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Radiation-hard KS-4V glass and opticalfiber, manufactured on its basis, forplasma diagnostics in ITERI. I. Cheremisin a , T. A. Ermolenko a , I. K. Evlampiev a , S. A.Popov a , P. K. Turoverov a , K. M. Golant b & M. O. Zabezhajlov ba The I.V. Grbenshikov Silicate Chemistry Institute of the RussianAcademy of Sciences , Odoevskogo str., 24-2, St. Petersburg,199155, Russiab Fiber Optics Research Center at the A.M. Prokhorov GeneralPhysics Institute of the Russian Academy of Sciences , Vavilov str.,38, Moscow, 199991, RussiaPublished online: 11 Oct 2011.

To cite this article: I. I. Cheremisin , T. A. Ermolenko , I. K. Evlampiev , S. A. Popov , P. K.Turoverov , K. M. Golant & M. O. Zabezhajlov (2004) Radiation-hard KS-4V glass and optical fiber,manufactured on its basis, for plasma diagnostics in ITER , Plasma Devices and Operations, 12:1, 1-9,DOI: 10.1080/10519990410001652593

To link to this article: http://dx.doi.org/10.1080/10519990410001652593

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Plasma Devices and Operations

Vol. 12, No. 1, March 2004, pp. 1–9

RADIATION-HARD KS-4V GLASS AND OPTICALFIBER, MANUFACTURED ON ITS BASIS, FOR

PLASMA DIAGNOSTICS IN ITER

I. I. CHEREMISINa, T. A. ERMOLENKOa, I. K. EVLAMPIEVa, S. A. POPOV,

P. K. TUROVEROVa, K. M. GOLANTb,* and M. O. ZABEZHAJLOVb

aThe I.V. Grbenshikov Silicate Chemistry Institute of the Russian Academy of Sciences,Odoevskogo str., 24-2, St. Petersburg, 199155, Russia;

bFiber Optics Research Center at the A.M. Prokhorov General Physics Institute of theRussian Academy of Sciences, Vavilov str., 38, Moscow, 199991, Russia

(Received 31 October 2002; Revised 26 March 2003)

Transmission spectra and optical losses before and after g-irradiation (60Co) at a dose up to 11 MGy in 1 cm thickglass plates and in ‘‘all-silica’’ type optical fibers were examined. KS-4V, KU-1, KUVI, Suprasil F-300 glasses andfluorine-doped glass (vapor-phase axial deposition (VAD) technology, Japan) were used as a core material in thefibers.

Comparison tests of four types of high purity fused silica (HPFS) showed that the highest ORR is characteristic ofKS-4V (the lowest chlorine content).

Optical Radiation Resistance (ORR) testing of ‘‘all-silica’’ type fibers (KS-4V glass, Japan VAD glass, SuprasilF-300 and KU-1 glasses as a core material) showed that the highest ORR is characteristic of KS-4V glass, whichcontains minimum of impurities. The level of optical losses in the visible band of the KS-4V spectrum is twicelower than the level of fluorinated glass (VAD), and 4–5 times lower as the level of losses of Suprasil F-300 andKU-1 respectively.

Keywords: Radiation-hard glass; Optical fiber; Plasma diagnostics

INTRODUCTION

The target development of the original technology of high purity hydrogen-free KS-4V silica

synthesis was aimed at the material employment in optical fiber telecommunications, as well

as at its specific application in aerospace engineering, electronics, medicine engineering [1].

Obtaining maximum Optical Radiation Resistance (ORR) in addition to all other character-

istics of the material, was of primary importance. The technology has been developed in the

Silicate Chemistry Institute of the Russian Academy of Sciences, and is permanently

improved. Currently industrial production of KS-4V glass in large-size units (up to 50 kg

in weight) undergoes a setting up stage under control of the developers.

High purity fused silica (HPFS) is characterized by approximately similar (at a level of

<0.01 ppm) content of colour centers and alkaline impurities. In addition, the amount of

* Corresponding author. E-mail: golant@fo.gpi.ru

ISSN 1051-9998 print; ISSN 1029-4929 online # 2004 Taylor & Francis LtdDOI: 10.1080=10519990410001652593

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OH-groups in KS-4V glass is extremely low (<0.1 ppm), and chlorine content does not

exceed 20 ppm. That is what distinguishes KS-4V glass from its Russian (KU-1 and

KUVI) and foreign (Suprasil F-300, F-100 by Heraeus and HPFS 7980 by Corning) analogs,

which are manufactured on the basis of high-purity silicon tetrachloride as a raw material. As

a result, chlorine content reaches �100 ppm in KU-1 and �400 ppm in KUVI, OH-groups

1000 ppm and �1 ppm correspondingly. Obtaining hydroxyl-free glass of a low chlorine

content is a difficult technological task, that has been solved in the course of KS-4V glass

developing.

The given task was resolved primarily by choosing an utterly new type of raw material

as compared to domestic and foreign HPFS obtained from silicon tetrachloride. KS-4V silica

is synthesized from high purity sol of polysilicon acid undergoing the following gradual

transformations: high purity sol of polysilicon acid SiO2 high purity crystobalite KS-4V

glass.

The glass obtained is of high transmission in the entire band of transparency window of the

glassy SiO2 (Fig. 1), optically homogeneous, free of gas bubbles and outer inclusions in the

volume of the entire unit, and is characterized by a possibly lowest content of OH-groups and

chlorine.

Characteristics are practically not changed in the subsequent high temperature treatments:

extrusion shaping of tube preforms and rods 60 mm in diameter, units and preforms molling,

rods redrawing for fiber optical aims, preforms and substrate tubes manufacturing and pulling

fibers out of preforms.

FIGURE 1 Optical transmission spectra in the three high-purity domestic silica. Reflection losses are notsubtracted. Samples thickness is 5 mm. 1 – KS-4V; 2 – KU-1; 3 – KUVI.

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The study of glass spectrum-optical characteristics in the vicinity of the UV-absorption

edge permitted us to conclude that the structure of glass-forming network of KS-4V glass,

when comparing it with other high purity glasses is the most resembling to the structure

of crystal, that is to say it has a lower concentration of oxygen vacancies in comparison to

other high purity glasses. KS-4V structure is the most ‘hard’, which is revealed through a

larger glass viscosity (by half an order of magnitude, approximately) and through high acous-

tic figure of merit of KS-4V (almost by an order of magnitude higher (2.5�107) than the one

of its analogues (3�106)).

EXPERIMENT AND DISCUSSION

A standard test on silica ORR includes comparative examining of optical loss spectra before

and after 1 MGy dose g-irradiation of samples.

In this study, a maximum of g-irradiation dose reached 11 MGy.

Glass samples in the form of 5–10 mm thick plates and pieces of fibers having various

silica cores were exposed to g-irradiation in specially prepared temperature-controlled

cells. 60Co was used as a source of g-radiation dose rate being of 4–7 Gy=s.

Domestic KS-4V, KU-1, KUVI and foreign Suprasil F-300, as well as two-stage made

Japanese fluorine-doped high purity glass (VAD technology) with OH-groups content of

about 3 ppm were examined as a core for ‘‘all-silica’’ type fibers. Reflecting cladding of

hydroxyl-free fluorine-doped silica glass was deposited on the lateral surface of the rod by

the microwave-induced plasma torch [2].

Comparison of KS-4V optical transmission spectra, before irradiation, with KU-1 and

KUVI spectra is presented on Figure 1.

Absorption band centered at 163 nm is usually associated with the presence of defects in

silica’s structure which are related to oxygen atoms vacancies. The origins of absorption in

UV, visible and IR optical spectrum bands, where all the three types of (not irradiated) silica

are characterized by a rather high transmission are thoroughly examined in Silins and

Trukhin monograph [3], and currently remain the topical issue of discussion. KU-1 and

KUVI absorption bands in IR-region are connected to longitudinal oscillations and OH-

groups overtones.

Figure 2 shows the analogous spectra for the same three glass types after g-irradiation at

1 MGy dose.

Figure 3 reflects optical loss spectra for irradiated all-silica fibers with cores made of these

glasses.

Relatively insignificant transmission differences in all the three high purity silicas (Fig. 1)

become radical after irradiation (Fig. 2). KS-4V transmission in the region of 200 nm remains

at a level of 55–60%, whereas KU-1, KUVI glasses (containing a considerable chlorine con-

centration resulting from SiCl4 synthesis) transmission in the wavelength region of 200–

250 nm drops almost to zero. Optical losses in fibers (Fig. 3) well correspond to transmission

characteristics of samples cut from units (Fig. 2). This testifies to the fact that these high-tem-

perature transformation changes of the KS-4V glass, when recycling by extrusion into sub-

strate tubes blanks, rods, when pulling preforms cores, when applying fluorine-doped silica

cladding and when pulling fibers from preforms did not have much influence on the optical

spectral characteristics of the glass in the core, including its ORR.

Comparative analysis of optical loss spectra before and after irradiation permitted us to

conclude that admixed chlorine atoms, the content of which is significantly higher in

KU-1 and KUVI glasses (see above), are immediate precursors or they promote production

RADIATION-HARD KS-4V GLASS 3

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FIGURE 2 Transmission spectra in silica samples after g-irradiation (60Co gamma source, dose rate 4 Gy=s).Reflection losses are not subtracted. Samples thickness is 10 mm. 1 – KS-4V; 2, 3 – KU-1; 4 – KUVI.

FIGURE 3 Optical loss spectra in all-silica fibers with the KS-4V, KU-1, KUVI silica in the core andfluorine doped cladding after g-irradiation (60Co gamma source, dose rate 4 Gy=s) to 1 MGy. 1 – KS-4V; 2 – KU-1;3 – KUVI.

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of g-activated colour centers, responsible for induced absorption at 163, 220, �250 and

630 nm during irradiation. KS-4V glass, which is of the smallest chlorine content (20 ppm

against �100 ppm in KU-1 and up to 400 ppm in KUVI), is characterized by a lower level

of losses at the wavelengths more than 400 nm. UV-absorption edge of 1 MGy irradiated

KS-4V glass is 50–100 nm shifted to the short wavelengths band, if compared to the two

other glass types. The position of g-induced absorption edge correlates to chlorine doping

concentration in the glass shifting towards the long waves band from KS-4V to KU-1 and

KUVI.

In addition, it should be noted that direct evidence for progressing silica ORR lowering

along with chlorine concentration increase was obtained by means of KS-4V chlorine doping

up to a level of �100 ppm. After that transmission curve of chlorine-doped 1 MGy irradiated

KS-4V practically coincided with the position of curve 4 on Figure 2, that is to say ORR of

chlorine-doped KS-4V glass became identical to those of KUVI and KU-1 glasses.

To explain the obtained results, a detailed analysis of the SiO2 glass-forming network

structure in the vicinity of extrinsic as well as intrinsic defects was carried out.

On Figure 4a, a fragment of an ‘ideal’ SiO2 structure (oxygen vacancies and non-bridging

oxygen atoms, i.e. the intrinsic defects, are not shown) and the schemes of incorporation in

the structure of molecules of water (4b), fluorine (4c), hydrogen (4d) and chlorine atom (4e)

are presented.

FIGURE 4 Impurity defects in silica structure. a – perfect structure; b – H2O incorporation in SiO2 structure;c – F2 incorporation accompanied by oxygen shifting into interstice; d – H2 incorporation; e – Cl incorporation withoxygen position and non-bridging oxygen formation.

RADIATION-HARD KS-4V GLASS 5

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Water (as well as HF, HCl) is incorporated in SiO2 structure resulting in one Si��O bond

rupture and two OH-radicals formation, bonded to the neighboring Si-atoms (OH-radicals

and Si��F and Si��Cl bonds respectively).

This does not generate charge irregularities, however, continuous spatial Si��O network is

disrupted in the place of oxygen unit substituted by non-bonded with one another pairs

Si��OH HO��Si, or Si��OH F��Si, and Si��OH Cl��Si.

At significant concentrations (up to 1000 ppm of OH and 100 ppm of HCl) water and

chlorine incorporation into the KU-1 glass structure is accompanied by a strong (in the

third sign) density decrease. KU-1 glass viscosity, as compared to KS-4V, is by half an

order of lower magnitude.

Molecular fluorine, taking into consideration a medium ion F� size (see Tab. I), joins the

structure by substituting oxygen with two F� ions and by displacement of oxygen atom into

interstice.

In the amount of up to 1–2 at.%, fluorine incorporation into SiO2 is accompanied by dis-

ruption of the continuous structure in a great quantity of network’s units, which in its turn

leads, in particular, to radical viscosity and glass transformation temperature Tg decrease.

Tg lowers from 1250 �C for undoped glass to �900 �C, and softening temperature Tsoft

goes down from 1750 to 1100 �C correspondingly at F concentration in SiO2 �1 at.%.

Chlorine solubility (Figure 4e) in water-free glass is relatively low, and Cl� ion size (see

Tab. I) apparently does not let two chlorine atoms substitute the oxygen atom in the unit of

glass-forming network without disruption of great amount of Si��O��Si bridges. That is why

chlorine atom incorporation in the position occupied by oxygen atom should lead to disrup-

tion or stretching of large quantity of the neighbouring Si��O- bonds apart from non-bridge

oxygen formation.

This consideration goes hand in hand with the data on ORR of chlorine-containing and

chlorine-free (KS-4V) glasses, described above.

Incorporation of large diameter atoms, such as chlorine atom, unlike OH radicals and

F-atoms, generates a higher concentration of non-bridging oxygen atoms and strained

S��O bonds and in conditions of intense g-irradiation, concentration of these defects, oxygen

vacancies and three-fold coordinated silicon atoms as precursors of centres responsible for

optical losses, turns out significantly higher than in chlorine-free silica.

In Figure 5, evolution of optical loss spectra in the visible spectrum band for the four types

of silica in fibers in the course of g-irradiation dose increasing from 17.0 kGy to 11.0 MGy

are shown. The key impurities concentrations for these silica types used as a core material for

fibers [5] are presented in Table II.

From Figure 5, it follows that KS-4V glass is characterized by the highest ORR practically

throughout the entire interval of g-irradiation doses. On irradiation of 11.0 MGy dose, KS-4V

TABLE I Host and Impurity Ions Sizes and Energies of Bonds in TheStructure of SiO2 Glass Forming Network [4, 5].

Species O2 (OH)� F2 Cl2

Ion diameter, A [2] O2� (OH)� F� Cl�

3.50 3.06 2.72 3.6

�DH�f 298, kJ=mol [3] SiO2 SiF4 SiCl4

880.1 1550.0 628.0

Bonds energy, a.u. Si��O Si��OH Si��F Si��Cl1.0 1.0 �1.8 �0.7

SiO2: interstice diameter 3.0� 0.3 A

6 I. I. CHEREMISIN et al.

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FIGURE 5 Induced loss level and spectrum evolution as a function of g-irradiation dose (60Co gamma source,dose rate 6.9 Gy=s). Data correspond to four fibers with various silica types as the core materials.

TABLE II ORR-Tested Fibers Characteristics [6].

#

Silicatype inthe core

OH-groupscontent,ppm

Fluorinecontent,ppm

Chlorinecontent,ppm

Core=claddingdiameters, microns,

numerical aperture, NA

1 KS-4V 0.5 50.0 20.0 100=120NA¼ 0.16

2 KU-1 800 7 80.0 400=440NA¼ 0.16

3 Fluorine-doped silica 3.0 5400 20.0 200=240NA¼ 0.16

4 Suprasil F-300 0.25 7 1200 200=220NA¼ 0.16

RADIATION-HARD KS-4V GLASS 7

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losses (in its maximum at 600 nm wavelength) are approximately twice lower than of the

closest by the ORR characteristics fluorine-doped glass, synthesized by VAD technology

and 4–5 times lower than the losses of Suprasil F-300 and KU-1.

Furthermore, unlike KU-1, Suprasil F-300 and fluorine-doped (0.54 wt.% F) glasses, the

losses in KS-4V remain practically unchanged at g-irradiation doses from �3 MGy to

11.0 MGy, whereas with the dose increase, the progressive level of losses increase in this

band is observed in all other ‘competitors’.

This can be explained by minimal impurities content in KS-4V and by maintaining struc-

ture balanced condition under g-irradiation, that is to say by its maximum randomization,

staring from a level of 3 MGy dose.

The presence of the great amount of disrupted units due to fluorine, chlorine and

water incorporation into the structure of glass-forming network apparently

opens additional possibilities for progressing appearance of color centers in the course of

g-irradiation.

Structure conservation in the conditions of ionizing irradiation provides the stability of

optical–spectral characteristics and other material properties when using it in control systems

of nuclear and thermonuclear set-ups, all this is intrinsic to high-purity KS-4V glass.

Significant ORR silica increase in the visible spectrum band, which is most crucial for

application in the ITER, was obtained by loading fibers with molecular hydrogen as it is

shown in Figure 6 [7]. Fibers of core and fluorine-doped silica cladding with diameters of

100 and 120 microns, respectively, were being H2-loaded under a pressure of 24 kg=cm2 dur-

ing 7 days at room temperature. As a result of molecular hydrogen ‘‘doping’’, subsequent

gamma irradiation leads to Si��OH � Si�centers formation (see Figure 4e), the presence of

which decreases ORR of glass in the UV-region and increases losses in the IR-region because

of absorption at 1.38, 2.2 and 2.73 microns. However, ORR considerably increases in

the visible band probably, resulting from hydrogen compensation of dangling bonds of

non-bridging oxygen atoms.

FIGURE 6 Optical losses in fibers, measured after g-irradiation (60Co gamma source) at 1.7 MGy and 3.4 MGydoses. H2 symbol stands for hydrogenated glass in the core by means of direct hydrogen loading.

8 I. I. CHEREMISIN et al.

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CONCLUSION

High-purity silica glasses and optical fiber, manufactured on its basis, are potential candi-

dates for use in conditions of intense ionizing irradiation, including nuclear and thermonuc-

lear reactors.

By its nature, the structure of glass is considerably randomized and in the course of g- and

neutron irradiation can not alter radically, unlike metals and non-metal crystal materials.

In the conditions of maximum possible SiO2 purification from impurities, the induced loss

level is basically defined by equilibrium for the irradiation conditions concentration of intrin-

sic defects of SiO2 glass-forming network. Maximum ORR at g-irradiation doses of up to

11.0 MGy is observed in KS-4V glass, characterized by the lowest content of impurities, pri-

marily of chlorine.

Further investigation of doping (firstly of fluorine) and nature of defects-precursors of opti-

cally active centers responsible for induced absorption at g- and neutron irradiation is neces-

sary for obtaining materials with a pre-assigned low-loss level in the desired wavelength band

for the objectives of nuclear and thermonuclear energetics.

The example of such doping is induced loss lowering in high-purity silica in the visible

band resulting from molecular hydrogen loading, which leads to ORR increase at g-irradia-

tion dose of up to 3.4 MGy.

In addition to induced loss, a very important parameter for the system of optical diagnos-

tics in impulse installations of ITER type is the presence of distortions caused by lumines-

cence of the optical materials used under the radiation streaming. The comparative

investigation of luminescent characteristics of silica and fibers in intense radiation fields is

therefore a topical issue.

References

[1] A. V. Shakhanov, K. M. Golant, A. N. Perov, S. D. Rumyantsev, A. G. Shebunyaev, I. I. Cheremisin andS. A. Popov, All-silica optical fibers with reduced losses beyond two microns, Proc. SPIE, 1893, 85–89 (1993).

[2] A. S. Biriukov, E. M. Dianov, K. M. Golant, R. R. Khrapko, A. V. Koropov, A. N. Perov, A. V. Shakhanov andS. A. Vasiliev, Synthesis of fluorine-doped silica glass by means of an outside deposition technique using amicrowave plasma torch, Sov. Lightwave Commun., 3 (1), 1–12 (1993).

[3] A. R. Silins and A. N. Trukhin, Point defects and elementary excitations in crystalline and glassy SiO2, Riga(Zinatne, 1985), pp. 13–14, 34–49, 51–73, 74–106, 109–129 (in Russian).

[4] Brief Handbook on Physical and Chemical Constants (Khimiya, 1965), pp. 123–125 (in Russian).[5] Handbook. Thermodynamical Properties of Non-organic Substances (Atomizdat, 1965), pp. 82–93, 96–97,

126–127 (in Russian).[6] A. L. Tomashuk and K. M. Golant, Radiation-resistant and radiation sensitive silica optical fibers, In: E. M.

Dianov (Ed.), Advances in Fiber Optics. Proc. SPIE, 4083, 188–201 (2000).[7] A. L. Tomashuk, E. M. Dianov, K. M. Golant and A. O. Rybaltovskii, g-radiation-induced absorption in pure-

silica-core fibers in the visible spectral region: The effect of H2-loading, IEEE Trans. Nucl. Sci., 45 (3), Part 3,1576–1579 (1998).

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