U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 1

Be deposition on ITER first mirrors: Be deposition on ITER first mirrors: layer morphology and influence on layer morphology and influence on

mirror reflectivitymirror reflectivity

G. De Temmerman, M. Baldwin, R. Doerner, D. Nishijima, R. SeraydarianCenter for Energy Research, University of California at San Diego, USA

K. Schmid, C. LinsmaierMax Planck Institut für Plasma Physik, Garching, Germany

L. MarotInstitute of Physics, University of Basel, Switzerland

Work performed as part of US-EU Collaboration on ITER MaterialsPresented to LIDAR Cluster Meeting, 8/8/7

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 2

MotivationMotivation• Deposition of impurities may strongly affect the First Mirror reflectivity

• Studies have focused on the effects of carbon deposition on the reflectivity

Presence of Be in the divertor plasma may mitigate erosion of the graphite targets (and subsequent carbon migration) [1]

Formation of Be-rich layers in direct line-of-sight locations from the plasma (no long-range migration for Be) [2]

• Until now, it was assumed that Be deposition will make the coated mirror behave like a Be mirror beyond a given deposited thickness [3]

1M.J. Baldwin, R.P. Doerner, Nucl. Fusion, 46 (2006), 444; 2M.J. Baldwin et al, J. Nucl. Mater.,337-339 (2005) 5903J.P. Allain et al, 12th meeting of the ITPA TG on diagnostics, March 2007, Princeton, USA

Aim of the experiments:

• Assess the effect of Be deposition on Mo and Cu mirror reflectivity

• Collect data on the “optical quality” of deposited Be layers

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 3

Experimental setupExperimental setup

Magnetic field

• Metallic mirrors (Mo and Cu) exposed to a Be-containing D2 plasma

• ion flux (target): 2.5-3.5·1018 cm-2s-1

• Te~ 6 eV

• ne ~ 2.5-3.5·1018 m-3

• target bias -50 V

• Ttarget~ 700°C (graphite target)

• Be concentration: 0.03-0.1%

Only material eroded/reflected from the graphite target is collected by the mirror

Experiments simulate the situation of mirrors looking at the ITER divertor targets (under the dome for example)

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

• Incidence angle 10°

• Wavelength range 450-950 nm (CCD spectrometer)

Reflectivity measurementsReflectivity measurements

reference

samplerelative I

IR Reference: unexposed sample

from the same material

4

Enclosure window

Calibrated white lightsource

Mirror

Spectrometer with linear CCD array

Optical fiber withfocusing lens

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 5

• Mirror temperature monitored by a thermocouple installed at the backside of the mirror

• Heating of the mirror arises due to plasma radiations

2 cases

Deposited layerDeposited layer

XPS depth profile on a W witness plate exposed with cBe=0.1%

Deposited layer consists mainly of Be

Film thickness 90 nm

Deposition rate 0.005-0.02 nm/s

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

Mirror initially at 240°C

Target bias -50 V]4[from

R

R

Mo

Be

Case of a pure Be layer deposited on a Mo mirror

Reflectivity of coated mirrors (Mo)Reflectivity of coated mirrors (Mo)• Mirrors exposed to similar plasma conditions

Mirror reflectivity strongly decreased by thin Be-rich layers

Layers are quite dark…

4Handbook of optical constants of solids, ed. E.D. Palik, Acad. Press, 1985 and 1991

6

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

Layer morphology and reflectivityLayer morphology and reflectivityMirror initially at 240°C

7

Well developed layer morphology.

Layers deposited at RT exhibit the same aspect.

]4[fromR

R

Cu

Be

4Handbook of optical constants of solids, ed. E.D. Palik, Acad. Press, 1985 and 1991

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

Surface roughness Surface roughness

MaterialTemperature

(°C)Be film

thickness (nm)Roughness

(nm)

W 240-320 90 40.6

Cu RT-120 41 11.3

Cu RT-120 41 21.9

Cu 240-320 75 16

Cu 240-320 55 14.3

Mo RT-120 32 7.6

• Does the layer roughness explain the reflectivity measurements ?

2

2

0

)4(exp

RMS

S

RRR• Influence of surface roughness on

reflectivity: Bennett’s formula

In the present case, roughness of the coated mirrors does not significantly differ from the roughness of the mirrors before exposure. Roughness certainly plays a minor role here…

8

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

Deeper look in the layer morphologyDeeper look in the layer morphology• Be layer 75 nm thick on Cu at 240-300 C ; P=5·103 torr, ne=3·1012 m3

9

• High level of porosity

• Layer seems to be made of small-size crystallites separated by void

• How does it fit with the reflectivity measurements ?

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

Influence of the porosity (1)Influence of the porosity (1)

volumefilm

volumeporef

02

)1(2

effair

effair

effBe

effBe ff

• Bruggeman effective medium approximation: polydispersed spheres in a continuous medium [5,6]

•εeff calculated from this relation. Then n and k of the effective medium are calculated using

22

122

21

nk

kn

i

• Reflectivity of the porous Be coated surface calculated for the couples (n,k)

10

5D.A.G. Bruggeman, Ann. Phys., 24 (1935) 636; 6M.T. Othman et al, J. Appl. Phys., 99 (2006) 083503

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

Influence of the porosity (2)Influence of the porosity (2)• 75 nm of Be deposited on Cu (interface effects and roughness are neglected)

• Film porosity strongly decreases the layer reflectivity !!

• Good agreement between calculated and measured reflectivity

11

400 600 800 10000

20

40

60

80

100 f=0% f=20% f=40% f=50% f=80% Cu mirror+75nm Be

Rel

ativ

e re

flec

tivi

ty (

%)

Wavelength (nm)

Simulations

0 10 20 30 40 50 60 70 80 90 1000

1

2

3

4

n k

n,

k

Porosity (%)

=632nm

400 800 1200 1600 20000

20

40

60

80

Ref

lect

ivit

y (%

)

Wavelength (nm)

f=0% f=10% f=20% f=30% f=40% f=50%

Be 75 nm on Cu

U C S DU niversity o f C alifo rn ia S a n D iego

R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008

T=RT-110°C, P=6mtorr T=250-320°C, P=6mtorr

T=RT-110°C, P=1.5mtorr T=250-320°C, P=1.5mtorr

P

T

• Increasing the pressure during deposition favours the formation of porous layers

• Increasing the surface temperature (to 300 °C) enhances the crystallite growth

• In agreement with the Structure-Zone model for physical deposition

J.A. Thornton, J. Vac. Sci. Tech., 11 (1974) 666

Formation conditions influence porosityFormation conditions influence porosity

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R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 13

• Deposited layers are Be-rich and contain almost no carbon

• Reflectivity of the coated mirrors strongly decreased by Be deposition.

Roughness, porosity Porosity of the layers seem to explain the observations

• Deposition rates observed in PISCES-B experiments in the range 0.005-0.02 nm/s

Formation of porous layers:

• According to the Structure Zone Model [7], high neutral pressure during deposition favours high levels of porosity

• Increasing the surface temperature acts the other way

• What should we expect in ITER ?

7J.A. Thornton, J. Vac. Sci. Technol., 11 (1974) 666

SummarySummary