Abstract.  ELI will be a new scientific infrastructure devoted to scientific research in lasers' field, dedicated to the investigation and applications of laser-matter interaction at the highest intensity level (more than 6 orders of magnitude higher than today's laser intensity).

The ELI project, a collaboration of 13 European countries, will comprise three branches:

1. Ultra High Field Science that will explore laser-matter interaction in an energy range where relativistic laws could stop to be valid,
   
2. Attosecond Laser Science designed to conduct temporal investigation of electron dynamics in atoms, molecules, plasmas and solids at attosecond scale,
   
3. High Energy Beam Science devoted to the development and usage of dedicated beam lines with ultra short pulses of high energy radiation and particles reaching almost the speed of light (100 GeV).

Using DPSSL (diode pumped solid state lasers) as pumping technology PW-class lasers with enhanced repetition rates are developed. Each of the Yb YAG amplifiers will be diode-pumped at a wavelength of 940 nm. This is a prerequisite for achieving high repetition rates. At 10 Hz, the diode stacks will be operate in pulsed mode with a duty cycle of 1 10^-2. The efficiency of DPSSL is inversely proportional to the temperature, for this reason the slab amplifier have to be cooled at a temperature in the range of 100K-170K with a heat flux of 1 MWm^-2.

This paper describe the thermo-mechanical issues of such a device and which will be the extrapolation for the future if inertial fusion would be used as a new energy source. An economic approach has been done to improve the fact that at low temperature the efficiency of high pulsed laser is better than at room temperature.