1 Holey Silica-Core Fibres: An Alternative Fibre Type for Plasma Diagnostic Systems A.L.Tomashuk, A.F.Kosolapov and S.L.Semjonov 10th Meeting of the ITPA Topical Group on Diagnostics, Moscow, Russia, April, 2006

2 OUTLINE 1.Retrospective glance at the progress in the development of fibers for plasma diagnostics in ITER 2.Silica-core holey fibres: properties and potential advantages 3.In-situ radiation hardening of a holey fibre with H 2 gas: the first experiment 4. CONCLUSION

3 1.Retrospective glance at the progress in the development of fibers for plasma diagnostics

– US and RF HT comparison experiment. Low-OH, low-Cl fibres are found to be most rad-hard: Russian KS-V4 and 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 (1996)) Radiation-induced optical loss, dB/m/MGy Wavelength:

– H 2 -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 (1998)) Radiation-induced optical loss, dB/m/MGy Wavelength:

– EU–JA–RF HT ‘round-robin’ experiment: H 2 -loaded and pre-irradiated KU-1-silica fibre (“KU-1H2G”) showed the lowest radiation-induced absorption. (B.Brichard et al, J. Nucl. Mater., vol , pp (2004); T.Kakuta et al, J. Nucl. Mater., vol , pp (2002)) Radiation-induced optical loss, dB/m/MGy Wavelength:

– H 2 -loaded hermetically Al-coated 100-μm-core fibre. Very high H 2 content of 5.7∙10 20 cm -3 (A.L.Tomashuk, K.M.Golant, E.M.Dianov et al., Patent RU (2004)) Radiation-induced optical loss, dB/m/MGy Wavelength:

– H 2 -loaded hermetically Al-coated 200-μm-core fibres. Testing in EU and RF. H 2 content of ~ 1∙10 19 cm -3 (B.Brichard, 7th ITPA Meeting, Hefei, China, Oct. 2004) Radiation-induced optical loss, dB/m/MGy Wavelength:

9 Radiation-induced optical loss, dB/m/MGy Wavelength: 2003 – H 2 -loaded hermetically Al-coated 200-μm-core fibres. Testing in EU and RF HT. H 2 content of ~ 1∙10 19 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, Feb. 2004; A.V.Bondarenko et al., Instruments and Experimental Techniques, vol. 49, pp (2006));

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

11 H-associated colour centres H 2 -loading may give rise to H-associated colour centres (!!!), FORMATION by hydrogen: H 2 + γ → H + H =Si: + H → =Si –H ≡Si–Si≡ + H + γ → ≡Si–H…≡Si H(I)-centre E’ β -centre

12 H-associated colour centres H 2 -loading may give rise to H-associated colour centres (!!!), FORMATION by hydrogen: H 2 + γ → H + H =Si: + H → =Si –H ≡Si–Si≡ + H + γ → ≡Si–H…≡Si H(I)-centre E’ β -centre SUPPRESION by hydrogen: =Si –H + H → =Si –H │ H ≡Si–H…≡Si + H → ≡Si–H + ≡Si–H

13 H-associated colour centres H 2 -loading may give rise to H-associated colour centres (!!!), Practical conclusions: 1) The initial H 2 content should be as high as possible (≥ 1∙10 20 cm -3 ), or, what is better, 2) the H 2 reservoir in silica should be replenished in-situ.

14 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 H 2 -loading; 2. H 2 -loaded hermetically Al-coated fibres are expected to withstand the radiation field inside the cryostat; 3. The initial H 2 concentration should be as high as possible (≥ cm -3 ); otherwise, growth of loss in the blue region may occur; 4. It is desirable to have the possibility to load fibres with H 2 in-situ, directly in the process of their operation in ITER.

15 2. Silica-core holey fibres: properties and potential advantages

16 2. Silica-core holey fibres: properties and potential advantages possibility to supply H 2 gas into the core in-situ through the holes low cost of preforms in quantity production high aperture

17 60 Co rods fibre coils H 2 pressure of 42 atm. 3. In-situ radiation hardening of a holey fibre with H 2 gas: the first experiment Fibre coils were immediately in the water: unexpected effect of radiolythic H 2 coming from the water pool H 2 pressure was just 42 atm. It can be increased by many times. holey fibre is connected to a hole-free fibre holey fibre is spliced with a hole-free fibre

18 3. In-situ radiation hardening of a holey fibre with H 2 gas: the first experiment H 2 -loaded holey fibre Very efficient suppression of the 610 nm band. Big H-associated absorption at short wavelengths, which decreases with dose!!! H 2 pressure and dose should be increased in the next experiment (≥ 120 atm. and > 1 MGy).

19 Comparison with H 2 -free KU-1 fibres irradiated at the same time λ=400 nm H 2 -loaded holey fibre holey fibre without H 2 POD-fibre without H 2 H 2 -loaded holey fibre POD-fibre without H 2 holey fibre without H 2 λ=400 nmλ=610 nm Non-monotonic behavior of the ‘H 2 -free’ fibres is due to H 2 penetration from the water pool. H 2 -loaded holey fibre: not only the 610 nm band, but also the short-wavelength absorption are gradually suppressed even at just 42 atm. H 2 pressure in the holes.

20 Radiation-induced optical loss, dB/m/MGy Wavelength: 2006 – Silica-core holey fibre loaded with H 2 in-situ. Ultra-low induced loss at 610 nm! Suppresion of the short-wavelength absorption is likely to be achieved at a higher H 2 pressure (≥ 120 atm.) The 610 nm band is suppressed almost completely (< 0,01 dB/m)!

21 CONCLUSION A closer look should be taken at silica-core holey fibres. Loading such fibres with H 2 in-situ using the longitudinal holes opens up a possibility to significantly prolong the fibre’s life-time inside the cryostat. Increasing the H 2 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.