LASER TEST OF LASER TEST OF MoMoAND AND Cu Cu MIRRORSMIRRORS

I. Bel’bas, A. Gorshkov, V. Sannikov, K. Vukolov.I. Bel’bas, A. Gorshkov, V. Sannikov, K. Vukolov.

This work is supported by Nuclear science and Technology Department of Minatom RF.This work is supported by Nuclear science and Technology Department of Minatom RF.

INTRODUCTIONINTRODUCTION

The main goal of frequency-operated laser test is an The main goal of frequency-operated laser test is an investigation of the laser damage threshold and lifetime for the investigation of the laser damage threshold and lifetime for the prototypes of diagnostic mirrors manufactured from Mo and Cu prototypes of diagnostic mirrors manufactured from Mo and Cu that will be used in Thomson scattering diagnostics in X-point that will be used in Thomson scattering diagnostics in X-point and divertor zones of ITER. and divertor zones of ITER. In ITER this mirrors must save their high reflectance In ITER this mirrors must save their high reflectance under influence of under influence of ~~101088 of laser shots during the experimental of laser shots during the experimental run. run. Our experiments were carry out for Our experiments were carry out for 2.2 x 102.2 x 1055 laser pulses and will be continued up to total number laser pulses and will be continued up to total number of 1 x 10of 1 x 1066 laser shots. laser shots. The mirrors with different The mirrors with different structures and qualities of Mo and Cu were investigated. This structures and qualities of Mo and Cu were investigated. This laser tests give us an information to predict life-time of laser tests give us an information to predict life-time of diagnostic mirrors. diagnostic mirrors.

Lay-out of laser setup to test Lay-out of laser setup to test laser damage threshold of the laser damage threshold of the

mirrors.mirrors.

ConvexMirror

Profile Meter

Metallic Mirror

Focusing Lens

PMT

to Data AcqusitionSystem (DAQ)

to DAQ

to DAQ

HeNe Laser

ChopperShutter

to DAQ

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.

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 Hzwavelength of radiation –1,06 µ; repetition rate –12.5; 25; 50 Hz

energy of radiation – 5-30 mJ/pulse; pulse duration – 12 nsenergy of radiation – 5-30 mJ/pulse; pulse duration – 12 ns

Frequency operated high power YAG laserFrequency operated high power YAG laser..Two types of regimes: impulse-periodic mode and frequency Two types of regimes: impulse-periodic mode and frequency

operation. Wavelength of radiation –1,06operation. Wavelength of radiation –1,06µ; repetition rate –10 Hzµ; repetition rate –10 Hzenergy of radiation – 1J/pulse; pulse duration – 15 nsenergy of radiation – 1J/pulse; pulse duration – 15 ns

(a) (b) (a) (b) Behaviour of Mo mirror’s optical parameters under multiple laser shots. Behaviour of Mo mirror’s optical parameters under multiple laser shots.

Specular reflection (a) and diffusion scattering (b) Specular reflection (a) and diffusion scattering (b) of polycrystal, single crystal and Mo/Mo mirrors via number of laser shots. 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/cmThe density of laser energy on mirror’s surface is about 1 J/cm22. .

Diffusion scattering coefficients of single-crystal molybdenum mirror Diffusion scattering coefficients of single-crystal molybdenum mirror via number of laser shots. The mirror was tested under different energy via number of laser shots. The mirror was tested under different energy densities and total number of laser shots achieved 220 000 pulses. densities and total number of laser shots achieved 220 000 pulses.

The arrows show the beginning of mirror’s surface damage. The arrows show the beginning of mirror’s surface damage.

(a) (b)(a) (b)Behaviour of copper mirror parameters during laser radiation: Behaviour of copper mirror parameters during laser radiation: (a) – specular reflection and (b) – diffusion scattering . (a) – specular reflection and (b) – diffusion scattering . The density of laser energy on mirror’s surface is about 5.7 J/cmThe density of laser energy on mirror’s surface is about 5.7 J/cm22. . The total number of laser shots achieved 64 000 pulses. The total number of laser shots achieved 64 000 pulses.

(a) (b) (a) (b) The dependence of multipulse laser damage thresholds upon The dependence of multipulse laser damage thresholds upon

the number of irradiating pulses: (a) – for single crystal Mo and (b) – the number of irradiating pulses: (a) – for single crystal Mo and (b) – for diamond turned copper mirror. for diamond turned copper mirror.

The predictive model of laser damage threshold FThe predictive model of laser damage threshold FN N for N for N laser pulses is defined as Flaser pulses is defined as FNN = F = F11x Nx NS-1S-1 [1] [1]

(a) (b)(a) (b) Scan-electron microscope photos of mirror surface.Scan-electron microscope photos of mirror surface.

(a) – molybdenum single crystal, (b) - molybdenum polycrystal.(a) – molybdenum single crystal, (b) - molybdenum polycrystal. Horizontal line gives a scale (10 µm).

(a) (b)(a) (b) Traces from laser radiation on the surface in excess of laser damage Traces from laser radiation on the surface in excess of laser damage

threshold: (a) – Diamond turned Cu mirror, radiation interaction zones in 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 the laser spot on a mirror: an area of melting and a zone resulting from cleaning of oxide films (light area). cleaning of oxide films (light area).

(b) – The destructions on the surface of the mirror with low oxygen (b) – The destructions on the surface of the mirror with low oxygen contamination in the copper. contamination in the copper.

CONCLUSIONCONCLUSION

1. 1. Measured single laser shot damage thresholds are equal to Measured single laser shot damage thresholds are equal to 22±0.4 J/cm±0.4 J/cm2 2 for polished Mo and for Mo/Mo mirrors, 3±0.5 J/cmfor polished Mo and for Mo/Mo mirrors, 3±0.5 J/cm2 2 forfor single crystal Mo.single crystal Mo.2. 2. The single shot laser damage thresholds of the copper mirror The single shot laser damage thresholds of the copper mirror under 10 ns laser pulsed radiation are 15.0under 10 ns laser pulsed radiation are 15.0±3 J/cm±3 J/cm2 2 for diamond turned for diamond turned mirror, 10.1±2J/cmmirror, 10.1±2J/cm2 2 for copper coated mirror and 12.5±1.3 J/cmfor copper coated mirror and 12.5±1.3 J/cm2 2 for for copper with low oxygen contamination. copper with low oxygen contamination. 3. 3. Degradation of metallic mirror surfaces under multiple pulse Degradation of metallic mirror surfaces under multiple pulse laser irradiations are described with good accuracy by predictive model laser irradiations are described with good accuracy by predictive model [1] for multipulse laser damage of metal mirrors up to 1.5x10[1] for multipulse laser damage of metal mirrors up to 1.5x105 5 laser laser shots. shots. 4. 4. Measurements of the degradation of “first mirror” prototypes Measurements of the degradation of “first mirror” prototypes made in “Kurchatov Institute” to a maximum number of repeated stable made in “Kurchatov Institute” to a maximum number of repeated stable laser shots up to 220 000, corresponding to one plasma duration of laser shots up to 220 000, corresponding to one plasma duration of ITER FEAT. ITER FEAT.

[1]. M.F. Becker, C. Ma, R.M. Walser, et al., Predicting multipulse [1]. M.F. Becker, C. Ma, R.M. Walser, et al., Predicting multipulse laser-induced failure for molybdenum metal mirrors, Applied Optics 30-laser-induced failure for molybdenum metal mirrors, Applied Optics 30-36, (1991), 5239-524536, (1991), 5239-5245