The ultrashort chirped pulse amplification (CPA) laser technology opens the path to high intensities of 1021 W/cm2 and above in the laser focus. Such intensities allow laser-matter interactions in the relativistic intensity regime. Within the PENELOPE project (Petawatt ENergy-Efficient Laser for Optical Plasma Experiments) a High-peak power operation at the Petawatt level and High-average power performance at a repetition rate of 1 Hz are combined. The scientific goal of laser experiments with PENELOPE is to generate laser accelerated proton and ion beams with energies of more than 100 MeV, relevant for future medical applications.
The PENELOPE project, a fully, direct diode-pumped laser system under development aims at 150 fs long pulses with energies of up to 150 J at repetition rates of up to 1 Hz. A mode-locked oscillator generates pulses of 60 fs, which are stretched to 5 ns. Subsequent amplification in several stages (High-Gain Broadband Amplifier, HBGA I to HGBA III) increases the energy to the sub-J level. The final amplification stages (High-Energy Power Amplification stages, HEPA I and II) are designed to yield 200 J before final recompression takes place. With increasing energy, concepts for amplification change mainly due to the drastically increased required pump power (120 kW and 1.2 MW peak pump power respectively for HEPA I and II). One of the central challenge is to maintain the necessary bandwidth to support ultrashort laser pulses, minimize the B-Integral and to preserve the quality of the laser wave front at the last amplification stages.
Both amplifier sections HEPA I and II rely on fully imaged, reflective 12 extraction passes using spherical mirrors. While HEPA I uses 4, 55 mm diameter gain medium disks pumped at 120 kW for up to 4 ms, this increases to 110 mm and 1.2 MW respectively for HEPA II. The He gas temperature for cooling of the gain medium disks can be adjusted between 200 K and 300 K. PENELOPE relies on purely imaged and fully reflective amplifier designs using spherical mirrors. In case of HEPA I and II those mirrors are aligned on a circular structure compensating the introduced astigmatism. The whole setup is kept under vacuum condition avoiding breakdown near the focal spots and any air turbulences while further reducing accumulated phase distortion.
Stretcher and compressor system
The stretcher setup is based on an Oeffner design where two spherical concentric mirrors are used. A multilayer dielectric (MLD) grating with a groove density of 1760 l/mm and a size of 550×100mm2 is implemented having a diffraction efficiency of 97%. The stretcher is designed for 72° angle of incidence (AOI) and a hard-clip bandwidth of 50 nm at a center wavelength of 1035 nm. A vertical and a horizontal roof-top-mirror are implemented resulting in an 8-pass configuration and a stretching factor of 195 ps/nm.
The compressor is based on a standard Treacy setup comprising of two 940×460 mm2 multilayer dielectric (MLD) gratings. A grating separation of 2200 mm and an input beam diameter of 250 mm was chosen. The compressor operates in a double-pass configuration with a folding end mirror, while a throughput of 88% is achieved for a bandwidth of 12 nm around the center-wavelength. However, the geometrical clipping for the entire hard-clip bandwidth is kept lower than 50%.