Modulations of the plasma scald on the downstream beam

• The downstream modulations of the plasma scald are simulated based on its damage characteristics.
• The typical plasma scald has little influence on the downstream beam.
• The influence of maximum plasma scald on the downstream modulation is significant.
• The uniformity of the beam gets worse as the number of the plasma scald is increased.

The plasma scald damage is the most typical laser damage morphology presented on the laser conditioned HfO2/SiO2 mirrors. In order to provide guidance for defining the acceptance quantitative specification for the plasma scald on these laser conditioned mirrors, the simulations about the downstream beam modulations of the plasma scald are investigated based on its damage characteristics and the diffraction theory. The single plasma scald with the typical size, induced by the laser conditioning, can hardly cause damage to the downstream optics and its influence on the beam contrast can be reduced easily with the increment of the propagation distance. Those with small size and large depth, or those with big size and shallow depth can also hardly induce downstream damage, but the beam contrasts become worse with the increment of the size of the concave part as well as its depth. For a maximum plasma scald observed on the laser conditioned mirror, the intensity modulation peak is close to 1.3 and the beam contrast reaches to 25% even at 2 m downstream from the mirror. For multi plasma scalds, more diffraction rings show up in the downstream beam and further decrease the overall uniformity of the beam. The uniformity of the beam gets worse as the number of the plasma scald is increased. When the density of the plasma scald with the typical size in these 1ωHfO2/SiO2 mirrors reaches 20 per 1 cm2, the beam contrast is less than 6%.