Influence of the laser prepulse on proton acceleration in thin-foil experiments

Proton and ion acceleration using high-intensity lasers is a field of rapidly growing in-terest. For possible applications of proton beams produced in laser-solid interactions, the generation of beams with controllable parameters such as energy spectrum, brightness, and spatial profile is crucial. Hence, the physics underlying the acceleration processes has to be well understood. After the first proof-of-principle experiments [1, 2], systematical studies were carried out to examine the influence of target material and thickness [3, 4]. To establish the influence of the main laser parameters such as intensity, pulse energy, and duration over a wide range, results from different laser systems have to be compared, since usually each system covers a small parameter range only. Besides these parameters, strength and duration of the prepulse due to amplified spontaneous emission (ASE) play an important role, too [3]. The origin of the most energetic protons is still debated. There are at least two acceleration scenarios able to explain the occurance of MeV-protons. (i) They may come from the front surface of the target, i.e. the side irradiated by the laser pulse [1] or (ii) from the rear surface [2, 4]. Recent results indicate that both mechanisms act simultaneously [5], in accordance with the predictions of particle-in-cell (PIC) simulations [6, 7, 8]