J. Plasma Fusion Res. SERIES, Vol.3 (2000) 270-273

Effect of Thin Film Goating on Reflectance ofIn-Vessel Metallic MirrorvoITsENYA Vladimir*, JACOB Wolfgangr, SAGARA Akio2, BARDAMID Alexandra3,
BONDARENKO Vladislav, FUKAREK Wolfgangr, KONOVALOV Vladimir, MASUZAKI Suguru2,
MOTOJIMA Osamu2, POPERENKO Leonid3, SATO Kuninori2, TSUZUKI Kazuhiro2
and VINNICHENKO Mykola3
Institute.of Plasma Physics NSC KIPT. 61108 Kharkov. Ukraine'Institute for Plasma Physics, Garching, Germany
2National Institute for Fusion Science. Toki. 509-5292Ja.pan3Kiei Universitv, Kiev. Ilkraiine
4''F o r s c hu g s ze nt rum Ro s s e ndo rf, D re s de n, G e rmany(Received: l8 January 2000 / Accepted:23 May 2000)

Abstract
It was observed in many fusion experiments the appearance of a not absolutely transparent deposit
on diagnostic windows of fusion devices. The deposit composition was shown to correlate withcomposition of materials of the most strongly eroded in-vessel components, or with the material of wall
coating used for improving vacuum conditions and suppressing the influx of heavy atoms into the coreplasma. The appearance of analogous deposit on diagnostic mirrors located inside the vacuum vessel will
result in changing the reflectance which has to be taken into account when measuring and registering theradiation that comes out of plasma by use of these in-vessel mirrors. In the present paper the role ofcontaminating film on mirror reflectance is analyzed.

Keywords:
in-vessel mirror, contaminating film, reflectance

1. Introduction

graphite protection of wall areas subjected to strongest
plasma impact. Thus the coating appearing on vesselWith an aim to suppress the metal atom influx fromthe vessel walls of fusion devices into the plasma and towalls and other inner components is the amorphousboron-carbon film with some amount of hydrogen (a-Blimprove discharge performance, the protection of walls

by low-Z materials is widely used. After successfulprocedure in great many fusion devices. Several years

C:H) [3]. On those inner components that are notsubjected to strong plasma impact, like diagnostic
windows and the in-vessel mirrors, the film grows not
only as a result of boronization but also during work

later on this device for the first time the boronization

discharges [2,4].

procedure has been used to coat the vessel walls with aboron-based film [2], and later the boronization came up
to take carbonization place in many fusion devices. In

There are two publications with experimental data
concerning effects of deposit on a transmissivity ofdiagnostic window in a fusion device [5,6]. In bothresults with carbonization of tokamak TEXTOR [] thismethod of carbon film coating became a routine

nowaday practice, the boronization is combined with"Corresponding author's e-mail: voitseny @ ipp.kharkov,ua
2'70@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Voitsenya V. et al., Effectof Thin Film Coating on Reflectance of In-Vessel Metallic Minor

cases the vessel walls were regularly carbonized. In [6]boron-carbon film, a-811 - *y/C^:H, deposited in different
conditions were measured by ellipsometry for several xvalues. In Table I such data at the wavelength 632.8 nm
are presented for three samples with different C/B ratio.

the chemical composition of deposited film wasmeasured and it was found that the film is ofpolyac ry lonit rile type.

As seen, increasing the carbon component of film
(samples #2 and #3) reveals a significant change of n

At the same time, effects of deposit on reflectanceof mirrors situated inside the fusion device chamber
were not described in publications up to now, on our
knowledge, and in this paper we are trying to analyze
this role of deposit. We limit ourselves by study of

but not k values.

The thickness of carbon filrns and their opticalconstants were measured at the wavelength 632.8 nm
and for the clean Mo surface - in a wide spectral range.
For these films, n and k from ellipsometric measurements were slightly different for different thickness, inspite of identical depositing conditions, and therefore theaverage n = 2.7 and ft = 0.5 values were used whencomparing measured R"fiL,) with calculation for thissubstrate-film pair.

boron-carbon film, as such films are most frequently
met in large-scale fusion experiments. The wavelength
range studied is 200-1100 nm, which is of interest forspectroscopy and Thomson scattering methods ofplasma diagnosing.

2. Ettect of Thin Film on Specular
Reflectance (Experiment and Calculationl
It was shown (e.g. t7l) that effective reflectance,The results of measurements for SS mirror coatedwith boron film (open points) and calculations forsample #2 (solid lines with solid points) presented inFig. I demonstrate a quite reasonable qualitativeagreement. Calculations for samples #l and #3 give
similar dependencies with a little shifted positions ofminima tor R"xQ) curves. The data of Fig. 1 indicatefi(X.), of metal mirror coated by a partly-transparent filmof any given thickness (d) can be calculated using
optical constants, i.e., refractionn(h) and extinction k(i)indices, of both - the substrate material and the film. In
the present study for comparison with measured
dependencies R(1,), the calculations were provided using

the following formula [8], which gives the effective
reflectance of the metal mirror coated with thin partly
transparent film at the normal incidence of light:

Table l.Characteristics of boron films deposited under
different conditions.
SampleA; exp(ft )+ Al exp ( -/c )+ Ai cos(n ) + Ao sin (n )^"Ai exp(/c)+A, exp(-ft#1)+A, cos(n)+ Ai sin(n )'where ft = (4nki)l L, n = (4nnp)l ).,A11 = [(1 * nr)2 + kr'l I@t + n2)2 + (kt + k)21,
A2x = l(l r n)2 + kr']1 [@, - nz)2 + (k, - k)2],#3

Af = 2[(l - n? - k,') (rr' + kr2 - n22 - k22)

nk

300

2.58

0.058

0.15

135

2.37

0.065

0.48

147

2.190.065

c{Bd,ratio, xnm0Boron film on SS mirror+ 4k(ntkz - nzk)1,Af = 4[(l - nr2 - ktz) (nrk2 - nzk)

! k{n12 + kr2 - n22 - k2\1.

oo

The complex indices n1 - iky and n2 - it2 represent data
for a film and substrate, correspondingly.
Effect of thin hlm on specular reflectance at nomal
incidence was checked using two stainless steel (SS)oC'ooomirrors (of 316 type) coated with boron film, andmolybdenum mirror coated with carbon film. The SS
samples were coated up to -7 nm and -17 nm film
thickness in the surface modification test stand (SUT)ooE

200 300 400 500 600 700[9]. The Mo mirror was coated by means of vacuum arc

Wavelength, nm

between graphite electrodes, and the deposit thickness at
different parts of mirror surface was -23 nm, 35 nm andFig. 1 Effect of boron film on reflectance of SS mirror.
Open points - results of measurements, lines withsolid points - calculations.

56 nm.

The spectral dependencies of optical constants of a271

Voitsenya V. et al., Effect of Thin Film Coating on Reflectance of In-Vessel Metallic Minorat 7 = 6323nm.

As Fig. 2 shows, the agreement between measured
and calculated data is more or less reasonable only forthe thinnest film (d = 23 nm), but for two other film
thicknesses the difference between calculated andmeasured R(/r) is not small. The possible reason of thiscan be the difference between the real spectral
dependencies of optical constants for carbon film

oocot)
oEE

deposited in this experiment and for carbon film
investigated by authors of [0].

o.:t€uJ

Nevertheless, it may be concluded that a formul R
can indeed be used for estimation of effective
reflectance of mirror coated by contaminating film, if
optical constants of both materials (film and metalsubstrate) are known.

As an example of such estimation, in Fig. 3 areshown results of calculations using the above-mentioned

Fig. 2 Effect of different thickness carbon film on

formula, of effects of boron film on copper substrate. As
seen, in the visible part of spectrum a strong distortion

reflectance of Mo mirror. Closed points - resultsof measurements, lines with open points -calculations.

of initial reflectance R(,i,) takes place already with d =10-20 nm. At the same time, the typical thickness of BC film deposited on the inner walls of fusion devices
during every boronization significantly exceeds 20 nm,
as it follows from publications presented at 9'h and l0'h
Conferences on Plasma Surface Interaction in Fusion
Devices [correspondingly, J. Nucl. Mater. vol. 1761177

sEgocG860

and vol. 196-1981.

oieo.:Ezo

3. Discussion

UJthin (< 10 nm) boron-carbon film can change strongly

The above shown results indicate that the deposit of

400 500 600 700

the effective reflectance of a metal mirror in visible and800

nearest UV regions. Note that film appearance on the in-Wavelength, nm

vessel mirror of a fusion device have to deteriorate themirror reflectivity much stronger than the transmissivity

Fig. 3 Effect of boron film coating on reflectance ofof diagnostic window. This is because in practice theeffective film thickness for reflected light is about 3
times of the real thickness: the light passes the film twotimes and not perpendicular to the surface. The effect ofcopper mirror.

that boron film effects on a SS mirror reflectance areespecially strong in the UV part of spectrum.
The measured and calculated RAL) dependencies
for Mo substrate coated with carbon film of threedifferent thickness are presented in Fig. 2 together withdata for the clean Mo surface. It is seen that carbon film
influences drastically the effective reflectance of C/Mo
mirror in the whole spectral range where measurements
were provided. Because of lack of n and ft measurements for our carbon film in the wide spectral range, thespectral dependencies for both indices from [10] wereused, corrected in accordance with the differences founddeposit growth on effective reflectance depends on thelight wavelength, chemical composition of film and its
thickness.

Principally, the change of reflectance can bepredicted and taken into account if both the film
thickness and film chemical composition are controlled,on the one hand, and if the correlation between film

composition and optical properties is known fromindependent preliminary measurements, on the otherhand. However, the latter is difficult to realize incommon, due to strong influence of film composition on272

Voitsenya Y . et aI., Effect of Thin Film Coating on Reflectance of In-Vessel Metallic Minor- The knowledge of optical properties of boron-carbon-

dependencies n(L) and ft(l.), as seen from data presentedin Table. In addition, some data relating to the effect ofbased films is not enough for the adequate prediction ofvariation of H concentration in the boron film on the nmagnitude at (L = 632.8 nm can be found in I I I ].
The best way to overcome the problem related todeposit growth is to find the effective methods to clean
up the surfaces from a deposition. For optical windows,

final result of film deposition on mirror surface even if

in addition to laser ablation method [6,12,13], thefusion devices.

the film thickness is known from independentmeasurements. There is a need to obtain data for films

with chemical and structural composition variation in awide range following the most probable composition in- The simple and reliable methods for the regularly
provided in-situ callbration and control of reflectance of
the in-vessel mirrors, as well as the methods of deposit
cleaning have to be developed. The plasma properties ofeffectiveness of chemical cleaning by using the local
ECR discharge in hydrogen was demonstrated in [4].
For the in-vessel mirrors the methods of cleaning up theboron-carbon deposit were not suggested yet. Therefore,
for the in-vessel mirrors the methods of in-situ controlof thickness and composition of B-C films have to be

an ECR discharge in hydrogen (or deuterium) lookrather promising for both these applications. However,the special experiments in large-scale fusion devices aredeveloped as well as methods of the in-situ recalibration of the whole optical scheme for devices
where the boron-carbon materials are used for the firstnecessary to be carried out to optimize thecharacteristics of ECR discharge for solving the problem

wall protection. The scheme tested on TFTR with steadyof low Z material deposition on operation of innerstate glow discharge in hydrogen as a light source [5]mirrors.

looks quite promising for regular providing the in-situ
calibration of optical characteristics of the in-vessel
components of spectroscopy and laser plasma
diagnostics. The low temperature plasma of a cw ECR

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4. Gonclusion- The deposition of thin boron-containing or other[10] E.T. Arakawa et al.,Phys. Rev. 31, 8097 (1985).
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contaminating films on the in-vessel mirrors of fusion

devices deteriorates the mirror reflectivity andtransmissivity of window. The effect of deposit onreflectance depends on the light wavelength, film
thickness and film chemical composition.

(l987).

273