Temperature measurement and real-time validation

Temperature measurement and real-time validation A. Herrmann, B. Sieglin, M. Faitsch, P. de Marné, ASDEX Upgrade team st IAEA Technical Meeting on Fusion Data Processing, Validation and Analysis

ITER- diagnostics categories The ITER plasma diagnostics are required to provide accurate measurements of plasma behaviour and performance. They are typically classified in different categories from operations point of view: Group a Group a Group b Group machine protection basic machine control advanced plasma control measurements required for evaluation and physics studies. The machine is unable to operate without a working diagnostic providing every Group a parameter (CIS & PCS). Thermography is part of the machine protection (surface temperature) Tore Supra: D. Guilhem, G. Martin, R. Reichle, H. Roche, M. Jouve, L. Ducobu, P. Messina, Infrared surface temperature measurement for long pulse; real-time feedback control in an actively cooled machine, Review of Scientific Instruments 70 () (999) 47 430. ASDEX Upgrade: Herrmann, A., R. Drube, T. Lunt, et al., Real-time protection of in-vessel components in ASDEX Upgrade. Fusion Engineering and Design, 0. 86(6-8): p. 530-534 Talk by Sven Martinov st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann

(optical) temperature measurement and machine protection Actively cooled targets W7-X target tile (cross section) Stationary temperature profiles on short time scales (τ eq << Δt Discharge ) Typical heat fluxes q = 0-0 MW/m. Where are the critical temperatures? Surface temperature (local melting, cracks, recrystallization) Interface temperatures, cooling channel The sensitive component is inside the target But the surface temperature is measured. Correlation to the temperature inside the bulk. T K Heat resistance: 00 q MW / m The machine protection is as good as the temperature measurement and the thermal model of the target. st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 3

Critical T s is time dependent steady state: Ts given by the interface T ( T T ) s s cool qs d transient (short vs. transition time): Ts limit due to surface temperature -> Energy impact T s ( T T ) 0 s s q d a ~ 0. 4 mm ms, W t c t Tungsten : MJ 40 m s Graphite : MJ 0 m s st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 4

Temperature calculation M e ( T, ) c 4 c exp( ) T st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 5

Temperature calculation M p ( T, ) c 4 c exp( ) T + Parasitic radiation Bremsstrahlung Marfes (impurity radiation) Arcs Dust Planck radiation Surface morphology Layers/deposits Reflections st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 6

Additional contributions - Examples Overestimation of the bulk temperature due to: Surface morphology - Deposits/layers. st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 7

Additional contributions Examples (II) Arcs Bremsstrahlung/Reflections Dust st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 8

Problems - summary The measured surface temperature is biased by Additional Planck radiation Other sources of radiation Both contributions result in a too high temperature (T measured > T surface )! Inherent safe but might reduce the operation range significantly Are there parameters that are applicable for real time data validation? YES Time behaviour Spectral dependence st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 9

Layers and temperature Tokamak experiments (JET) Layers at the inner target. Verified by spectroscopic measurements (background) About 50 K / MW/m P. Andrew et al. Journal of Nuclear Materials 337 339 (005) 99 03 st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 0

Layer effects (I) q s Temperature gradient in top of the bulk T s T b T layer T s T b dl l q s Bulk material: thermal data known Numerical: W d 50 m b 00 l / 0b m K The additional ΔT is 45 K/MWm - The time constant for such a thin region is short. d 50 s a After this time the time behaviour of the surface temperature follows the heating of the bulk. T c.. l b (Nearly) no effect on the measured surface temperature l b The surface temperature is increased The derived heat flux is too large if the surface effect is not considered. st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann

Layer effects (II) + st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann

Hot spots result in an artificial temperature increase EK 98 T l T h R_T temperature ratio hot spot/bulk R_a area ratio hot_spot/total area The measured temperature is calculated from the photons belonging to two (ore more) temperatures. The microscopic temperature patterns are fixed over many heating cycles. T l T h T l T h st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 3

spectral radiant exitance [W/(m^ m)] temperature [K] Wavelength selection (I) Planck s formulae for radiation from a black body into the half space M e ( T, ) c 0 c 5 c exp( ) T 6 hc 3,74 Wm 7 0 0 6 0 5 0 4 0 3 3000 000 000 W [ M e ] m m ch c.4380 k 3 T.898 mk max 0 4 mk Planck s law Unique relation between radiation/photon emission of a body and temperature. Depends on the wavelength (broad band radiation). Select an optimum wavelength: Temperature range. Environment (vacuum, air). Available detectors (costs). 0 500 0 Vis/NIR MWIR 300 LWIR 0.5 5 0 0 0 wavelength [m] st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 4

0 8 0 7 0 6 0 5 0 4 Wavelength selection (II) c ( T, ) 5 c exp( ) T 0 3 0 500 000 500 000 500 3000 c S det S t arget + SBck; St arget K /(exp( ) ) T 0 % cal. error MWIR LWIR vis/nir M e amplific. mitigation 3500 Typical wavelengths regions for T measurement: Vis/near infrared (vis/nir, ~ μm) Mid wave infrared (MWIR, ~ 5μm) Long wave infrared (LWIR, ~ 0 μm) MWIR and LWIR cover temperature range from 500 to 3500 K. Vis/NIR covers a small T-range. T measurement error: T T K ( + + T c K S + S t arget Bck Strong error mitigation in the vis/nir wavelength region: Comparator like behaviour. robustness against change of system parameters (emissivity). st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 5 Bck S S Bck )

Bremsstrahlung T brems < 500 K T brems >500 K Strong decrease of Bremsstrahlung contribution between and 5 μm (/λ ). Optimum wavelength 5 μm Cold and dense plasmas contribute to Bremsstrahlung. Reduced target load due to divertor detachment. Temperature equivalent for Bremsstrahlung. A constant pressure of n e T e = x0 evm -3 is assumed. dwb d dv const Z eff e / e T n G ff hc Exp Te st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 6

Reflections IR images (radiometric units), simulated by SPEOS CAA V5 Based, looking at the outer divertor with W monoblock. (Left) Image in direct radiance when the flux coming on the sensor is only the thermal emission of hot targets. (Right) Real image of the camera when the flux picked up by the camera includes also the reflection effects. Target acts as a mirror in the optical system. Time behaviour of reflections can t be used to discriminate between reflections and target. Reduce reflections as much as possible. Simulation of reflections for an ideal 3D geometry. Identification of critical regions dominated by reflections. st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 7

Reduction of reflections Reflectivity of both parts, % 00 80 60 40 0 0 500 000 500 000 500 Wavelength, nm R tot, shiny, 90 o! R tot, shiny part R tot =R diff, matt R diff, shiny, 90 o R diff, shiny part Sand blasted compared to as manufactured as manufactured dominant direct reflection (mirror) sand blasted dominant diffusive reflection, suppression of direct reflections Moderate increase of the emissivity (0. -0.3) @ about 4 μm sand blasted as manufactured BB /5 st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 8

f_e wavelength (ratio) measurement M M (, T ) ( ) M BB (, Tobj ) + BB Bck object reflected background radiation M ( T, ) c 5 c exp( ) T M M ( ) M ( ) M Assumptions for the ratio measurement: T obj >> T Bck λ < λ max (Wien), i.e NIR range Grey body c BB BB (, T (, T obj obj M ln ( ) M ) + ) + W W M BB (, TBck ) M (, T ) BB ( ) + 5ln ( ) Bck Single band vs. ratio measurement color vs. single color obj f used T 0 obj 0,8 0,9,,,8,6,4, 0,8 0,6 0,4 0, single ratio ratio epsi_ratio ε /ε single st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 9

Machine protection add. information q s (t,x) target heat load Δx Cooling media channel - Calorimetry for measurement of global energy removal. T s (t,x) T b T b Thermocouples at different positions. Localised Limited time response Not affected by surface effects TCs for direct heat flux recording q T T x b b Optical diagnostic to measure surface temperature evolution high spatial resolution (millimetres) high time resolution (microseconds) detection wavelength selectable measured temperature is sensitive to surface modifications st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann 0

Summary Surface temperature measurement is part of machine protection. The cooling structure has to be protected against overheating The tolerable surface temperature is time dependent Short term events (ELMs) Degeneration of thermal parameters. The measured photon flux is falsified by additional photon sources and reflections. The resulting (measured) surface temperature is too high reduction of the operational space. Real time validation is possible by: Considering the temporal evolution dt/dt or heat flux calculations. Typical time constants are ms. Measuring @ (or more) wavelengths to eliminate Bremsstrahlung contribution. Using vis/nir data points for point calibration (comparator like behaviour). st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann

complexity alt. Summary Temperature Temperature evolution and power calculation (transient like ELMs, TCs) Characterize the surface of the material (hot spot fraction) for on-line T correction. In situ surface characterization. Multi-wave, multi-band measurement, single detector chip (hot spots, Layer, reflections, ε, τ). Spectral measurements (D profile -> D chip). Photothermal methods Multi-color pyroreflectrometry The measured temperature is usually not the bulk temperature. Machine safety (inherent safe, restrictions for the operation range) Verify the measured temperature, deduce the true bulk temperature. Keep the diagnostic as simple as possible! st IAEA TMFDPVA, Nice, -3 June 05 A. Herrmann