Transcript LASER TEST OF Mo AND Cu MIRRORS

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.
to Data Acqusition
System (DAQ)
HeNe Laser
Chopper
Shutter
Convex
Mirror
Focusing Lens
ser
a
l
G
YA
Profile Meter
to DAQ
to DAQ
PMT
Metallic Mirror
to DAQ
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.
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.
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

(a)
(b)
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]

(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 3036, (1991), 5239-5245