K. Vukolov

Frequency laser damage of Mo mirrors

Abstract Laser tests were performed on first mirror (FM) prototypes manufactured from single crystal and polycrystal Mo and with reflective Mo coatings on Mo substrate. Data on laser damage thresholds under the influence of the frequency-operated pulsed YAG laser (1064 nm wavelength, 12.5 Hz repetition rate, 10–30 mJ per pulse energy, 12 ns pulse duration) were obtained for single laser shot and after about 2.2×10 5 laser shots. The output beam of the laser operating in the TEM oo mode has a Gaussian profile. The single shot damage thresholds were measured and were equal to 2±0.4 J/cm 2 for polycrystal Mo and Mo/Mo mirrors, 3±0.5 J/cm 2 for single crystal Mo mirror. The long-time effect of frequency-operated YAG laser on mirror reflectivity was studied. It was shown that diffusion scattering is a very sensitive instrument to check the surface condition. Different types of Mo mirrors show various values of multipulse laser damage threshold. It may be connected as with initial quality of the surface and with different structures of the reflection layer for investigated samples of Mo. The degradation of a single crystal Mo mirror under multiple pulse laser irradiation is described with good accuracy by a predictive model for multipulse laser damage of metal mirrors [Appl. Optics 30–36 (1991) 5239] up to 1.5×10 5 laser pulses.

Experimental Study of First Mirror Degradation Under Influence of Iter Relevant Conditions

Being placed in a straight view of burning plasma the First Mirror (FM) is one of most critical elements of ITER diagnostics. The sputtering by charge exchange atoms (CXA) and the deposition of contaminating films and dust on a mirror surface will be the main reasons of FM reflectivity degradation under ITER conditions, and for laser diagnostics the thermomechanical stresses occurring at every laser shot may accelerate the mirror destruction [1]. The materials traditionally used for optical mirrors are not suitable for building-up the ITER diagnostic mirrors. The use of mirror with reflective coating from metals with low sputtering yield and sufficiently high reflectivity (e.g., Rh, Mo, W, Ag) on metallic substrate is one of the possible variant of FM choice for a survival in extreme ITER environments [2]. The production practice of mirrors with Rh and Mo coatings on SS and Mo substrates (Rh/SS and Mo/Mo, correspondingly) by means of sputter deposition of reflective coating on polished substrates is elaborated using a dc planar magnetron [3]. The influence of main damage factors on FM prototypes and experiments on a remote cleaning of mirrors from carbon films by pulsed laser radiation are considered in the paper.