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Fusion Engineering and Design 84 (2009) 1472–1474

Contents lists available at ScienceDirect

Fusion Engineering and Design

journa l homepage: www.e lsev ier .com/ locate / fusengdes

ptically transmissive window on ITER port cell door

unal Patela,∗, Mike Walshb, Guy Phillipsa, Phil Parsonsc, Nick Balshawd, Simon Hanke

EURATOM/UKAEA, Culham Science Centre, J20/0/65, Abingdon, Oxfordshire OX14 3DB, UKUKAEA, Culham Science Centre, K1/0/52, Abingdon, Oxfordshire OX14 3DB, UKUKAEA, Culham Science Centre, D3/2.79, Abingdon, Oxfordshire OX14 3DB, UKUKAEA, Culham Science Centre, J20/0/81, Abingdon, Oxfordshire OX14 3DB, UKUKAEA, Culham Science Centre, D4/G.05, Abingdon, Oxfordshire OX14 3DB, UK

r t i c l e i n f o

rticle history:vailable online 3 March 2009

eywords:ptical diagnosticsressure windowptical windowaser path

a b s t r a c t

In the ITER Thomson Scattering Core LIDAR system there are currently three concepts for collecting scat-tered light from the plasma and delivering it to the diagnostic room. An option to use a broadband mirrorin Cassegrain configuration has been explored. In this concept a large optical window, 1.5 m in diameter,is required in the port cell door. This could be manufactured in a number of ways, including using a singlethick window or mosaic arrangement with a supporting structure. Challenges associated with this designinclude maintaining the secondary confinement barrier, withstanding a pressure differential of 0.5 baracross the window in foreseeable fault conditions, and maintaining the optical properties of the window.It must also be possible to protect the window when work is being carried out in the area. Fracture anal-ysis for brittle material was carried out, for a flaw size with a length to depth ratio of 80/20, which gavea safety reserve factor of 4. The flaw size was based on a standard optical transmissive lens for collectionlight. A static analysis was carried out on both options of having a single thick glass window or a mosaic

arrangement. The principal stress for a single window arrangement is 12 MPa and for a proposed mosaicarrangement is 12.5 MPa. The analyses have demonstrated that a single glass window with a thicknessof 42 mm would be mechanically acceptable but expensive to manufacture and it would be difficult topolish glass of that size to lens quality. By comparison in the proposed mosaic window arrangement, theglass thickness is reduced by 50% with the aid of the supporting structure. As a result of the supportingstructure the geometrical optical losses are 4%, which are mechanically and optically acceptable, whilst

cost

significantly reducing the

. Introduction

There are three concepts of light collection from the biologicalhield area to the diagnostics area. As part of the conceptual opti-isation of the ITER core Thomson scattering system, an option

o couple the light from the biological shield area to the diagnos-ics area with one broadband mirror in Cassegrain configuration isxplored. Use of this mirror would require an optically transmissiveoundary on the port cell door.

The objective is to design a window to fit in the port cell doorhat will satisfy the functional window specification. The diame-er of the window is 1.5 m with the support structure taking up

ess than 4% of the window area with an aperture of 200 mm iniameter is required for laser calibration. The window must be ableo transmit wavelength from 270 to 1200 nm. In order to complyith the wavelength transmission requirement Fused Silica KU1 is

∗ Corresponding author. Tel.: +44 1235464913; fax: +44 1235 465102.E-mail address: kunal.patel@jet.uk (K. Patel).

920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.fusengdes.2009.01.088

of manufacturing.© 2009 Elsevier B.V. All rights reserved.

used as a window material. Neutronics has not been consideredfor the window design because it is situated outside the bio-shieldwall. Other assumptions made on basis of best available informa-tion from ITER as follows, window would be subjected to a pressuredifferential of ±0.5 bar [1], which would be applied slowly andcontinuously. Cyclic pressure loading on the window is negligiblebecause this would only occur in the event of failure of rupturedisc. The material used for support structure to reinforce the win-dow is structural steel grade S185 or similar. For stress calculationsa worst case scenario was considered, the glass is fixed rigidly at allsupports and linear elastic analysis was carried out. Fracture analy-sis for brittle material was not relevant for an assumed flaw size of0.100 mm length and 0.050 mm, typical value for polished opticallens.

2. Window design solution

Two concepts were explored.The first concept was using a single glass window. The minimum

required glass thickness was calculated analytically to be 42 mm.

K. Patel et al. / Fusion Engineering and Design 84 (2009) 1472–1474 1473

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Fig. 1. Arrangement of split window.

he maximum stress on the glass was 12 MPa on the outer edgend a maximum deflection of 0.5 mm was found in the centre.

In order to minimise the size and costs involved in forming a sin-le glass of this thickness, a second concept of splitting the windownto nine segments, in a 3 × 3 grid, with supporting steel structuren between the glass segment was investigated, see Figs. 1 and 4.sing the analytical solution, the thickness of glass was reduced by0% to 21 mm. The maximum stress on the glass was 8 MPa on theuter edge and a maximum deflection of 0.08 mm was found in theentre. This was based on 0.5 m × 0.5 m square glass panel.

Fracture analysis for brittle material was carried out, for a flawize with a length to depth ratio of 80/20, which gave a safety reserveactor of 4. The flaw size was based on a standard optical trans-

issive lens for collection light and the recommended stress for aroposed mosaic arrangement is 12.5 MPa.

Finite element analysis was carried out verifying the analyticalolution for both options. As expected this shows significant stress

oncentrations on the corners of the supporting structure and glass.or this initial analysis, these areas have been ignored, because thisan be improved by good design practise, by removing sharp cor-ers on the glass and the supporting steel frame. When ignoring thetress concentrations, the peak stress on the glass was 25 MPa and

Fig. 2. Principal stress on a split window.

Fig. 3. Total deflection on the split window.

average principal stress on the glass was approximately 12.5 MPa.Stress on the supporting structure was 100 MPa with deflection of1 mm in the centre. The peak stress can be eliminated by having adamping material between the glass and the supporting steel struc-ture, see Figs. 2 and 3. Note that with this option the visible areawas reduced by 4%.

3. Manufacturing viability

After consultation with industry, it was deemed that a singleglass with a diameter of 1.5 m was not practicable to meet ourrequirements, hence the second option of 0.5 m × 0.5 m square glasswas analysed above.

4. Window shutter concept

The window on the inter-space door is designed against thedifferential pressure, in order to comply with the requirement. A

shutter is proposed to protect the window from transmitting radi-ation into the gallery area and against scratches to the opticallytransmissive glass during maintenance periods. It is assumed thatthe shutter is same thickness as the door design to protect againstradiation because the door becomes a radiation shield when the

Fig. 4. Illustrating the shutter in the gallery area with a linear actuator in open orclosure position.

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474 K. Patel et al. / Fusion Engineer

ort plug or bio-shield is removed. Opening and closure of the shut-er is achieved by rotary or linear actuators. It order to limit thetraight line gamma streaming when the bio-shield is disassem-led, the shutter design will consist of stepped overlap on the edgesnd uses the same fire resistant steel plate used in the door design.oth shutters will be closed during shutdown and reopened whenhe machine is started.

. Design concept

There are two types of shutters; first shutter is situated on theide facing the port cell area, which acts as a radiation shield. Theecond shutter is located on the side facing the gallery area, whichill protect the glass from any scratches. The weight of the shutter

n the port cell side is 10% of the door weight. This is based on thehutter thickness being the same as the current door thickness for

he shielding purposes and on the gallery side it is 0.5% of the dooreight for protection of window. The mechanism used for opening

nd closure of the shutter on the port cell side is achieved using aeared rotary actuator and on the gallery shutter is achieved usinglinear actuator on both right and left sides. Illustrated in Fig. 4.

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d Design 84 (2009) 1472–1474

6. Conclusion

If the Cassegrain configuration is adopted, the overall outcome ofthe two analyses indicates the optimum solution would be to splitthe window into segments due to manufacturing considerations.Protection for the window is recommended to avoid risk of damageduring maintenance periods and maintain integrity of the door asa radiation shield during shutdowns.

Acknowledgements

The author would like to thank Mike Walsh, Guy Phillips, PhilParsons, Nick Balshaw, and Simon Hanks. Funded by EuroatomFusion Training Scheme, Engineering of Optical Diagnostics forITER.

Reference

1] J. How, ITER Project Integration Document, ITER D 2234RH, 26th January 2007,Version 3.