If we focussed all the solar radiation coming on earth on a tiny spot as small as the cross section of a human hair we would obtain intensities as high as 1022 Watts per cm2. However, with any optical lens one can only achieve the surface intensity (or temperature) of the thermal radiator itself which corresponds to 104 W/cm2 or 6000 K when focussing the sun. A laser can be focussed down to spot sizes as small as the wavelength of light (several micrometers). With a pulse duration of 150 fs and pulse energy of 150 J (similar to a potential energy when climbig one step of a staircase) a power of 1 Petawatt (1015 W) can be achieved. This corresponds to the total power of all power plants together in Europe. Our lasers deliver those power only for femtoseconds and once a second. Therefore, the power consumption of PENELOPE's pumping diode system remains at the Kilowatt level.
Petawatt ENergy-Efficient Laser for Optical Plasma Experiments
The PENELOPE system is a direct diode pumped laser with an overall pump power of 1.3 MW, while the peak pump energy for the system is fixed to 5 kJ within a maximum pump pulse duration of 4 ms. An oscillator generates pulses of 60 fs, which are stretched by 200 ps/nm with a hard clip of 50 nm resulting in 4 ns (FWHM) pulses. Subsequent amplification in several stages (High-Gain Broadband Amplifier, HGBA I to III) increases the energy to the joule level. The two High-Energy Power Amplification stages (HEPA I and II) are designed to increase the energy to 200 J before final compression takes place. While the first amplification stages rely on an active-mirror approach, the last two amplifiers work in transmission with several He-gas cooled gain medium slabs. Yb3+:CaF2 is used as gain medium throughout the laser in order to maintain the necessary pulse bandwidth for 150 fs pulses. Recent activation of HEPA I showed 12.6 J together with a spectral bandwidth supporting 150 fs pulses. The impact of amplified spontaneous emission (ASE) on the energy storage of both large aperture Yb:CaF2 amplifiers is estimated with the HASEonGPU code and found to be negligible.
Laser Amplifiers under Vacuum Conditions
Both final two main amplifiers rely on 12 fully imaged, reflective extraction passes using spherical mirrors. The optical setup of HEPA I and II is cunstructed and aligned within vacuum chambers (total length: 6.5 m and 13 m) in order to prevent breakdown near the focal spots and any air turbulences. Spherical and planar mirrors are aligned on circular structures, while the full circular setup compensates the introduced astigmatism. Current work is dedicated to activate the cooling solution for the final amplifiers supporting a 1 Hz operation. Furthermore, the construction of HEPA II was initiated with the installation of its vacuum vessel.