Instrumentation Department Report on radiation effects on optical fibres at SCKCEN: H2-loading, infrared-fibres and fibre current sensor Nuclear Belgian Research Center Boeretang, 200 2400 Mol Belgium Benoît Brichard, Hans Ooms, Stan van Ierschot, Jean Pouders, Stan Hendrieckx Instrumentation Department Tel: +32 14 33 26 40 Secr: +32 14 33 26 07 Fax: +32 14 31 19 93 bbrichar@sckcen.be www.sckcen.be

Overview Progress in on-going EFDA-IRRCER fibre-related tasks Development & Irradiation testing of radiation-resistant fibres TW5-TPDC/IRRCER-Deliverable 1 & 2 IR fibres for thermography application: gamma radiation-sensitivity TW4-TPDC/IRRCER-Deliverable 16 fiber current sensor behaviour at cryogenic temperature TW5-TPDC/IRRCER-Deliverable 9 New fibre optic technology for ITER: a proposal

Defects and imperfections cause photons to be absorbed at specific wavelengths in fibres Intrinsic spectral absorption in silica fibre dB/km UV edge Total intrinsic optical absorption l< 0.6 µm > 10-2 dB/m 1.8 µm > l > 1 µm < 1x10-3 dB/m 104 IR phonon absorption edge Drawing defects Si-OH 102 An important optical parameter when dealing with fibres is the absorption; There are several factors that cause photons to be absorbed in glass, like Rayliegh scatterinf, lattice vibration, impurities, or even the effect of drawing. Radiation creates additional defects that will further absorb light at specific wavelength 1 Rayleigh scattering ~l-4 Waveguide imperfections Radiation creates additional or new defects 200 600 1000 1400 1800 2200 Wavelength [nm]

Two categories of defects in SiO2 Diamagnetic Centres Paramagnetic Centres Oxygen Deficient Centre (ODC) Si-Si OA: 5 eV PL: 7.6 eV Peroxy Linkage (POL) Si-O-O-Si OA: 7.1 eV ? (recent assignment) E’ NBOHC POR (SPOR) STH Only indirect optical evidences Can act as defect precursors for paramagnetic defects Detected by EPR/ESR spectroscopy

Radiation affects the optical properties of silica Radiation-Induced Absorption (RIA) Due to defect formations: E’,NBOHC,POR,STH,… Radiation-Induced Luminescence (RIL) Due to Photoluminescence Due to Cherenkov effect in SiO2 Radiation-Induced Refractive Index Change (RIRIC) Compaction-densification Colour centres

H2-treatment drastically reduces the 2 eV RIA band formation in all type of fibres Fission–reactor irradiation of High OH silica fibres 200 µm core – Acrylate coated RIA in dB/m Wavelength in nm 410 500 600 700 800 900 4 8 12 KU1 treated with H2 KU1 80°C 12 MGy (E>0.1 MeV) 2.7x1017 n/cm2 B. Brichard, A. L. Tomashuk & al., SCKCEN, J. of Nucl. Mat., 329, p1456, 2004

H2 slows down the RIA growth at 600 nm while OH content is enhanced at the same time Low OH silica with H2 Low OH silica without H2 RIA [dB/m] 0.25 0.5 0.75 1 1.25 1.5 394 kGy 1.6 MGy 1.6 MGy 55 kGy 394 kGy 55 kGy + 0.4 eV 2 H OH Si O + - º ® · 0.1 eV  550 670 790 1030 1390 550 670 790 1030 1390 Wavelength [nm] Wavelength [nm]

At “low” dose the H2-STU fibre showed the best radiation-resistance RIA [dB/m] 5x1015 n/cm2 ; 200 kGy 330 Gy/s ; 60°C ~3 MGy (pre-ionised) 10 Radiation-hardness factor @ 600 nm STU 5. STU-200 µm SSU+H2 (600 µm) 4. SSU-600 µm 10 5 Ranking @ 600 nm STU+H2 1. STU-200 µm + H2 KU1+H2 3. KU1-200 µm KSV4+H2 2. KS4V-200 µm OH growth 400 600 1000 1400 Wavelength [nm]

When the H2 is exhausted RIA quickly re-increases RIA [dB/m] 30 STU KU1+H2 and STU 5 15 With these Al-coated H2-loaded fibres we can enter the cryostat but probably not far into the diagnostic block, unless we could change the fibres. Looking for improvement ? STU+H2 KS4V+H2 7.12x1017 n/cm2 23 MGY 80°C 600 1000 1400 Wavelength [nm]

H2 into the glass network ? How to keep H2 into the glass network ?

We follow two complementary but different strategies The previous results demonstrate the clear advantage of treating silica optical fibres with hydrogen to improve the radiation resistance of the optical transmission in the visible spectral region. However, the optical transmission start degrading again as soon as the hydrogen is exhausted. Heavy H2 PRE-LOADING Hermetic coating 300°C, 300 bars This approach is currently on-going in Troistk, Moscow (cf collaboration EU/RF) in-situ H2 RE-FUELING Permeable coating H2 filling-line This approach is currently under study at SCKCEN (cf TW5-TPDC/IRRCER-Del 1 & 2)

=> licensing OK for reactor test SMIRNOF VI – irradiation device upgrade for handling depleted-H2 atmosphere in reactor Fibres Thermocouples In-pile capsule Fibres are protected in stainless steal tubes filled with H2-depleted in anaerobic atmosphere pressure 10-20 bars temperature max 100°C Continuous H2 flow => licensing OK for reactor test

A two step irradiation neutron (~ 1 MeV): 1.5 1016 n/cm2 Irradiation conditions in BR2-SIDONIE irradiation channel at full power (56 MW) Irradiation 1 10 % Irradiation 2 40 % time 2 h Neutron flux: 1.7x1014 n/cm2s Epithermal flux: 4.6x1013 n/cm2s Fast neutron flux (>1 Mev): 1.9x1013 n/cm2s Gamma Heating: 3 W/g[Al] neutron (~ 1 MeV): 1.5 1016 n/cm2 gamma : 2.2 MGy neutron (~ 1 MeV): 5.7 1016 n/cm2 gamma : 11 MGy

Overview Progress in on-going EFDA-IRRCER fibre-related tasks Development & Irradiation testing of radiation-resistant fibres TW5-TPDC/IRRCER-Deliverable 1 & 2 IR fibres for thermography application: gamma radiation-sensitivity TW4-TPDC/IRRCER-Deliverable 16 fiber current sensor behaviour at cryogenic temperature TW5-TPDC/IRRCER-Deliverable 9 New fibre optic technology for ITER: a proposal

Divertor thermography with IR fibres Divertor Cassette is a high temperature region to be continuously monitored for machine protection IR thermography proposed by CEA-Cadarache Tore-Supra IR fibre ? Divertor Cassette

IR-Fibres could be used to transport IR radiation from the divertor port to the bioshield Low OH Silica 1-2 µm Sapphire 1-3.5 µm max 3 m ZrF4 1-4 µm Up to 250°C Chalcogenide 1-11 µm Up to 150°C Metal-coated fibres 3-17 µm Low NA PBG fibres / Bragg Fibres ??? Line of Sight mirrors Fibres At the divertor port ~1019 n/cm2 (E>0.1 MeV) ~ 1 Gy/s ; >10 MGy 8.5 m up to Bioshield Large Wavelength Span Cassegrain Telescope

Experimental set up to measure on-line radiation-induced absorption in IR fibres Lock-in IR Spectrometer Lamp RITA Irradiation container Labview DAQ CEA Acquisition (R. Reichle)

IR1- Zirconiumfluoride IR1 RIA decreases with increasing wavelength Radiation sensitivity depends on the wavelength and type of “fluoride” compound material used 3 kGy 5.2 kGy IR1- Zirconiumfluoride 2 µm 1 2 3 4 RIA in dB/m IR1 0.0 4.0x10 8.0x10 1.2x10 5 1.6x10 Time in hours 3.5 µm recovery RIA decreases with increasing wavelength IR2 3.5 µm 2 µm IR2- Hafniumfluoride RIA dB/m 1500 2000 2500 3000 3500 4000 1 2 3 3 kGy Recovery ~17 h Wavelength in nm

Similar RIA in ZrF4 fibre from other manufacturer. 3 kGy 2.5 20 40 60 80 100 120 Temperature C° Temperature increase favours RIA decrease. 5.2 kGy Zirconium Fluoride (RA6) - Polymicro 2.0 1.5 RIA dB/m 2 µm 1.0 0.5 Recovery 0.0 4 0.0 4 5 5 5 5 5 5 -5.0x10 5.0x10 1.0x10 1.5x10 2.0x10 2.5x10 3.0x10 3.5x10 Time in seconds

Hollow Waveguide Fibre: good radiation resistance but extremely sensitive to bending Hollow Waveguide: 750 µm core – 2 meters Hollow Waveguide From Polymicro No change observed after 27 kGy !

Preliminary Conlusion on IR fibres For 1-2 µm, low-OH pure silica is a good candidate. However, we need more data on neutron damage at 2 µm Above 2 µm, Zirconium/ Hafnium Fibre much more radiation-sensitive than silica Hollow-Waveguide, good candidate but high-intrinsic loss and difficult to handle Still to test Saphirre Fibre (and Chalcogenide ?) Also looking for PBG (Bragg) silica fibre operating in 2-3 µm

Overview Progress in on-going EFDA-IRRCER fibre-related tasks Development & Irradiation testing of radiation-resistant fibres TW5-TPDC/IRRCER-Deliverable 1 & 2 IR fibres for thermography application: gamma radiation-sensitivity TW4-TPDC/IRRCER-Deliverable 16 fiber current sensor behaviour at cryogenic temperature TW5-TPDC/IRRCER-Deliverable 9 New fibre optic technology for ITER: a proposal

Optical Fiber Current sensor in ITER ? Conventional plasma current measurement system like Rogowski coils looses sensitivity in quasi steady state plasma Interest for Fibre Current Sensor ? Faraday Effect Need to assess the influence of radiation and low temperature on the Verdet Constant Magnetic Field rotates the incident polarization state by an amount proportional to the Verdet Constant µV.

Few publications talking about Fibre current sensor in TOKAMAK and … S. Kasai, I. Sone, M. Abe, T. Nishitani, S. Tanaka, T. Yagi, N. Yokoo and S. Yamamoto, On-line Irradiation Tests on Sensing Fiber of Optical-fiber Current Transformer, JAERI-Research 2002-007, p130-144 N.M. Kozhevnikov, Y. Barmenkov, V.A. Belyakov, A. Medvedev, G. Razdobarin, Fiber-optic sensor for plasma current diagnostic in tokamaks, SPIE vol. 1584 Fiber Optic and Lasers IX (1991), p 138-144 Y. Barmenkov, F. Mendoza-Santoyo, Faraday plasma current sensor with compensation for reciprocal birefringence induced by mechanical perturbations, J. Appl. Research and Technology, Vol 1, No2, 2003, p157-163 Commercially Available System exists for electrical power industry In the US, NxtPhase : http://www.nxtphase.com In Switzerland, ABB, Baden-Dättwil CH-5405, K.Bohnert, optics and lasers in Engineering, 43 (2005), 511-526

Verdet constant in silica as function of wavelengths The fibre current performance will depend on wavelengths, temperature and radiation… Verdet constant in silica as function of wavelengths Faraday Effect as  We prefer to operate the fibre current sensor in the low sensitivity region, i.e. 1.3-1.5 µm, because at these wavelengths: we reduce the combined effect of radiation and low temperature we can more easily use an all-fibre optic sensor system A.H. Rose, JLT, Vol 15,n°5,1997

No data on Verdet Constant in Liquid Nitrogen… Mini-ITER Fibre Polarisation controller Current Source Light Source Liquid Nitrogen

Cryogenic Temperature induces Fiber current sensitivity slightly decreases when subjected to liquid nitrogen temperature 5 10 15 20 25 30 35 Time in seconds I = 40 A => DOP ~ 3% - I + I 300 K Cryogenic Temperature induces decrease in sensitivity additional noise I = 40 A => DOP ~ 2% - 77 K

Preliminary conclusion on fibre current sensor Preliminary result is encouraging At liquid nitrogen temperature we observed a slight decrease of the fibre sensitivity with an increase of the noise in the measure => need optimizatiion Need to verify now the RIA of the fibre at 1.5 µm at -77K if the radiation could degrade the Verdet constant

Overview Progress in on-going EFDA-IRRCER fibre-related tasks Development & Irradiation testing of radiation-resistant fibres TW5-TPDC/IRRCER-Deliverable 1 & 2 IR fibres for thermography application: gamma radiation-sensitivity TW4-TPDC/IRRCER-Deliverable 16 fiber current sensor behaviour at cryogenic temperature TW5-TPDC/IRRCER-Deliverable 9 New fibre optic technology for ITER: a proposal

New fibre optic technology for ITER ?

Photonic Crystal Fibres can be classified in two different families I. High Index Guiding Fibers - HICF Light is guided in a solid core with higher refractive index than cladding Strong wavelength dependence of the effective refractive index Low intrinsic absorption 1.5 dB/km @ 1550 nm 30 dB/km @ 500 nm High NA > 0.6 II. High Index Guiding Fibers - LICF Due to the Band Gap feature, light is exclusively guided into a hollow core characterised by low index Low intrinsic absorption is now available 0.1 dB/m @ 1550 nm 1 dB/m @ 500 nm

Luminescence in 200 µm core fibre Radiation induces Photo and Radio Luminescence in silica based material Luminescence in 200 µm core fibre 600 800 1200 dB Function of fibre diameter Wavelength [nm] 600 µm 200 µm Photoluminescence (O2) 0.1 nW level at detector side Light intensity @ detector side [pW] RIA Cerenkov Wavelength [nm] Cherenkov and Photoluminescence Function of reactor power Reactor Time 80 % 60 % 40 % 20 %

Cf results of A.L. Tomashuk (FORC) High-Index Core Fibres (HICF) should reduce Cherenkov yield while holding good light coupling With HICF we can reduce the fibre diameter while increasing the numerical aperture Cerenkov Yield Y ~ D2 Coupled Power P ~ D2 NA2 In addition holes provide a way to inject efficiently hydrogen to “repair” the fibre transmission Cf results of A.L. Tomashuk (FORC)

Fibres might simplify design and maintenance in many diagnostic … Small and compact space PMTs suffer Radiation and EMI => Move away PMTs and Use fibres Liquid Scintillators response After D. Marocco, ENEA Lower Vertical Neutron Camera -LVNC 10 collimators ;  35 mm

Conclusion, perspectives and expectations R&D work will carry on … Hydrogen-loading technique with engineering emphasis Outlook to new fibre technology, like Photonic Crystal Fibres… Now, real need to interact with designers to implement fibre pathways in ITER