Modeling laser conditioning of KDP crystals

Abstract

When potassium dihydrogen phosphate crystals (KH 2 PO 4 or KDP) are illuminated by multi-gigawatt nanosecond pulses, damages may appear in the crystal bulk. One can increase damage resistance through a conditioning that consists in carrying out a laser pre-exposure of the crystal. The present paper addresses the modeling of laserinduced conditioning of KDP crystals. The method is based on heating a distribution of defects, the cooperation of which may lead to a dramatic temperature rise. The conditioning is assumed to be due to a decrease in the defect absorption efficiency. Two scenarios associated with various defect natures are proposed and these account for certain of the observed experimental facts. For instance, in order to improve the crystal resistance to damage, one needs to use a conditioning pulse duration shorter than the testing pulse. Also, a conditioning scenario based on the migration of point (atomic-size) defects allows the reproduction of a logarithmic-like evolution of the conditioning gain with respect to the number of laser pre-exposures. Moreover, this study aims at refining the knowledge regarding the precursor defects responsible for the laser-induced damage in KDP crystals. Within the presented modeling, the best candidate permitting the reproduction of major experimental facts is comprised of a collection of one-hundred-nanometer structural defects associated with point defects as for instance cracks and couples of oxygen interstitials and vacancies.