Holey Silica-Core Fibres: An Alternative Fibre Type for Plasma Diagnostic Systems

1. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Holey Silica-Core Fibres: An Alternative Fibre Type for Plasma Diagnostic Systems A.L.Tomashuk, A.F.Kosolapov and S.L.Semjonov tomashuk@fo.gpi.ru 10th Meeting of the ITPA Topical Group on Diagnostics, Moscow, Russia, 10-14 April, 2006

2. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) • OUTLINE • Retrospective glance at the progress in the development • of fibers for plasma diagnostics in ITER • Silica-core holey fibres: properties and potential advantages • In-situ radiation hardening of a holey fibre with H2 gas: • the first experiment • 4. CONCLUSION

3. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) • Retrospective glance at the progress • in the development of fibers for plasma diagnostics

4. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Radiation-induced optical loss, dB/m/MGy Wavelength: 1995 – US and RF HT comparison experiment. Low-OH, low-Cl fibres are found to be most rad-hard: Russian KS-V4and Japanese F-doped-silica fibres. (D.L.Griscom, K.M.Golant, A.L.Tomashuk, D.V.Pavlov, Yu.A.Tarabrin, Appl. Phys. Lett., vol. 69, pp. 322-324 (1996))

5. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Radiation-induced optical loss, dB/m/MGy Wavelength: • 1997 – H2-loading and pre-irradiation of polymer-coated fibres: • significant reduction of radiation-induced optical absorption • regardless of the type of silica in the core • (A.L.Tomashuk, E.M.Dianov, K.M.Golant, A.O.Rybaltovsky, • IEEE Trans. Nucl. Sci., vol. 45, pp. 1575-1579 (1998))

6. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Radiation-induced optical loss, dB/m/MGy Wavelength: • 2000 – EU–JA–RF HT ‘round-robin’ experiment: • H2-loaded and pre-irradiated KU-1-silica fibre (“KU-1H2G”) • showed the lowest radiation-induced absorption. • (B.Brichard et al, J. Nucl. Mater., vol. 329-333, pp. 1456-1460 (2004); • T.Kakuta et al, J. Nucl. Mater., vol. 307-311, pp. 1277-1281 (2002))

7. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Radiation-induced optical loss, dB/m/MGy Wavelength: 2001 – H2-loaded hermetically Al-coated 100-μm-core fibre. Very high H2 content of 5.7∙1020 cm-3 (A.L.Tomashuk, K.M.Golant, E.M.Dianov et al., Patent RU2222032 (2004))

8. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Radiation-induced optical loss, dB/m/MGy Wavelength: 2003 – H2-loaded hermetically Al-coated 200-μm-corefibres. Testing in EU and RF. H2 content of ~ 1∙1019 cm-3 (B.Brichard, 7th ITPA Meeting, Hefei, China, 11-15 Oct. 2004)

9. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Radiation-induced optical loss, dB/m/MGy Wavelength: 2003 – H2-loaded hermetically Al-coated 200-μm-corefibres. Testing in EU and RF HT. H2 content of ~ 1∙1019 cm-3 A better result in RF HT owing to a higher temperature ? It was concluded that fibres CAN be used inside the cryostat. (A.Krasilnikov, 6th ITPA Meeting, Naka, Japan, 19-21 Feb. 2004; A.V.Bondarenko et al., Instruments and Experimental Techniques, vol. 49, pp. 190-198 (2006));

10. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) •H2-loading may give rise to H-associated colour centres (!!!), if 1) the initial H2 content is small, or 2) all H2 molecules have entered into the silica network under radiation at high doses. •Fortunately, these centres are healed by hydrogen as well. H-associated colour centres

11. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) •H2-loading may give rise to H-associated colour centres (!!!), FORMATION by hydrogen: H2 + γ → H + H =Si: + H →=Si• –H ≡Si–Si≡ + H + γ → ≡Si–H…≡Si• H-associated colour centres H(I)-centre E’β-centre

12. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) •H2-loading may give rise to H-associated colour centres (!!!), FORMATION by hydrogen: H2 + γ → H + H =Si: + H →=Si• –H ≡Si–Si≡ + H + γ → ≡Si–H…≡Si• H-associated colour centres H(I)-centre E’β-centre SUPPRESION by hydrogen: =Si• –H+ H → =Si –H │ H ≡Si–H…≡Si•+ H → ≡Si–H + ≡Si–H

13. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) •H2-loading may give rise to H-associated colour centres (!!!), Practical conclusions: 1) The initial H2 content should be as high as possible (≥ 1∙1020 cm-3), or, what is better, 2) the H2 reservoir in silica should be replenished in-situ. H-associated colour centres

14. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Thus, the state of the art of the development of rad-hard fibres is: 1. Fibres to be installed inside the cryostat should be radiation-hardened via H2-loading; 2. H2-loaded hermetically Al-coated fibres are expected to withstand the radiation field inside the cryostat; 3. The initial H2 concentration should be as high as possible (≥ 1020 cm-3); otherwise, growth of loss in the blue region may occur; 4. It is desirable to have the possibility to load fibres with H2in-situ, directly in the process of their operation in ITER.

15. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) • 2. Silica-core holey fibres: • properties and potential advantages

16. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) 2. Silica-core holey fibres: properties and potential advantages • possibility to supply H2 gas into the core in-situ through the holes • low cost of preforms in quantity production • high aperture

17. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) 3. In-situ radiation hardening of a holey fibre with H2 gas: the first experiment holey fibre is spliced with a hole-free fibre • Fibre coils were immediately in the water: unexpected effect of radiolythic H2 coming from the water pool • H2 pressure was just 42 atm. It can be increased by many times. fibre coils holey fibre is connected to a hole-free fibre 60Co rods H2 pressure of 42 atm.

18. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) 3. In-situ radiation hardening of a holey fibre with H2 gas: the first experiment H2-loaded holey fibre • Very efficient suppression of the 610 nm band. • Big H-associated absorption at short wavelengths, which decreases with dose!!! • H2 pressure and dose should be increased in the next experiment (≥ 120 atm. and > 1 MGy).

19. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Comparison with H2-free KU-1 fibres irradiated at the same time λ=400 nm λ=400 nm λ=610 nm H2-loaded holey fibre holey fibre without H2 holey fibre without H2 POD-fibre without H2 POD-fibre without H2 H2-loaded holey fibre •Non-monotonic behavior of the ‘H2-free’ fibres is due to H2 penetration from the water pool. • H2-loaded holey fibre: not only the 610 nm band, but also the short-wavelength absorption are gradually suppressed even at just 42 atm. H2 pressure in the holes.

20. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) Radiation-induced optical loss, dB/m/MGy Wavelength: The 610 nm band is suppressed almost completely (< 0,01 dB/m)! 2006 – Silica-core holey fibre loaded with H2 in-situ. • Ultra-low induced loss at 610 nm! • Suppresion of the short-wavelength absorption is likely to be achieved at a higher H2 pressure (≥ 120 atm.)

21. Fibre Optics Research Centre of the Russian Academy of Sciences (FORC) CONCLUSION A closer look should be taken at silica-core holey fibres. Loading such fibres with H2in-situusing the longitudinal holes opens up a possibility to significantly prolong the fibre’s life-time inside the cryostat. Increasing the H2 gas pressure in the holes from 42 atm. used in our experiment to ~ 120 atm. and over will allow further reduction of the radiation-induced loss in the red region and is likely to lead to suppression of the short-wavelength radiation-induced absorption. Such an experiment under high-dose (> 1 MGy) γ- or reactor irradiation would be of much physical and practical interest.