New perspectives have been opened up in the field of laser–matter interactions due to recent advances in laser technology, leading to laser systems of high contrast and extreme intensity values, where the frontier of maximum intensity is pushed now to about 1022 W/cm2. Many striking phenomena such as laser-acceleration of electrons up to the GeV level, fast moving ions with kinetic energies of several 10s of MeV, as well as nuclear physics experiments have already actuated a broad variety of theoretical as well as experimental studies. Also highly relativistic effects like laser induced electron-positron pair production are under discussion. All these activities have considerably stimulated the progress in understanding the underlying physical processes and possible applications. This article reviews recent advances in the experimental techniques as well as the associated plasma dynamics studies at relativistic intensities performed at the Max-Born-Institute (MBI). Interactions of a laser pulse at intensities above 1019 W/cm2 with water- and heavy-water droplets, as well as, with thin foils are discussed. Rear and front side acceleration mechanisms, particle dynamics inside the dense target, proton source characteristics, strong modulations in proton and deuteron emission spectra, and finally generation of quasi-monoenergetic deuteron bursts are the topics covered in the article.

Small water droplets (20 micron in diameter) have been exposed to intense (∼ 1019 W/cm2) laser pulses in order to study ultrashort (∼ 35 fs) laser pulse driven ion acceleration. Ion emission spectra registered simultaneously in forward and backward direction in respect to the incident laser beam carry similar integral ion energy but show different ion cutoff energies. With simple model estimations on basis of the confined and spherical geometry of the droplet-target, we inferred acceleration field strengths of about (0.7–2) MV/μm. Up to 9% of the incident laser energy is converted to kinetic energy of ions, which have been accelerated to energies above 100 keV and up to 1.5 MeV. A laser pedestal at an intensity of about 10−7 of the peak intensity at 1–2 ns in front of the pulse peak still limits the achievable cutoff energies of emitted protons from the droplet. The observed increase of cutoff energies with an enhanced temporal contrast of the laser pulse is elucidated within a simple acceleration model.