Interim progress summary:ITER Imaging X-ray crystal spectrometer design

Sam Davis - UKAEA

Robin Barnsley - ITER

Aims

Aims of current design activity:

• Determine most appropriate location of crystals and detectors – i.e. within or behind equatorial port plug 3

• Determine required and achievable bandwidths ….

•and the corresponding shielding required

• Determine whether the full plasma cross section can be viewed from equatorial port, simplify etc

Factors affecting these decisions include

•Diagnostic: n-γ flux – noise, activation, component life

•Others: ease of maintenance

The main focus so far has been to create a CAD model suitable for efficiently investigating the effects of various features on nuclear performance using Atilla. Results are expected for the next ITPA.

Design for wavelength & bandwidthChosen parameters

fm Crystal-detector length Want this high for good resolution, but constrained by available space. 1m max for inside the plug

θB Central Bragg angle 50° - chosen based on line spectra to be observed, available crystals and spherical crystal imaging properties

Δλ Bandwidth 0.5% - minimum for Doppler width and shift of a single spectral line giving temperature and rotation2% sufficient for a group of lines

δθB Crystal width aberration Require <10-4 for resolution – drives crystal width

Derived parameters

R Rowland circle radius

ΔθB Variation in Bragg angle

w Crystal width

fs Distance to focus in plasma

B

ms

ff

2cos

B

B

B

tan

BwRw tan8 2

B

mfRsin

tunnel to plasma

neck

crystal

Conical shape – basic profile is rotated around the normal to the crystal centre to extent required for spatial coverage

View tunnel for 5% bandwidth, θB=60°

(illustrative only) View tunnel for 0.5% bandwidth, θB=50°Note narrower neck

Basic tunnel shape generation

• The geometry of the tunnels through the port plug through which the spectrometers view the plasma directly affects the neutron and gamma flux

• The tunnel shape required to accommodate all the required X-rays is generated by CAD by constructing the nominal ray paths from the chosen instrument parameters.

• The appropriate geometric constraints and previous equations are included in the model, allowing rapid and efficient modification.

tunnel to detector

crystal

detector

Rowland circle

Design for spatial coverage

Plasma coverage by toroidal viewsPlasma coverage by radial views

Necessary to reduce the crystal-detector distance for the furthest-forward toroidal view spectrometer

• Yellow represents view tunnel within the port plug and its virtual extension into the plasma• Aim is to view the tangent to all plasma flux surfaces• Spatial coverage drives detector height

View from top of plug

radial

toroidal

1) Maximum shielding Δλ=0.5%

2) Minimum shielding Δλ=2%

3) Straight shield

(not shown right)

4) ‘Squared off’ shield rather than conical shape for easier construction

Shielding tunnel liners to be investigated

• Shielding options for the tunnel through the plug to the plasma form nested layers

• These can be modelled as vacuum or steel in Atilla to investigate their effect

• Liners could be fitted later for high neutron flux operations, allowing higher bandwidth during earlier campaigns

Other options under investigation

Different thicknesses of different materials around detector

Different widths for tunnel to detector depending on the tuning required – again modeled as adjacent layers

Option to model vacuum / neutron absorbers behind crystal to avoid direct line-of-sight scattering into the detector

crystal

detector

Atilla modelling considerations

• Atilla is able to accept solid models for nuclear analysis, however every volume must be occupied by precisely one component – i.e. no voids or clashes

• Port plug structure - current solid model has been simplified to remove features which complicate geometry (and hence meshing) but which either

– have negligible effect on radiation – e.g. bolts– can be approximated – e.g. by modelling components as water/steel mix cooling

systems can be omittedThis will be used by other analyses and remains linked in VPM to the detailed

model so it will be automatically updated.• Other diagnostic systems are modelled as vacuum to ensure their

neutron transparency is included • All other volumes are modelled as homogeneous water/steel mix with

more steel at the front and more water at the back of the plug.