TECHNOLOGICAL CHALLENGES OF ITER DIAGNOSTICS

1. TECHNOLOGICAL CHALLENGES OF ITER DIAGNOSTICS 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

2. 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 includingmagnetics, 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.

3. The physicsof the operation of the diagnostics, as established on past and present machines is, in many cases, directly applicable to ITER. On the other hand, thetechnology and engineering differs substantiallyand 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

4. OUTLINE  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

5. 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 ! )

6. Consequentially arange of phenomenahave 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 deposition Radiation induced absorption Radioluminescence Heating Change in other properties such as activation, transmutation and swelling Moreover, thenuclear environmentsets stringent demands on the engineering of the diagnostic systems – for example onneutron shielding, Tritium containment, vacuum integrity, RH compatibility.

7. SELECTED DIAGNOSTICS

8. 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.

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

10. Location of a pick-up coil behind a blanket module Vacuum vessel Coil Manifold Blanket Module

11. 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 EMF • Radiation induces currents between the sensor wire and its • surroundings. • -> 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 EMF • Seen in MI cable, possibly due to manufacturing non- • uniformity. • -> Can cause spurious EMF arising from nuclear heating.

12. Radiation effects – delayed RIED: Radiation-Induced Electrical Degradation.Not fully understood but maybe associated with metal colloid formation 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

13. 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

14. Measurement of Thermally Induced EMF Coil nv 30 mm Heated MI Cable Mineral Insulated Cable heated by 55 °C over 30 mm at intervals of 0.5 m Measurements by E. Hodgson and R. Vila, CIEMAT

15. 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.

16. Magnetic Diagnostics: Areas Requiring Further DevelopmentDesignGood 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).

17. TECHNOLOGICAL CHALLENGES: NEUTRON DIAGNOSTICSIn 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 DetectorsThe confined alphas will probably be measured with a microwave system.

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

19. 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.

20. 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

21. 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 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

22. Neutron Diagnostics:Areas Requiring Further DevelopmentDesignFor 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: Improvedcompact 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.

23. 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 PolarimeterSome 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.

24. The principal design requirement with the optical systems is to provide high optical throughput while maintaining neutron shielding. 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. Scheme of LIDAR Thomson Scattering

25. 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 mirrorsOptical materials (windows, fibres) suffer Radiation Induced Absorption and Radioluminescence: Maintaining the performance of the windows, optical fibres etcThe 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

26. An extensive R&D program is on going in which candidate mirror materials are subject toenergetic particle bombardment with ion sourcesandplasma simulators, and toenvironmental tests intokamaks. The degradation of the performance of the mirrors is measured. Molybdenum Single crystal Polycrystal Degradation of reflectivity of candidate first mirrors materials under energetic ion bombardment. After V Voitsenya, et al, RSI, 2001

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

28. Similarly an extensive programme is ongoing with candidate optical materials for windows and optical fibres. Absorbed dose is 0.3GGy(Si). The sample dimension is 16 mm in diameter and 8 mm in thickness.

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

30. Optical Diagnostics: Areas Requiring Further DevelopmentDesignThe 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

31. 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.

32. 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)

33. Other systems require viewing lines inside the divertor. Viewing Fans of the Divertor Impurity Monitor in the Divertor RegionThe mirrors are enclosed in a box and baffles are incorporated to reduce the deposition on the mirrors.

34. Additional views are installed in the upper and equatorial ports

35. 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.

36. Spectroscopy/Bolometry: Areas Requiring Further DevelopmentDesignThe 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.

37. 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).

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

39. In vessel waveguide of the plasma position refelectometer

40. Reflectometry: low field side. Differential movements taken in in the waveguide joints. Port Cell (air) Antenna groups Diagnostic block Shield module

41. Differential movements taken in the waveguide joints.

42. Microwave diagnostics:Areas Requiring Further DevelopmentDesignThe 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).

43. 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 TV Thermocouples Pressure Gauges Residual Gas Analyzers IR Thermography Divertor Langmuir ProbesThe 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.

44. Divertor systems

45. 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.

46. 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.

47. 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

48. TECHNOLOGICAL CHALLENGES: INTEGRATION AND INSTALLATION In-vesselHave to accommodate diagnostic sensors, cables and waveguides.Challenges Installation of diag components some of which are delicate Vacuum compatibityFor 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.

49. Double Window Arrangement for LIDAR system Part of the inboard Vacuum Vessel with Blanket and Divertor removed

50. 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