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A E Costley, T Sugie, G Vayakis and C Walker*

ITER International Team, Naka, Japan*ITER International Team, Garching, Germany

23rd SOFT Symposium, Venice, September, 2004

TECHNOLOGICAL CHALLENGES OF ITER DIAGNOSTICS

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Like all modern tokamaks ITER will require an extensive diagnostic system to provide the measurements necessary for

Machine protection -> separatrix/wall gap, first wall temperature, etc

Plasma control-> plasma shape and position, plasma current, etc

Physics studies-> confined alpha particles, alpha driven modes, etc

About 40 individual measurement systems drawn from the full range of plasma diagnostics including magnetics, neutron systems, optical, spectroscopic, bolometry, and microwave systems.

Installed in multiple locations - Vacuum Vessel, Upper and Equatorial ports, Divertor, Port Cells and in the remote Diagnostic Building.

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The physics of the operation of the diagnostics, as established on past and present machines is, in many cases, directly applicable to ITER. On the other hand, the technology and engineering differs substantially and involves many difficult challenges. These arise from:

The relatively harsh environment phenomena new to diagnostic design have to be handled;

The control role of the measurements requires high accuracy and reliability;

The long plasma pulse length

requires high stability; The nuclear environment

sets stringent demands on the engineering.

The technological challenges and the progress made so far in overcoming them are presented in this paper

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Environment - new phenomena that have to be considered in diagnostic

selection and design- new engineering requirements

Technological challenges- diagnostic systems- integration and installation- maintenance and operation

Summary

OUTLINE

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ITER ENVIRONMENT

Relative to existing machines, on ITER the diagnostic components will be subject to (relative to JET)

High neutron and gamma fluxes (up to x 10)

Neutron heating (essentially zero)

High fluxes of energetic neutral particles from charge exchange processes (up to x5)

Long pulse lengths (up to x 100)

High neutron fluence (> 106 ! )

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Consequentially a range of phenomena have to be considered that are new to diagnostic design including:

Radiation-induced conductivity (RIC) Radiation induced electrical degradation (RIED) Radiation-induced electromotive force (RIEMF)Erosion and depositionRadiation induced absorptionRadioluminescenceHeatingChange in other properties such as activation, trans

mutation and swelling

Moreover, the nuclear environment sets stringent demands on the engineering of the diagnostic systems – for example on neutron shielding, Tritium containment, vacuum integrity, RH compatibility.

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SELECTED DIAGNOSTICSMagnetic Diagnostics Spectroscopic and NPA Systems Vessel Magnetics CXRS Active Spectr. (based on DNB) In-Vessel Magnetics H Alpha Spectroscopy Divertor Coils VUV Impurity Monitoring (Main Plasma) Continuous Rogowski Coils Visible & UV Impurity Monitoring (Div) Diamagnetic Loop X-Ray Crystal Spectrometers Halo Current Sensors Visible Continuum ArrayNeutron Diagnostics Soft X-Ray Array Radial Neutron Camera Neutral Particle Analysers Vertical Neutron Camera Laser Induced Fluorescence (N/C) Microfission Chambers (In-Vessel) (N/C) MSE based on heating beam Neutron Flux Monitors (Ex-Vessel) Microwave Diagnostics Gamma-Ray Spectrometers ECE Diagnostics for Main Plasma Neutron Activation System Reflectometers for Main Plasma Lost Alpha Detectors (N/C) Reflectometers for Plasma Position Knock-on Tail Neutron Spectrom. (N/C) Reflectometers for Divertor PlasmaOptical Systems Fast Wave Reflectometry (N/C) Thomson Scattering (Core) Plasma-Facing Comps and Operational Diag Thomson Scattering (Edge) IR Cameras, visible/IR TV Thomson Scattering (Divertor region) Thermocouples Toroidal Interferom./Polarimetric System Pressure Gauges Polarimetric System (Pol. Field Meas) Residual Gas Analyzers Collective Scattering System IR Thermography DivertorBolometric System Langmuir Probes Bolometric Array For Main Plasma Diagnostic Neutral Beam Bolometric Array For Divertor

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TECHNOLOGICAL CHALLENGES: MAGNETIC DIAGNOSTICS

Magnetic diagnostics typically consist of a number of coils and loops mounted on the inside of the vacuum vessel and, in some case, in the divertor. They are used to measure parameters fundamental to the plasma operation such as the plasma position, shape and current.

On ITER the magnetic diagnostic consists of:sets of pick-up coils, saddle loops and voltage loops on the inner wall of the vacuum vessel;sets of pick-up coils and steady state sensors on the outer surface of

the vacuum vessel;continuous poloidal (Rogowski) loops on the TF coil case;sets of coils in the divertor diagnostic cassettes;a diamagnetic system comprising poloidal loops on the inner wall of the VV and compensation circuits inside and outside the vessel;Rogowski coils mounted around earth straps of the blanket/shield modules and divertor structures for measuring the 'halo’ currents.

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MHD-dedicated saddle loops mounted on all nine machine sectorsLoops made from mineral insulated (MI) Cable

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Location of a pick-up coil behind a blanket module

Blanket Module

Coil

Manifold

Vacuum vessel

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Main Technological Challenges

Radiation effects – prompt RIC: Radiation-Induced Conductivity

-> Loads the signal but can be made negligible by careful choice of insulator and can be compensated.

RIEMF: Radiation-Induced EMFRadiation induces currents between the sensor wire and itssurroundings.

-> Expected to generate < 100 nV signals across the ITER coils.

Nuclear Heating-> 0.1 – 1 W/cm3 cooled by conduction so special construction

needed to reduce peak temperature to acceptable levels.

TIEMF: Thermally Induced EMFSeen in MI cable, possibly due to manufacturing non-uniformity.

-> Can cause spurious EMF arising from nuclear heating.

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Radiation effects – delayed

RIED: Radiation-Induced Electrical Degradation. Not fully understood but maybe associated with metal colloidformation in insulator.

-> In ITER coils limits design electric field < 100 kV / m (cf. typical design values ~ 1 MV / m) and leads to rather large

coils.

RITES: Radiation-Induced ThermoElectric Sensitivity. -> Nuclear heating supplies the temperature differences-> A variety of effects can supply the material property changes

that generate in turn thermocouples

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Measurements of RITES and RIEMF

Measurements with two protoype coils of different parameters in the Japan Materials Test Reactor (JMTR)

Coil A, 0.5 mm dia Cu core, 0.25 mm St.St. sheath, Magnesia

Coil B, 0.8 mm dia Cu core, 0.23 mm St.St. sheath, Magnesia

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nvCoil

30 mmHeated

MI Cable

Measurement of Thermally Induced EMF

-2500

-2000

-1500

-1000

-500

0

500

1000

0 1 2 3 4 5 6 7 8

RIEMF task T effect

Cable A+B fixed T (nV)

Distance (m)

Mineral Insulated Cable heated by 55 °C over 30 mm at intervals of 0.5 m

Measurements by E. Hodgson and R. Vila, CIEMAT

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Other possible sources of parasitic voltage for the coils include The various thermomagnetic effects (arising when strong magnetic field gradients interact with thermal gradients). RF pickup. This can occur in the coils (if the RF response of the coil is high enough), but also in any exposed connectors. Electrochemical voltage generation, due to oxidation or similar

processes occurring in the conductor creating a weak but significant battery.

All these effects can be handled by: Careful selection of sensor parameters especially to avoid prompt effects due to RIC, RIEMF and RITES. Choice of materials to avoid mechanical damage (cracking and

swelling) and electrical damage (RIED). Design of suitable cooling mechanisms. Inclusion of special measures (eg shields over exposed connectors).

The designs are based on modeling of the effects using data obtained in dedicated R&D.

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Magnetic Diagnostics: Areas Requiring Further Development

DesignGood prototype designs exist but further optimization of the sensor designs, number and location is needed. Some detailing has been performed but this needs to be taken further.

R&DNeed more information on the radiation and thermoelectric effects on the candidate materials.Some specific developments needed, for example integrators with a CMRR that can handle the expected level of RIEMF.Small steady state sensors for use outside the vessel.

On the basis of the work carried out so far, we can expect that the needed information will be obtained to complete the design and that the system will meet the measurement requirements for the initial pulse length (300s) and with further development the planned extended pulse operation (3600 s).

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TECHNOLOGICAL CHALLENGES: NEUTRON DIAGNOSTICS

In order to demonstrate the generation of fusion power in ITER it will be necessary measure the total neutron source strength with high accuracy and high reliability. Measurements of the fusion products, especially the confined and escaping alpha particles, are also needed to achieve a full understanding of the ignition physics. In order to make these measurements six neutron diagnostics are planned:

Internal and External Neutron Flux Monitors (NFMs)

Radial and Vertical Viewing Neutron Cameras (RNC and VNC)

Neutron Activation Systems

Lost Alpha Detectors

The confined alphas will probably be measured with a microwave system.

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Radial Camera

Additional channels with the detectors mounted in the port for measuring the plasma edge region. Requires development of improved compact detector/spectrometers.

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Two particularly difficult areas, currently unsolved, are:

the provision of a Vertical Viewing Neutron Camerathis is difficult because there are no vertical ports.

the development and installation of devices to measure the escaping alphas (Faraday Cups or Scintillator probes)

such devices would have to be very close to the plasma and

may require electrical connections to the BSM.

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Schematic of proposed concept for the Vertical Neutron Camera mounted in a divertor port at the lower level presently under consideration.

The plasma would be viewed through the gap in the divertor cassettes and the Blanket Modules (BM). The gap between the divertor cassettes may have to be enlarged.Figure courtesy of A Krasilnikov, TRINITY Inst. Moscow

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View of the Faraday cup detector array inside the JET vacuum vessel. The detectors are mounted on five “pylons” which are supported by a curved I-beam mounted to the vacuum vessel. After D Darrow et al, 15th High Temp. Plasma Diag, Conf, San Diego, April, 2004

Exploded view of a single pylon, showing the top plate with aperture holes, a stack of alternating Ni foils and mica insulators, terminal blocks, foil stack mounting recesses, backing plate and spring, and carbon-carbon composite protective tile with mounting hardware

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Neutron Diagnostics:Areas Requiring Further Development

DesignFor the Flux Monitors, Radial Neutron Camera and the Activation Systems, no serious design issues are foreseen. The VNC requires more work especially on the interfaces.

R&DSome specific developments are needed:

Improved compact spectrometers for use in the RNC. High-resolution spectrometer (for DT measurements) High efficiency spectrometer for measurement of alpha knock-on

tails. ITER compatible detectors for lost alpha measurements.

We can expect that the main measurements will be made to the required specification – in particular the neutron source strength – but it may not be possible to satisfy all the target measurement specifications on the spatial profile of the neutron emission and on the escaping alpha particles.

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TECHNOLOGICAL CHALLENGES: OPTICAL DIAGNOSTICS

There will be several major optical systems on ITER, and some of them will require multiple access to the plasma. The main systems are:

Thomson Scattering (Core) (LIDAR type)

Thomson Scattering (Edge) (Conventional)

Toroidal Interferometer/Polarimeter

Poloidal Plane Polarimeter

Some of the Spectroscopic systems, for example, the Visible Continuum Array, and the First Wall and Divertor Viewing systems that operate in the near IR, share some of the same implementation difficulties and will utilize the solutions that have been developed.

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This is achieved by using labyrinths in shielding blocks mounted in the ports. The first element of the system has to be a mirror because the high levels of radiation lead to enhanced absorption in refractive components.

The principal design requirement with the optical systems is to provide high optical throughput while maintaining neutron shielding.

Scheme of LIDAR Thomson Scattering

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The mirrors face the plasma and can suffer both erosion and deposition depending on their location and the plasma conditions. This leads to one of the most challenging problems in the designs:

Maintaining the performance of the mirrors

Optical materials (windows, fibres) suffer Radiation Induced Absorption and Radioluminescence:

Maintaining the performance of the windows, optical fibres etc

The systems typically have components in several locations such as the ports, port cells, galleries, and the diagnostic building which leads to another challenging problem

Maintaining the alignment and calibration

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An extensive R&D program is on going in which candidate mirror materials are subject to energetic particle bombardment with ion sources and plasma simulators, and to environmental tests in tokamaks. The degradation of the performance of the mirrors is measured.

Degradation of reflectivity of candidate first mirrors materials under energetic ion bombardment. After V Voitsenya, et al, RSI, 2001

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50

55

60

65

0 1 2 3 4 5 6 7

600 nm

Thickness of sputtered layer, μm

M o poly

W poly

W (111)block

W (111)

M o (111)

SS(111 )

Single crystal Polycrystal

Molybdenum

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A particularly difficult mirror is the first mirror in the Plasma and First Wall Viewing system since it has to be far forward to get the necessary views. This has been designed in some detail.

400 500 600 700 800 900 10000

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40

60

80

100 before after

wavelength, nm

Dielectric mirrors can be used for second and third mirrorsI Orlovskiy and K Vukolov, this conference paper P2C-D-159

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Similarly an extensive programme is ongoing with candidate optical materials for windows and optical fibres.

0

20

40

60

80

100

160 240 320 400 480 560 640 720 800

Annealing Effects on KU-1

Initial

500 C

372 C

235 C

100 C

Transparency (%)

Wavelength (nm)

600 C

Absorbed dose is 0.3GGy(Si). The sample dimension is 16 mm in diameter and 8 mm in thickness.

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Active alignment schemes are used to maintain the alignment, for example in the Divertor Impurity Monitor.

Opticsin Divertor Cassette

Collecting & FocusingOptics CCD

Mirror

CCR

DiffuserPlane Mirror

from Plane Mirroron CCD

from CCR

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Optical Diagnostics: Areas Requiring Further Development

DesignThe system designs are generally well advanced at the concept/feasibility level. Detailing is needed.

R&DMore measurements on effects of erosion and deposition on mirrorsMore measurements needed on radiation induced absorption especially for windows carrying high power laser radiation.

Development of mitigation techniques, eg shuttersDevelopment of in-situ cleaning techniquesDemonstration of active alignment/calibration systems

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TECHNOLOGICAL CHALLENGES: SPECTROSCOPY AND BOLOMETRY

An extensive array of spectroscopic instrumentation will be installed covering the visible to X-ray wavelength range. Both passive and active measurement techniques will be employed. The four main regions of the plasma - the core, the radiation mantle, the scrape-off layer (SOL), and the divertor - will be probed.

Instrument Wavelength Range

Function

Hsystem Visible region ELMs, L/H mode indicator, nT/nD and nH/nD at edge and in divertor.

Visible continuum array 523 nm Zeff(r), line averaged electron density indicator.

VUV (main/divertor plasma) 2.3 – 160 nm Impurity species identification.

X-ray (survey and high resolution)

0.1 – 2.5 nm Impurity species identification, plasma rotation, Ti.

Divertor impurity monitor 200 – 1000 nm Impurity species and influx, divertor He density, ionisation front position, Ti.

Charge Exchange Recombination Spec. (CXRS)

Visible region Ti (r), He ash density, impurity density profile, plasma rotation, alphas.

Motional Stark Effect (MSE) Visible region q (r), Er (r).

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Those spectroscopic systems that use the visible/IR region share many of the same challenges as the Optical Systems and the same solutions are adopted.

Some systems require direct coupling to the tokamak and special provisions are provided.

Direct coupled systems integrated on one port (Figure courtesy of R Barnsley, ITER IT)

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Other systems require viewing lines inside the divertor.

Viewing Fans of the Divertor Impurity Monitor in the Divertor Region

The mirrors are enclosed in a box and baffles are incorporated to reduce the deposition on the mirrors.

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Additional views are installed in the upper and equatorial ports

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Bolometry requires the installation of multiple detectors in the ports, in the divertor cassettes and at selected locations and inside the vacuumvessel.

The sensors must be radiation hard and require special development. Several types are under development. One is based on a resistive foil type and the another utilises a thin foil with a pinhole camera to form an image of the plasma using the plasma radiation.

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Spectroscopy/Bolometry: Areas Requiring Further Development

DesignThe system designs are generally well advanced at the concept/feasibility level. Detailing is needed.

R&DMirrors – same as for the Optical systems.Some of the techniques also require the maintenance of the polarization on reflection and this can be disturbed by deposits.Further development and validation of the radiation hard bolometers.

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TECHNOLOGICAL CHALLENGES: MICROWAVE SYSTEMS

Microwave diagnostics only require waveguides in the VV close to the plasma and since these can be made very robust these diagnostics are relatively well suited to the ITER environment. The principal microwave diagnostics are: Electron Cyclotron Emission (ECE) from the main plasma, and three Reflectometry systems for probing the main plasma, the divertor plasma, and for measuring the plasma position. A system for measuring the confined alpha particles based on Collective Scattering is also under consideration. The key technical challenges are

Maintaining good performance of the in-vessel waveguides with several complicated bends. The installation of the antennas in the VV and the interface with the BSMs. Coping with relative movements of different components of the systems. In-situ calibration for ECE (requires a source in the port).

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Waveguides for the Plasma Position Reflectometer mounted in the VV (with Blanket and Upper Port Plug removed)

Inboard Outboard

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In vessel waveguide of the plasma position refelectometer

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Reflectometry: low field side. Differential movements taken in in the waveguide joints.

Diagnostic block

Shield module

Port Cell (air)

Antenna groups

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Differential movements taken in the waveguide joints.

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Microwave diagnostics: Areas Requiring Further Development

DesignThe system designs are generally well advanced at the concept/feasibility level. Detailing is needed.

R&DFurther development and validation of the in-vessel waveguides.Development of a radiation hard calibration source (ECE).

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TECHNOLOGICAL CHALLENGES: PLASMA FACING COMPONENTS AND OPERATIONAL DIAGNOSTICS

Several diagnostics will be included for measuring the state of the plasma facing components - especially first wall and divertor – and for supporting the technical operation of the machine. The principal diagnostics are:

IR Cameras, visible/IR TVThermocouplesPressure GaugesResidual Gas AnalyzersIR Thermography DivertorLangmuir Probes

The IR/Visible systems have similar problems to the optical systems.There are no significant problems with the pressure gauges.The performance of the Thermocouples and Langmuir Probes can be affected by the environment and may have a limited lifetime.

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Divertor systems

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An outstanding challenge is the design of the the Langmuir probesBy using CFC as the probe tip the intention is that the probe performance will be maintained even as the divertor erodes but this has not been established.

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9.5

4

11

30

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Toroidal Direction Plasma Facing Surface

Probe Tip

(CFC)

Ceramic-Insulated

Surface

Brazed on the Side

of Plasma Facing

Tile

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Current

Heat Flow

Plasma Particles

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TECHNOLOGICAL CHALLENGES: DNB

Main Parameters: 100 keV, H0, pulse 1-3 s (modulated at 5 Hz)every 10-20 s.

Reference design:

The DNB uses a negative ion beam as the primary beam to achieve the required performance with acceptable system efficiency. It uses the same negative ion source as the ITER H&CD injectors, coupled to a single stage accelerator. The design concept and hence the geometry of the beam line components, i.e. the neutraliser and the residual ion dump and the calorimeter, are similar to those utilised for the H&CD injectors. Thus, it is possible to utilise largely the R&D and design performed for the H&CD NB injectors and to standardise the components, maintenance equipment and procedures. See E Di Pietro, et al, Proc 21st SOFT.

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The DNB shares a port with a H&CD injector and this has made it difficult to aim at the plasma centre. However, this has now been achieved. 　

.

DNB installed on port 4

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TECHNOLOGICAL CHALLENGES: INTEGRATION AND INSTALLATION

In-vessel Have to accommodate diagnostic sensors, cables and waveguides.Challenges

Installation of diag components some of which are delicate Vacuum compatibity

For example, there are ~75 Kms of MI cable which can potentially outgas. R&D is in progress on perforated cable.

Interfaces with other machine components, esp. the BSM and VV Preservation of component performance Limited opportunities for changing or maintaining component Nuclear heating – provision of cooling, and/or operation at

elevated temperature. Reliability of primary safety boundary (windows, electrical

feedthroughs) R H compatibilty Resistance against em loads, erosion and deposition

These challenges have to be met individually by a combination of design and R&D.

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Part of the inboard Vacuum Vessel with Blanket and Divertor removed

Double Window Arrangement for LIDAR system

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Proposed Collective Scattering diagnostic at 60 GHz for measuring confined alpha particles through forward and backward scattering. The forward scattering measurement requires an antenna on the inboard side.

H Bindslev et al, 15th High Temp. Plasma Diag, Conf, San Diego, April, 2004

QuickTime™ and aGraphics decompressor

are needed to see this picture. 30 mm

overmoded waveguides

mirror

horn array

Fund. waveguides

earthstrap

Blanket Module key

inner vesselwall

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In Upper and Equatorial Ports Have to accommodate diagnostic sensors, cables, waveguides, mirrors for activation systems.Challenges

Optimum use of available space which is consistent with role and importance of measurement Provision of high throughput transmission lines while maintaining effective neutron shielding Erosion and deposition on plasma facing components Resistance to em loads Nuclear heating – provision of cooling Feasible installation and maintenance procedures compatible with

R H equipment Use of a design which has common interfaces with the machine but can be customized for the different diagnostics

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Port distribution – eq level

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A generic port structure, which gives common interfaces with the machine, but has custom diagnostic modules, has been developed.

Equatorial port (#3) showing the flange face (contains components for the MSE, CXRS, Vis/IR viewing and H alpha diagnostics)

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Special BSM at each diagnostic port, based on Limiter design

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In Divertor Ports Have to accommodate diagnostic sensors, mirrors, cables, and waveguides.

Challenges Same as for in-vessel plus

Severely restricted access – many other components at this level. Limited modifications can be made to the divertor Highest levels of radiation (neutrons and gammas). Do not have the shielding and protection of the BSMs as in the main chamber. High levels of erosion of the divertor plates potentially leading to

high levels of deposition on the diagnostic components.

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Arrangement of divertor diagnostic cassette and diagnostic racks, shown as assembled in the torus and port.

A similar approach to that adopted in the equatorial plane and upper ports is used but some of the divertor cassettes are modified to take diagnostic equipment.

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Some components are installed in the port cells

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Transmission lines pass through the galleries to lasers, detectors, spectrometers etc in the diagnostic building

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TECHNOLOGICAL CHALLENGES: MAINTENANCE AND OPERATION

Several factors relating to the maintenance and operation add technical challenges, especially

Infrequent periods between maintenance breaks-> requires stability of operation for long periods

Components in the vessel, port and divertor will be radiation hard necessitating handling with RH tools and repair in the Hot Cell

See C Walker et al paper P2C-D515

Overall there is a requirement for a very high reliability in performance, for long times, of a somewhat remote complicated assembly, existing in a hostile environment. An approach similar to that taken for satellites is therefore appropriate. It is intended to procure the diagnostics through port based procurement packages. This could be thought of as a collection of ‘satellites’ provided by the different ITER partners.

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FINAL COMMENTS

The step to ITER Diagnostics is very large --> represents a sea change or even a cultural change. Diagnostics for next step devices are beginning to be referred to as a new discipline - Burning Plasma Diagnostics.

The base for ITER diagnostics is of course the techniques currently used on today’s large tokamaks, but these techniques have not been selected or developed for the BPX environment and are often not well suited to that environment. There is a need for the development of specific techniques which are better suited to the environment.

The benefits would be substantial. It is well established that the performance and knowledge gained in experiments on large machines are directly related to the diagnostic capability. This needs to be recognised and the development of BPX diagnostics should become a part of the programme of today’s large tokamaks.

ITER is flexible and can accommodate new techniques, but time is limited. Development time for a new technique is typically 3 - 5 years.

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SUMMARY

In order to support the operation and exploitation of ITER an extensive diagnostic system will be required.

While the physics of the operation of the diagnostics, as established on past and present machines is, in many cases, directly applicable to ITER, the technology and engineering of the implementation of the systems differs substantially and involves many difficult challenges.

The challenges arise from several different aspects. In particular many phenomena new to diagnostic design can occur due to the unavoidable location of many of the diagnostic components in the high radiation fields inside the vacuum vessel.

The challenges are being met individually through a combination of careful technique selection, dedicated R&D, and design. Good progress has been made but the full resolution of the challenges will require significant effort by the laboratories of the ITER parties. This should extend to the development of Burning Plasma Diagnostics as an accepted

part of current programmes.

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Thank you for your attention