MOLIBDENUM MIRRORS WITH COLUMN NANOGRAIN REFLECTING COATING AND EFFECT OF ION-STIMULATED DIFFUSION BLISTERRING

RRC «Кurchatov Institute»

А.V. Rogov, К.Yu.Vukolov

Influence of mirrors structure to physical sputtering resistant

Purpose of work: Design of experimental samples of Mo NC-mirrors, investigate its optical properties and stability to sputtering in ITER conditions.

Single crystal (SC) polished mirrors have the better resistant to sputtering .

Technological limit for SC mirrors size. For Mo SC mirror: 140 mm

As perspective large size mirrors for ITER we suggest :mirrors with reflected layer, formed by densely packed column nanocrystallites, oriented perpendicular to substrate surface, named: nanocrystal (NC) mirrors

a

b

constant of R during sputtering; optical quality equal to substrate; max spattering depth time stability.

1

2

[V.S. Voitsenia et. al.3-th Conference Fusion and Plasma Phis., 2003.].

Structure of NC mirror: (a) – reflecting layer; (b) – polished substrate.

Structure requirements:

Design and structure of experimental samples Mo NC-mirrors .

Substrate: single crystal Мо;Evaporated material: Мо;Method: DC- magnetron evaporation;Mirror size: 21 mm ×1.5 mm;Thickness reflecting layer: 1.5 m;Crystallites size: 50×100×1500 nm;

2

1 m

3

1

Column nano-structure

Crystallites size across

[Thornton J.A. // J. Vac. Sci. Technol, 1974, Vol. 11, № 4].

Parameters of Mo NC - mirrors:

Selection of modeling conditions for sputtering in ITER

• Critical diagnostic system: MSE and CXRS (= 470656 nm; Size: 500x300, 350x90 mm)

• Charge-exchange atoms: D, T, He, Be, C• Energetic distribution of charge-exchange

atoms on the first wall in ITER:• Model for calculate components composition:

nBe=nC

nHe= 0.1(nD+nT) Z = 1,69

Applied calculation model take account of:

Basic data for experimental simulation sputtering of first mirror in ITER:

• Fluens during 1 year (N=1000, Т=500 s): Ф = 1025 atoms m-2;• Middle energy charge-exchange atoms: E = 250 eV;• Erosion on account heavy elements (Be, C): 20%;• Optical probe irradiation : = 632,8 nm (He-Ne- laser)

[Mayer M. e.al. - Plenum Press, New York, 1998]

1

2

First wall

(E

), a

t m

-2 c

-1

E, eV

Modeling device – DC magnetron sputtering system

• Sputtering in gas mixture: D2 + small part of air (N2 + O2), for modeling heavy addition in ITER;

• Middle energy of sputtering ions (D+ , N+, O+):

• Conservation of fraction material sputtered by light components (D, T, He charge-exchange atoms in ITER and D+, D2

+ in magnetron):

DC-magnetron schematic diagram:1- magnetic field; 2 – ionization zone;3 - cathode (sputtering mirror); 4 – erosion zone, 5 – anode.

1

eVEE magiter 250

776,022

magtot

mag

D

mag

D

mag

D

mag

D

itertot

iterHe

iterHe

iterT

iterT

iterD

iterD

Y

YYY

2

Ions energy distribution in magnetron and

charge-exchange atoms in ITER

Ions fluence (F) distributions due to mirror diameter with different sputtering times (T)

3Modeling parameters:

Reflectance R, diffusion scattering D ( =635 nm) and surface morphology dependence from ions fluence and sputtering time Т for SC and NC mirrors.

Macroblisters on NC mirrors after sputtering during Т=135 min.10 m300 мкм10 m300 m

Dependence blisters evolution against ions fluence. Sputtering time Т = 10 min.

5 m 5 m 5 m

2 3 4

Reflectance R, diffusion scattering D ( =635 nm) and surface morphology dependence from ion fluence and sputtering time Т for SC mirrors.

1 m

1

Microblisters - Т = 10 min - Т = 135 min

F1,21023 ions/m2F1,21024 ions/m2 F2,01024 ions/m2

F, ions/m2F, ions/m2DR

Main conclusions: • Nanocrystal Mo mirrors have high stability to sputtering,

similar to single crystal Mo mirrors

• Large mirrors can be manufactured on polished polycrystalline substrate by means of proposal technology

• The effect of ion-stimulated diffusion blistering was discovered. The presence of surface film, with properties distinct from substrate properties, required for this effect

• Optical quality for nanocrystal and single crystal Mo mirrors was mainly determined by blistering in fluence range up to 1025

ions/ m2, that corresponds to sputtering during 1 year on the first wall in ITER

• The ion-stimulated diffusion blistering can be reason for dust formation in ITER

• The additional study of diffusion blistering is needed to understand this effect possibility in ITER