LASER TEST OF Mo AND Cu MIRRORS I. Bel’bas, A. Gorshkov, V. Sannikov, K. Vukolov. This work is supported by Nuclear science and Technology Department of Minatom RF.

INTRODUCTION The main goal of frequency-operated laser test is an investigation of the laser damage threshold and lifetime for the prototypes of diagnostic mirrors manufactured from Mo and Cu that will be used in Thomson scattering diagnostics in X-point and divertor zones of ITER. In ITER this mirrors must save their high reflectance under influence of ~108 of laser shots during the experimental run. Our experiments were carry out for 2.2 x 105 laser pulses and will be continued up to total number of 1 x 106 laser shots. The mirrors with different structures and qualities of Mo and Cu were investigated. This laser tests give us an information to predict life-time of diagnostic mirrors.

Lay-out of laser setup to test laser damage threshold of the mirrors. Due to moveable focusing lens the diameter of laser spot on mirror surface can be changed. The power density on the surface is variated by changing of spot’s size or the total energy of laser radiation. The shutter is controlled by computer and is used to interrupt the influence of YAG laser on mirror. The coefficients kspec (specular) and kdiff (diffusion) measured simultaneously after hahdpicked quantity of the YAG laser shots. Lay-out of laser setup to test laser damage threshold of the mirrors.

Multipulse YAG laser with three frequency regimes Multipulse YAG laser with three frequency regimes. wavelength of radiation –1,06 µ; repetition rate –12.5; 25; 50 Hz energy of radiation – 5-30 mJ/pulse; pulse duration – 12 ns

Frequency operated high power YAG laser Frequency operated high power YAG laser. Two types of regimes: impulse-periodic mode and frequency operation. Wavelength of radiation –1,06µ; repetition rate –10 Hz energy of radiation – 1J/pulse; pulse duration – 15 ns

The density of laser energy on mirror’s surface is about 1 J/cm2. Behaviour of Mo mirror’s optical parameters under multiple laser shots. Specular reflection (a) and diffusion scattering (b) of polycrystal, single crystal and Mo/Mo mirrors via number of laser shots. The density of laser energy on mirror’s surface is about 1 J/cm2.

Diffusion scattering coefficients of single-crystal molybdenum mirror via number of laser shots. The mirror was tested under different energy densities and total number of laser shots achieved 220 000 pulses. The arrows show the beginning of mirror’s surface damage.

(a) (b) Behaviour of copper mirror parameters during laser radiation: (a) – specular reflection and (b) – diffusion scattering . The density of laser energy on mirror’s surface is about 5.7 J/cm2. The total number of laser shots achieved 64 000 pulses.

(a) (b) The dependence of multipulse laser damage thresholds upon the number of irradiating pulses: (a) – for single crystal Mo and (b) – for diamond turned copper mirror. The predictive model of laser damage threshold FN for N laser pulses is defined as FN = F1x NS-1 [1]

Scan-electron microscope photos of mirror surface. (a) (b) Scan-electron microscope photos of mirror surface. (a) – molybdenum single crystal, (b) - molybdenum polycrystal. Horizontal line gives a scale (10 µm).

(a) (b) Traces from laser radiation on the surface in excess of laser damage threshold: (a) – Diamond turned Cu mirror, radiation interaction zones in the laser spot on a mirror: an area of melting and a zone resulting from cleaning of oxide films (light area). (b) – The destructions on the surface of the mirror with low oxygen contamination in the copper.

CONCLUSION 1. Measured single laser shot damage thresholds are equal to 2±0.4 J/cm2 for polished Mo and for Mo/Mo mirrors, 3±0.5 J/cm2 for single crystal Mo. 2. The single shot laser damage thresholds of the copper mirror under 10 ns laser pulsed radiation are 15.0±3 J/cm2 for diamond turned mirror, 10.1±2J/cm2 for copper coated mirror and 12.5±1.3 J/cm2 for copper with low oxygen contamination. 3. Degradation of metallic mirror surfaces under multiple pulse laser irradiations are described with good accuracy by predictive model [1] for multipulse laser damage of metal mirrors up to 1.5x105 laser shots. 4. Measurements of the degradation of “first mirror” prototypes made in “Kurchatov Institute” to a maximum number of repeated stable laser shots up to 220 000, corresponding to one plasma duration of ITER FEAT. [1]. M.F. Becker, C. Ma, R.M. Walser, et al., Predicting multipulse laser-induced failure for molybdenum metal mirrors, Applied Optics 30-36, (1991), 5239-5245