Considerations on thermal blooming in relation to modern HEL systems

Research into thermal blooming effects in laser beam propagation has been initiated within ten years after the first
demonstration of a laser. Results of these studies have been published in excellent review papers. Now several decades
later, developments in laser technology are bringing High Energy Laser (HEL) systems closer to actual operational
application and to power levels where thermal blooming might affect the system performance. In support of understanding
HEL performance and development of potential mitigation measures, it is of interest to be able to estimate the limits at
which a system will enter conditions where thermal blooming will affect performance. This paper discusses which parameters influence thermal blooming in a HEL system in several realistic scenarios. The influence of these parameters on the laser beam are illustrated analytically for a collimated HEL beam. We provide a simplified derivation of the distortion parameter as presented in the review papers. This distortion parameter will be used to explore the effect of thermal blooming on three generic HEL systems: short range (I), medium range (II) and large range (III), with corresponding power levels For more complex beam shapes, thermal blooming effects on the power on target are analysed with wave optics simulations. Thermal blooming critically depends on the atmospheric conditions. More specifically: the molecular absorption, the wind speed component perpendicular to the beam and the atmospheric transmission losses due to scatter and absorption by aerosols and molecules. The averages of these parameters and their variability over location and time, have been analysed by exploring a data base of weather parameters collected over the whole of Europe in 2019. For a specific location in the Mediterranean their effects on thermal blooming in a generic large range HEL system are calculated. The combined effect of turbulence and thermal blooming has been investigated as well, to explore where one or the other effect might dominate and how the combined effect degrades HEL system performance. Finally some considerations on mitigation of thermal blooming effects by system design choices or change in wavelength are given.