Control of turbulence in rectangular pipe flows: application to the cooling of Ultra High Intensity laser amplifiers.

The domain of Ultra High Intensity lasers has experienced a dramatic development these years, thanks to the discovery of the chirped pulse amplification technique invented by the Nobel laureate Gerard Mourou. We can now consider multi-petawatts lasers at high repetition rate, provided that we can master thermal effects.

Indeed, if each laser pulse has a modest energy (10 J for a commercial laser PW today), the high repetition rate results in the heating of the amplifiers, so that only forced convection can evacuate the heat generated in the amplifiers. Moreover, as the thermal conductivity of amplifier crystals is higher at low temperature, cryogenic cooling allows to optimise the homogeneity of the temperature in the amplifier crystal, as well as, sometimes (in the case of Yb-doped crystals), it permits to improve the efficiency of the laser. In the case of a "multislab" amplifier, the laser beam passes through the amplifiers and the cooling flow. It is therefore crucial to master and control the turbulence of the flow: indeed, if the turbulence improves the energy transfer between the amplifier and the cooling fluid, it could degrade the quality of the laser beam by scrambling the phase.

Therefore, this PhD project addresses the problem of the control of a turbulent flow between the amplifier slabs. During this PhD we explore different situations, most often relevant to the cooling of laser amplifiers (in cryogenic conditions, but not only…), also closely related to the onset and control of turbulence in pipe flows, and in boundary layer flows. Thus, in this essentially numerical thesis, we study different situations, between these two extremes:

• the situation in which we try to accelerate the turbulence by devices arranged upstream of the amplifiers;
• and the situation where, on the contrary, it is sought to delay as much as possible the onset of turbulence in the flow, by paying particular attention to the inlet conditions, and possibly by passive devices such as superhydrophobic surfaces (SSH) or large eddy break-up devices LEBUs).

The numerical simulations will be based on the Direct Numerical Simulation (DNS) of the Navier Stokes equations. From this study we expect to examin the potential and main characteristics of passive devices to accelerate turbulence in boundary layer flows, and to increase our knowledge in this rapidly growing field of research, namely SSH and LEBUS. This increased knowledge will help us design cooling systems for ultra high intensity lasers.