# Presentation: 138346

## The ITER radiation environment for diagnostics

- G. Vayakis
- With apologies to A. Costley, E. Hodgson, C. Walker, K. Ebisawa R. Reichle, T. Nishitani and the many contributors to the ITER Nuclear Analysis Report
- N 55 RI 38 04-05-06 W 0.1

## Outline

- Introduction
- Radiation environment
- Radiation effects and affected components
- Radiation R&D summary
- Specific examples
- Conclusions

## Warning!

- Radiation effects on diagnostics span a vast multidimensional space
- My experience in this area has sampled small volumes of this space, both as a user and a generator of data
- Your mileage will vary!

## ITER parameters

- Radiation identified as a problem at global level:

- From NAR

## Key environmental issues affecting  diagnostic performance

| Effect | Of concern ... |
| --- | --- |
| Radiation-induced conductivity (RIC) | When using insulators |
| Radiation-induced electrical degradation (RIED) | When using insulators held at electric field |
| Radiation-induced electromotive force (RIEMF)  | For any small voltage or current |
| Radiation-induced thermoelectric sensitivity (RITES) |  |
| Radiation-induced absorption | For optical transmission |
| Deposition | For any first mirror or any thin metal window close to the plasma |
| Erosion any first mirror |  |
| Radioluminescence;  | For fibers and windows |
| Nuclear heating | For most front end components with complex consequences |
| Change in other properties due to transmutation and swelling |  |

- After a table by A. Costley

## Radiation levels at ITER diagnostic locations

**Radiation levels at ITER diagnostic locations**

-  Where
- - First wall, in gaps, in divertor, behind blanket centre, behind blanket edge, in port, in port behind labyrinth...
-  What
- Assembly level: coils, mirrors, shutters, windows, more complex units such as bolometers
- Materials: metals, insulators, liquids,gases
-  How
- - Physical properties: strength, brittleness, transport (electrical, thermal, Hall and cross terms), heating, optical transmission, parasitic current and light generation
-  When
- During operation, over machine life, over maintenance period
- DIFFICULT TO MAKE GENERAL STATEMENTS:

- 40 + diagnostics addressing with 40 ++ measurements
- 20 + ports of three different types
- Large number of locations

## Flux gradients (1-D)

- From NAR

## Variation of the source

- From NAR

## Local variation behind blanket

- Variation of nuclear heating near blanket gap along the toroidal
- direction (example). Peaking factor x 5 - 10 over 1-D calculation

- Depth into vacuum vessel

- From NAR

## Variation within ports (early MSE model)

- From NAR

## Typical Numbers 

- From A. Costley et al, ITER Diagnostics R&D, Fusion Engineering and Design (2001)

## Lifetime examples

-  Life time of some key diagnostics components at neutron power density of 0.5 MW / m2 or neutron power of 450 MW
- The fluence of 0.3 Mwa / m2 gives
- 2 x107 s burn at 450 MW
- The life time is mainly limited by radiation effects
- except where shown with ().

- From A. K.Ebisawa  et al, Plasma Diagnostics fro ITER-FEAT, HTPD 2000

## R&D approach for diagnostics

- Use results of general ITER radiation R&D program
- Suggest specific materials to be tested typical of present diagnostics
- Look for specific effects mainly relevant to diagnostic (RIEMF, radioluminescence...)
- Test prototype assemblies in a few cases
- Testing has some limitations, so margins are needed...

## 1st wall n,  spectra

- Testing of materials and prototypes done using  sources, 14 MeV n irradiation facilities and fission reactors
- Spectrum and fluence cannot be reproduced simultaneously
- Residual uncertainty after R&D

- From NAR

## Key radiation effect Summary Sheets (mainly R&D)

- Ceramic insulators, and wires/cables
- Mirrors and reflectors
- Windows
- Optical fibres
- Complex assemblies
- Example: Bolometers
- Example: Magnetic coils

## Ceramic insulators and wires

- Several diagnostics will use ceramic insulators and wires; extensively investigated
- Key effects: RIC, RIED, RIEMF, RITES
- RIC can be rendered negligible by careful choice of materials
- RIED is still not understood, but an extensive database has been established to guide design
- RIEMF understood in general and can be modeled
- Additional effects arise when wires are made into real circuits (RITES)

## Mirrors and reflectors

- Plasma facing optical element often a mirror. Lifetime a key parameter.
- Extensive tests in which candidate mirror materials have been subject to different types and levels of radiation have been carried out
- Nuclear heating can be  a problem via distortion
- Bulk metal mirrors: Neutron and  radiation not a threat
- Dielectric coated mirrors + neutron irradiation can lead to flaking and blistering. Multi-layer mirrors ( X-ray ) can be affected by neutrons
- careful choice of mirror material
- guidelines have been developed.
- Inorganic X-ray crystals (e.g. graphite) not affected.

## Windows

- Key effects: radiation induced absorption (instantaneous / permanent component), radioluminescence.
- Optical properties tested in many materials
- passive diagnostics by wavelength range:
- < 200 nm: absorption is very high without irradiation and direct coupling is necessary
- < 400 nm: both the absorption and luminescence are enhanced;  further work required
- 400 nm to 5 m: suitable materials found
- >5 m: ZnSe and diamond can work.
- mm-microwave diagnostics: no problems are anticipated.
- Active diagnostics (high power): much lower tolerance to absorption < 5%. Further work is required to optimise the windows for such systems.
- Thermal annealing can work for some materials

## Example of annealing

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

## Fibres

- Used extensively for spectroscopy, Thomson scattering. Would like to use as far into the vessel as possible.
- Optical path much longer than for windows: radiation induced absorption and radioluminescence more significant. Mechanical damage (embrittlement) possible.
- Extensive R&D program, continuing
- Substantial radiation induced absorption and luminescence occurs especially at short wavelengths (< 800 nm).  Typical results are shown in figures 5.8-6 and 5.8-7.
- In general, optical fibres cannot be used inside the vacuum vessel.
- outside the bioshield may be able to use fibres at visible and longer wavelengths.
- In the intermediate region may be possible at infrared wavelengths.  More work is required to determine the optimum material and the precise magnitude of the optical properties. Should be application-specific.

## Example of fibre luminescence

**Example of fibre luminescence**

## Assembly Example: Magnetic coils

- RIC thought to be a problem
- Measurements of insulators found Al2O3 good enough
- RIED thought to be a problem
- Measurements have tentatively identified safe design window (E, T, radiation level); design within it
- RIEMF thought to be a problem
- Measurements and calculations suggest RIEMF levels may be tolerable but RITES can be a problem.
- More design and R&D needed

- From G. Vayakis et al, HTPD 2002 and  this meeting

## Assembly Example: Bolometers

- JET-type bolometer design target
- Mica initially thought to be a possible problem
- Test on mica films in reactors suggest may be OK in ITER
- Whole bolometer tested in reactor
- Gold meander thought to have broken by a combination of embrittlement / transmutation  and substrate swelling
- More design and R&D planned: alternative meander (Pt) and substrates, alternative principle (Ferroelectric)

## Lessons from personal radiation R&D involvement (user and generator)

- Generic results only a rough guide to a very complex field
- Can be difficult to find relevant results
- Tracing references can also be difficult
- Experts are heavily overloaded
- Need for a radiation database just to document the ITER R&D and modeling results (underway within the ITPA framework)
- Surprises await when whole assemblies are tested
- Unless margins are very large,
- Must make units maintainable or
- test in reactors
- But preferably both!
- Radiation R&D takes a very long time
- Planning must involve the key users
- must not be left to the end

## And finally, some more thoughts (on a tangent)

- ITER is test bed for a fusion reactor
- These numbers still 0.5 -1 order of magnitude away from typical projected reactors
- But diagnostics work at  1-2 orders of magnitude below 1st wall equivalent levels
- IFMIF volume is limited and timing uncertain; materials testing a priority
- Space at high flux available in ITER upper ports (even whole ports)
- Much could be learned by using space to test key  control diagnostic components (e.g. magnetic coils) near ITER 1st wall