**5. References**


**4** 

*France* 

Laurent Lamaignère

**Laser-Induced Damage Density** 

*CEA, DAM, CESTA, BP N°2, F-33114 Le Barp,* 

**Measurements of Optical Materials** 

The prediction of the lifetime of optics in high power fusion lasers is a key point for mastering the facilities (Bercegol et al., 2008). The laser damage scenario is seen as occurring in two distinct steps. The first one concerns the damage occurrence due to the first optic irradiation: the initiation step (Feit et al., 1999). Afterwards, damage sites are likely to grow with successive new shots: the growth step (Norton et al., 2006 ; Negres et al., 2010). The damage growth study requires the use of large beams due to the exponential nature of the process, leading to centimetre damage sites. At the same time, damage density measurements on large optics are mainly performed off-line by raster scanning the whole component with small Gaussian beams (~1mm) (Lamaignère et al., 2007), except for a wide range of results using larger beams which permit also a comparison between procedures (DeMange et al., 2004). Tests are currently done at a given control fluence. The goal is to irradiate a known area in order to reveal all the defects which could create damage. This procedure, named *rasterscan procedure*, gives access to the optical damage densities (Lamaignère et al., 2007). This measurement is accurate and reproducible (Lamaignère et al., 2010). In standard tests (ISO standards, 2011), results are given in terms of damage probability. This data treatment is size dependent; on the contrary, it is important to focus

The purpose of the present chapter is to explain how the knowledge of the entire test parameters leads to a comparable and reproducible metrology whatever beam and test characteristics. To this end, a specific mathematical treatment is implemented which takes into account beam shape and overlap. This procedure, which leads to very low damage densities with a good accuracy, is then compared with 1/1 tests using small beams. It is also presented with a peculiar attention on data reduction. Indeed an appropriate treatment of these tests results into damage densities gives at once good complementarity of the several procedures and permits the use of one procedure or the other, depending on the need. This procedure is also compared with tests realized with large beams (centimetre sized as used on high power facilities). That permits to compare results given by specific table-top test

In section 2, facilities using small and large beams are described. The several procedures used are presented in section 3. Developments of data treatment and analysis are given in section 4. Section 5 is devoted to the determination of error bars on both the fluence

the attention rather on the density of sites that damage at a given fluence.

facilities with those really obtained on laser lines (Lamaignère et al., 2011).

**1. Introduction** 

