing, positioning, and alignment. Currently, a wide range of targets has been developed for laser-plasma acceleration, such as thin solid foils, tapes [19], gas [20] or liquid jets [21,22], clusters [23], cryogenic targets [24,25], and liquid crystals [10,26]. Hereafter, we aim to address the need for high repetition rate planar thin target delivery as a continuous sheet of material (thin foil) that is refreshed by a motorized stage. A picture of the planar target delivery systems (PTDS) used in our experimental tests is presented in Figure 1a. The main part of the device is modular to allow holding of various types of target frames. The system can accommodate 900 foil targets, with an average distance of 5 mm between neighboring positions. The foils are secured using a special frame system made of two metallic plates that hold the foils in between them. Both plates contain a matrix of conical holes of about 1 mm in diameter. The conical shape helps to avoid the shock wave propagation from one target to the neighbors, permitting use of all the available targets during the laser irradiation. Moreover, this design provides good protection of individual targets from excessive evaporation or overheating effects occurring during laser-target interaction, or contaminations from neighboring targets after the shot. The current modification of the system uses nine of these frame holders and is able to hold any kind of foil (metallic, semiconductor, or plastic targets) with a very broad range of possible thicknesses (from foils of few tens of nanometers to few hundreds of micrometers thick). A motorized system of PTDS has five degrees of freedom (x, y, z, pitch, and tip) and allows the device to align and to hit each individual target at 1 Hz with an accuracy better than 10 \(\upmu\)m on each axis. To control the position of the device to avoid unwanted clashing with neighboring devices, each actuator is supplied with two linear limit switches to define the range of motion of the corresponding axis. The used motors are suitable for operation in a harsh environment where very strong electro-magnetic pulses (EMP), associated with high intensity laser-matter interaction, may affect the operation of electronics and high precision motorized systems. To reduce electrical discharge effects induced by laser-target interaction, the target holder is decoupled from the other metallic parts of the device with a polytetrafluoroethylene plate. The target holder itself is directly grounded to the vacuum chamber, thus the electrical conductor runs directly from the target plate to the ground. The use of sophisticated optical systems imposes additional stringent requirements in terms of cleanliness levels of the vacuum systems. All PTDS parts, such as frames, motors, actuators, and limit switches, are made of high vacuum compatible materials. The connection cables are encased in copper braid with Kapton insulation to reduce the outgassing of particles detrimental to the vacuum system cleanliness. A custom-made software is used for manual positioning of individual targets during the alignment phase and allows automatic refreshing of targets during a laser shot sequence. The graphical user interface shown in Figure 1b allows setting of either absolute or relative target positions, using five degrees of freedom with an accuracy of 5 \(\upmu\)m and recording an individual position of the target for the high repetition-rate laser shot sequence.