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Fusion Engineering and Design 84 (2009) 1702–1707

Contents lists available at ScienceDirect

Fusion Engineering and Design

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

esign and manufacturing of the ITER ECRH upper launcher mirrors

rancisco Sancheza,∗, R. Bertizzoloa, R. Chavana, A. Collazosa, M. Hendersonb, J.D. Landisa

Ecole Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association EURATOMConfédération, Suisse, CH-1015 Lausanne, SwitzerlandITER Organization, Cadarache Centre, Saint Paul Lez Durance, France

r t i c l e i n f o

rticle history:vailable online 27 February 2009

eywords:pper launcherirroreat loadanufacturing

a b s t r a c t

Four of the 16 ITER upper port plugs will be devoted to electron cyclotron resonance heating (ECRH)in order to control magnetohydrodynamic (MHD) instabilities [1,2]. In order to achieve the stabilisationof the neoclassical tearing modes (NTM) and sawtooth oscillation, a deposition of a very localized andpeaked current density profile over a broad poloidal steering range is required.

In the present optical configuration eight 2 MW mm-wave beams enter each of the four upper launch-ers (UL) through waveguides into the vacuum vessel. Each beam line comprises consecutive corrugatedwaveguide sections with two mitre bends, orientating the poloidal and toroidal directions and threesections of quasi-optical transmission [3].

The beam waist locations and beam shaping properties in free space propagation are defined by twoadditional mirrors, the first being a static focusing mirror and the second a plane poloidally steerablemirror. Each mirror reflects a group of 4 mm-wave beams.

The three types of UL mirrors (mitre bend, focusing and steering) absorb heat generated essentially bythree sources: the ohmic loss of the RF beam reflected at the mirror surfaces and the nuclear and thermal

radiation coming from the plasma. While the average heat load is within reasonable engineering limits,three elements condition the actual mirror design, the peak ohmic heat load (Gaussian or Bessel typeheat deposition profiles), the electromagnetic forces generated in vertical disruption events (VDE), andthe ITER cooling water requirements.

This paper provides an overview of the different upper port-plug mirror designs and cooling schemesand an outlook on the prototype manufacturing activities and the future test program. The optimized

he EC

mm-wave layout within t

. Introduction

A simplified view of the current front-steerable upper launcheresign is shown in Fig. 1. Eight circular HE11 63.5 mm waveguidessimilar to the waveguide used in the transmission line), enter theort plug entrance on the right (note that there are four waveguidesuperimposed in this poloidal cut). Prior to the closure plate a dia-ond window and an in-line isolation valve is placed to provide the

rimary tritium barrier [3]. The waveguide continue after the clo-ure plate to a free space propagation from a set of mirrors (lowerirror 1, 2 and upper mirror 1, 2) (from LM1 and LM2 to UM1 andM2) for a quasioptical configuration used to angle the 8 beams

both in toroidal and poloidal directions) to two focusing mirrors

UM3 and LM3) with the incident beams partially overlapping inoth toroidal and poloidal directions. The reflected beams are thenirected downward to two flat steering mirrors (UM4 and LM4),hich redirect the beams into the plasma with a toroidal injection

∗ Corresponding author. Tel.: +41 216933434.E-mail address: francisco.sanchez@epfl.ch (F. Sanchez).

920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.fusengdes.2009.01.013

H port plugs is also presented.© 2009 Elsevier B.V. All rights reserved.

angle of ˇ ≈ 20◦. Four beams are incident on every single mirror, theoverlapping of the beams permit the largest spot for a finite focus-ing mirror size within the confined space of the port plug, speciallyat the blanket shield module region.

The quasi-optical design here outlined [2] has replaced the tra-ditional waveguide and mitre bends with free space mirrors, andincreases flexibility in optics, reduces astigmatism, system com-plexity, component costs, volume occupied by beams and improvesthermal efficiency by

• Decreasing peak power densities: the highest cumulated peakpower density on the mm-wave components occurs on the mitrebend mirrors. Instead of this, the beams are expanded in freespace such that the peak power density is reduced.

• Reduction of thermal loading in the waveguide: the mitre bends

generate about 0.1% in higher order modes, half of which isabsorbed in the neighboring waveguide sections requiring activecooling of the waveguide. Removing the mitre bends opens upthe possibility to use radiative cooling techniques for the HE11waveguide, simplifying the design and maintenance.

F. Sanchez et al. / Fusion Engineering and Design 84 (2009) 1702–1707 1703

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The design of the steering mirrors (UM4 and LM4) is critical inhat it requires a light weight design for ease in rotating the mirror,nd a good balance between thermal and electrical characteristicsminimizing stress, temperatures and the total induced current dur-ng a disruption event). A majority of the test procedures will beentered on this mirror, assuming that the other mirrors have lessestrictive design parameters.

. Heat load on the mirrors

The main contribution to the heat load on the mirrors is thehmic loss of the reflected mm-wave beam. Every mirror is sub-ect to four partially overlapped 2 MW incoming Gaussian beamassumed circularly polarized), each beam with the followingarameters:

dP

dA= P0�abss

�w2m

[1 + cos2(�inc)]

here P0 is the input power (2 MW), wm the beam spot size onirror, �inc the beam incidence angle to mirror surface normal, s

he surface roughness factor and �abs is the RF absorption factorFig. 2).

.1. Material dependency of ohmic mirror loss

The ohmic loss depends on the surface resistance of the reflector,n the polarization and of the incident angle of the reflected wave.t the considered high frequencies the imaged current density (and

he absorbed power) concentrates within a thin layer (skin depth)t the surface of the conductor.

able 1eat loads on free space mirrors.

teering mirrors (UM4 and LM4)Heat flux from plasma: 10 kW/m2 = 0.6 kWVolumetric (neutronic) heating: 1 MW/m3 = 0.75 kWOhmic loss: (4 beams) = 26.8 kWTotal heat to be removed ∼28.15 kW per mirror

ocusing mirrors (UM3 and LM3)Volumetric (neutronic) heating: 1 MW/m3 = 7.35 kWOhmic loss: 4 × 2 MW × 0.2% = 16 kWTotal heat to be removed ∼23.35 kW per mirror

M1, LM1 and UM2, LM2Volumetric (neutronic) heating: 0.1 MW/m3 = 0.735 kWOhmic loss: 4 × 2 MW × 0.2% = 16 kWTotal heat to be removed ∼16.75 kW per mirrorTotal heat to be removed is ∼170 kW (in-launcher free space mirrors)Cooling water [7]: inlet at 100 ± 15 ◦C, 3.2 MPa

into the upper launcher.

Typically the skin depth is characterized by the mirror material,such as copper [8]. However, the plasma could erode the mirrorsurface, affecting to the absorption. In order to account this effect,the surface roughness factor s has been considered different for theplasma facing mirrors (UM4 and LM4) (s = 2) and for the focusingand free space mirrors (s = 1.3).

Table 1 shows the heat load to be removed per mirror for thecurrent optical configuration.

While the deposited averaged heat load is within reasonablelimits, to remove the cumulated peak ohmic load in compliancewith ITER structural design criteria and ITER cooling water require-ments (reference) is the main challenge. A first approach couldinduce to a thick copper/thin stainless steel mirror, but althoughit is a valid solution for most of the mirrors, in the case of a VDE theplasma facing steering mirrors are subjected to high electromag-netic forces, which set a limit for the size of the flexure pivots at thebalanced configuration of the steering mechanism [4].

The thickness of the steering mirrors was designed to minimizethe effective cross-sectional area where the induced current canflow [5]. Ideally, the reflective surface should have high thermaland low electrical conductivity, but as copper is to be used as areflective surface, it has to be relatively thin to minimize inducedcurrent during a VDE (Fig. 2). The EM forces related to the inducedcurrents during a disruption were estimated for the steering mirrorin the worst configuration and assuming no shielding effect fromthe port wall, dBP/dt = 25 T/s (plasma current 17.85 MA and linear

current decay time 0.04 s) and BT = 5.0 T. The latest values givenfor disruptions of type II and III were accounted and the resultinginduced torque on the mirror is <350 Nm, resulting in a force of<1.4 kN per flexure pivot on the current cantilevered configuration[5].

Fig. 2. Front steering mirror.

1704 F. Sanchez et al. / Fusion Engineering a

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The main steps for the fabrication of the front steering mirrorswithin this schema are shown in the figure and summarized asfollows (Fig. 5):

Fig. 3. SS tubes and copper surface plate before powder HIP.

The second layer of the mirror is relatively thick (∼15 mm)nd houses the cooling channels to evacuate the absorbed mm-ave power. The cooling channels will prevent current from being

nduced in the center of the mirror. Although, current can flowbove and bellow the cooling channels, the thickness has beeneduced to a minimum specially at the edges.. The thermal-echanical stresses of this mirror are within ITER Structural Design

riteria requirements [6].

. The fabrication route for the steering mirror

The particular stepped interface (designed for an optimal bal-nce between cooling performance and electromagnetic-inducedorces), requires special attention to the manufacturing method.he following manufacturing methods were evaluated for joininghe reflective copper surface, SS intermediate plate and SS coolinglate:

Hot isostatic pressing (HIP).Brazing.Electroplating.

.1. Hot isostatic pressing (HIP)

Fig. 3 shows the components used for a mirror HIP prototypeanufactured to insure that the powder HIP technique is a valid

Fig. 4. Mirror surface after electroplating.

nd Design 84 (2009) 1702–1707

method in assembling the mirrors. There are two issues in particularthat were being addressed, first the quality of joining of the SS316Lto the copper and second maintaining the geometrical integrity ofthe cooling lines and mirror structure.

The joining of solid copper, SS tubes and SS powder has beenshown to be enhanced by placing an inter-layer of nickel, and thequality of the join was good, with a high continuity and no voidsappreciated. The control of the interface geometry and shape isthe weak point of this method, which could drive to significantdeformation of the cooling channel structure or the Cu–SS steppedinterface.

3.2. Electroplating

Electroplating is an interesting and well-known option, but thesize of the mirror plate (roughly, 290 mm × 200 mm) and the needof a thin Cu deposition at the edges (0.3 mm) while a thick one atthe center (4 mm thick) imposes a long time (>250 h in a pyrophos-phate bath) and a difficult final machining. Due to the low currentdensity in the center respect to the edges, first tests only reached1.85 of 4 mm of deposition thickness at the mirror center after 100 hdeposition. The current density profile makes the deposition ratherin the edges than in the center, which is the opposite of the desiredCu profile (Fig. 4).

3.3. Vacuum brazing

Brazing is a well-known technology, but the large areas requirea careful control of the interface Cu–SS. In general, brazing defectshave two effects:

• Discontinuity in the exchange of the thermal stream between theabsorber and the heat exchanger body.

• The formation of pockets which are difficult to remove and whichconstitute eventual leaks or future hot spots/crack roots.

Fig. 5. Mirror bodies for brazing.

F. Sanchez et al. / Fusion Engineering and Design 84 (2009) 1702–1707 1705

term

4. Test programme

During the TW5 work programme, the mitre bend was identi-fied as the component that experiences the highest peak power

Fig. 6. Mirror surface and in

Step 1: Cutting the SS plates in the same laminar sense. In order toguarantee a continuous brazing, the SS pieces to be brazed have tobe cut in the same sense and follow an annealing heat treatmentcomprising high heating, followed by quick cooling in order tokeep the homogeneous austenitic structure after return to ambienttemperature.Step 2: Starting from rough block: elliptical contour and ellipti-cal pockets at bottom face (male of the previous piece): importantparameters of this operation are the required planity (0.02 mm)and the parallelism and perpendicularity <0.01 mm. Also to betaken into account, the last step of machining shall be done with-out heavy lubricating or with non-silicone-based oils, as they aredifficult to remove during the previous to brazing cleaning process.In our case, in order to obtain the required planarity (and avoidingan annealing treatment of the Cu, which would make machining

more difficult), an extra-thickness at the top-flat side was added.After a first (failed) attempt with 3 mm thick the required mini-mum extra-thickness for machining was determined to be 12 mm(Fig. 6).

Fig. 7. Cooling channels machined on bottom plate before brazin.

ediate plate before brazing.

Step 3: Joining a machined Oxygen/halogen free copper to themachined intermediate plate by means of brazing.

The ultrasonic tests showed that a large area (>50% of thetotal surface) was poorly brazed, which could produce thermaldiscontinuities and hot spots due to the peaked heat depo-sition. Due to this the mirror was subject to solid HIP afterbrazing.Step 4: Machining the interface groove on the intermediatepiece and machining the cooling channels onto the bottom plate(Fig. 7).

Fig. 8. Temperature distribution from ANSYS and time constant.

1706 F. Sanchez et al. / Fusion Engineering a

Table 2Test programme.

Thermal heat load Simulate 2 MW with artificially increased absorption(using TiO2 or Ni coating on the mirror surface) andmulti beam footprint (from a deformed single beam)(in collaboration with JAEA)

Pressure testing Proof pressure testing of the coolant circuits accordingto ITER requirements (6.25 MPa, equivalent to 1.25times the design pressure [6]

Surface reflectivity Measurement of the surface reflectivity of variousmirror materials to determining the expected thermalloading of the mirrors (performed by CNR and IPF)

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urface roughness Determine a realistic surface roughness factor s, forestimating the increased absorption beyond that ofnominal ohmic losses (performed by CNR and IPF)

ensity (≤4 MW/m2) of all mm-wave components in the upperauncher [9]. Therefore, a high power RF test program was devel-ped with the aim of demonstrating the feasibility of coolinguch peak power densities in the ITER environment. The programas established in collaboration with JAEA and GA and integrated

wo mitre bends into the transmission line of the 170 GHz, 1 MWyrotron test facility in Naka. The present optical (miter bend free)esign will use of the experience gained through that testing pro-ram.

The test consisted on integrating a mitre bend into the trans-ission line of the 170 GHz, 1 MW gyrotron test facility in Naka. A

econd mitre bend was installed with the mirror coated with nickelo increase the surface resistivity by a factor of 1.9 to simulate evenigher incident powers (up to ∼1.8 MW). Several diagnostic weresed to diagnose the temperature rise of various components:

Thermocouples embedded in mirror (provide cross-check withANSYS modeling).Absorbed power in mirror (estimates total absorbed power inmirror).Thermocouples on waveguide (estimates mode coupling increasedue to mirror thermal deformations at higher incident powers).Note that F4E and ITER are considering adding a fiber optic tomonitor the mirror temperature via infrared measurements. Thefiber optic would be useful for determining the amount of impu-rities on the mirror during plasma operation. Once the DD or Dtoperation occurs the neutrons will render the fiber optics opaque,but the time before it happens (around 4 years) will be sufficientto characterize impurities such as C, W or Be.Absorbed power in waveguide cooling clamps (estimates modecoupling).Calorimetric load (monitors incident RF power).

An ANSYS model of the mirror was developed prior to thenset of the tests, which estimated the peak temperature rise,hermal profile on and in the mirror, thermal deformation of the

irror surface and thermal time constants of the cooling circuitFig. 8).

The actual experiments occurred in late February 2007 with thearticipation of a CRPP representative. Incident powers of up to.7 MW and pulse lengths between 400 and 1000 s were achieved.ome results are shown in Figs. 3–6b, which are the measuredbsorbed power for both the copper and nickel mirror as a functionf incident powers. The percentage of absorbed power was 0.22% forhe copper and 0.42% for the nickel, which is approximately a 30%ncrease over the theoretical and low power absorption coefficients.

The test program for the free space mirrors (including theteering mirror ensemble) is summarized in Table 2. It includesanufacturability, thermomechanical hydraulic and optical tests

f each of the mirrors associated with the free space propagationf the beam through the launcher.

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nd Design 84 (2009) 1702–1707

The mirrors will be either flat or focusing and all are assumedto have an optically polished copper surface. Note that the opticalsurface is not required for propagation but is important for opticalalignment of the system using a laser.

The testing of the optical system at full power (four beamsof 2.0 MW) and CW operation would require an expensive testfacility with multiple gyrotrons. It is indeed more effective usingan e-beam that can be swept across the mirror surface withthe sweep speed varied to simulate the power density of thefour incident beams on a given mirror. Or using the high resis-tive coating and a lower power source. Such a simulated testreduces the infrastructure costs and speeds up the testing process.Therefore the test program (listed in Table 2) for the free spacemirrors will be adapted to existing test facilities, to reduce R&Dcosts.

5. Conclusions

The thermal loads of the ITER ECH upper launcher mirrorsare within reasonable limits, and they are lower than the onesexpected for already tested mitre bend mirrors. The design ofthe steering mirrors (UM4 and LM4) is critical in that is subjectto both high peaked thermal loads and electromagnetic forcesduring VDE events, requiring a light weight design for ease inrotating the mirror, and a good balance between thermal andelectrical characteristics (minimizing stress, temperatures andthe total induced current during a disruption event). Thicknessof the steering mirrors was designed to minimize the effec-tive cross-sectional area where the induced current can flow.The key of success in manufacturing of the front steering mir-ror, is a good joint between the multi-thickness layer of Cu(which variates from 0.3 mm at the edges to 4 mm at the cen-ter) and the SS cooling plate. While none single method is totallyadapted to that kind of geometry, a method that joins brazingwith a post-brazing solid HIP looks promising (at the momentof writing this paper, the ultrasonic test for the joint qualityare underway). As a brief resume of the difficulties encoun-tered, HIP is the most difficult manufacturing method becausethe difficulty of controlling the geometry, brazing has the riskof larger voids and internal discontinuities, and electroplat-ing such a Cu thickness (4 mm) in the center of a large area(200 mm × 300 mm) while needing a thin deposition (0.3 mm) atthe edges is a very demanding task for final machining, althoughlooks as the preferred option. Low power testing is envisioned in2009.

Acknowledgments

This work, supported by the European Communities under thecontract of Association between EURATOM-Confederation-Suisse,was carried out within the framework of the European FusionDevelopment Agreement (EFDA) and Fusion for Energy (F4E). Theviews and opinions expressed herein do not necessarily reflectthose of the European Commission.

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2] M.A. Henderson, et al., Overview of the ITER EC upper launcher, Nuclear Fusion

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3] R. Chavan, et al., The ECH front steering launcher for the ITER upper port, FusionEngineering and Design 82 (5–14) (2007) 867–872.

4] J.D. Landis, et al., Design of the critical components in the ITER ECH upperlauncher steering mechanism, Fusion Engineering and Design 82 (5–14) (2007)897–904.

ring a

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[8] R. Chavan, et al. Design and testing of the ITER ECRH Upper Launcher- thermo-

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