Fabrice Rigollet

Evidence of second order transition induced by the porosity in the thermal conductivity of sintered metals

In this paper, using both experimental data and theoretical modelling, we investigate the degradation of the thermal conductivity of sintered metals due simultaneously to the grain boundary thermal resistance and the porosity. We show that the porosity dependence of the thermal conductivity of sintered material from spherical particle powder, exhibits a critical behaviour associated with a second order phase transition. An analytical model with a single parameter is proposed to describe the critical behaviour of the thermal conductivity of sintered metals versus porosity.

Experimental study of the thermal conductivity of sintered tungsten: Evidence of a critical behaviour with porosity

Rear face flash experiments were performed in order to determine the thermal conductivity of sintered tungsten at room temperature. Ten different samples were synthesized with the spark plasma sintering technique. The microstructure obtained from the sintering is porous and consists of angular grains with medium sphericity. The average grain size (d) and the porosity (P) of the samples lie within the ranges of 2 mu m <= d <= 7 mu m and 0 <= P <= 0.35. We show that the dependence of the thermal conductivity of the sintered tungsten samples on the porosity shows a critical behaviour. A theoretical explanation of this behaviour and a predictive model for this porosity dependence are proposed

Surface temperature measurement of plasma facing components with active pyrometry

In fusion devices like ITER, plasma facing components will be in metal, (Tungsten and Beryllium), with emissivity in the range of 0.1–0.4. Therefore, surface temperature monitoring by infrared system will become more challenging due to low emissivity and consequently non negligible reflected flux. The active pyrometry method proposed in this paper allows surface temperature measurements independently of reflected and parasitic fluxes. A local increase of the surface temperature (ΔT(t)~10 °C) introduced by a transient heating source (pulsed or modulated) results in an additional component of the flux collected by the detector. A filtering of the signal allows extracting a temporal flux proportional only to the variation of the emitted flux. The ratio of simultaneous measurements at two wavelengths allows solving the unknown emissivity (same as for classical bicolour pyrometry). In this paper, it is described how the active pyrometry method is adapted to the surface temperature measurements of metallic PFCs independently of the reflected fluxes. Experimental results for carbon and tungsten samples are reported. Finally, it is shown how, by using the active pyrometry, the overall 2D standard IR perturbed by a reflected flux is corrected to recover the full 2D surface temperature close to the real surface temperature.

Simultaneous Identification of Thermophysical Properties and Surface Heat Flux on Plasma Facing Components Inside the Jet Fusion Reactor

We present here two thermal diagnostics (IR surface and embedded thermocouple temperature measurements) that outfit the same carbon tile inside the JET Fusion reactor and whose combination enables to identify, on one hand, a surface heat flux history and, on the other hand, one thermophysical parameter of an unknown deposited surface carbon layer (SCL). A sensitivity study of the thermal model of the SCL enables to show that only one parameter can be identified here : the thermal contact resistance between the SCL and the bulk substrate.Copyright

Methodology for heat flux investigation on leading edges using infrared thermography

During steady state plasma operation in fusion devices, leading edges of the actively cooled plasma-facing components can be impacted by plasma flux with nearly normal angle of incidence, causing local overheating. The overheating can be a critical issue in high-power machines, especially in the presence of mechanical misalignments. Due to heat diffusion through the material, the edge power overload also leads to a local increase of temperature on the top part of the tile that can be detected by the infrared imaging system (viewed from the top of the machine). In the Tore Supra tokamak, heat flux impinging on the top and the leading edge of the carbon fibre composite (CFC) flat tiles are characterized with both an infrared (IR) thermographic system and 2D thermal modelling of the tile. A specific sensor correction based on a laboratory blackbody-slit experiment has been developed to simulate the spatial resolution related effects (necessary here since the temperature gradient near the leading edge is smaller than the pixel size of the IR system). The transfer function of the IR system is characterized by a Gaussian distribution function. The standard deviation is found to be sigma = 1.75 mm for a pixel size of 3.1 mm. The heat flux calculation is applied to CFC flat tiles and, after being processed with the transfer function, compared to experimental IR data for two geometrical situations: one with 0.2 mm misalignment between two adjacent tiles and the other without misalignment (well-aligned tiles). The heat flux ratio between the leading edge and top is found to be similar to 25 in the case of the protruding tile, which is lower than the expected ratio using the guiding-centre ballistic approximation with no cross-field heat flux (57).

Identification of space and time varying thermal resistance: Numerical feasibility for plasma facing materials

Abstract The present paper deals with a non-linear unsteady calculation combined with the conjugate gradient method (CGM) and the adjoint state, in order to characterize in-situ the spatial and time variation of the thermal resistance of a surface layer. This paper presents the numerical feasibility of this method for the plasma-facing components (PFC), and precisely on the surface carbon layer (SCL), usually poorly attached to the PFC in the fusion machines, a realistic experiment design was used. The accuracy of the method is examined by using simulated inexact infrared measurements obtained on the SCL surface. The advantages of applying the CGM with the adjoint state in the present study, are that no prior information is needed on the time variation and for the initial guesses of the unknown thermal resistance.

Inverse heat conduction problem using thermocouple deconvolution: application to the heat flux estimation in a tokamak

Abstract Internal components of magnetic confinement fusion machines are subjected to significant heat fluxes. A large part of this power is directed towards plasma facing components. Even if these components are designed to receive about , surface temperature and heat flux measurements are important issues to guarantee safe plasma operations. In JET tokamak, few embedded thermocouples (TC) located 1 cm below the tile surface are used to measure the bulk temperatures of the Carbon Fiber tiles (coated with about 20 m of tungsten with the ITER-like wall). We propose here to use an inverse thermal calculation based on Thermal Quadrupole method to locally deduce the deposited heat flux. The calculation requires the location of the peak and the normalized 1D-shape of the heat flux deposited on the target.

Investigation on reduced thermal models for simulating infrared images in fusion devices

In fusion facilities, the in-vessel wall receives high heat flux density up to 20 MW/m2. The monitoring of in-vessel components is usually ensured by infra-red (IR) thermography but with all-metallic walls, disturbance phenomenon as reflections may lead to inaccurate temperature estimates, potentially endangering machine safety. A full predictive photonic simulation is then used to assess accurately the IR measurements. This paper investigates some reduced thermal models (semi-infinite wall, thermal quadrupole) to predict the surface temperature from the particle loads on components for a given plasma scenario. The results are compared with a reference 3D Finite Element Method (Ansys Mechanical) and used as input for simulating IR images. The performances of reduced thermal models are analysed by comparing the resulting IR images.

Surface temperature measurement of plasma facing components with pulsed active pyrometry

In fusion devices, like Tore Supra, the surface temperature of the carbon plasma facing components (PFC) is measured by standard infrared (IR) system in real time for control of integrity of PFCs. The emissivity of carbon is known and high (ε~0.8-0.9) and therefore the contribution of the reflected flux in the total flux collected can be neglected. For future tokamaks like WEST and ITER, PFCs will be in metal (Be/W) with low and variable emissivities (ε~0.1-0.4). Consequently, the surface temperature measurement will become challenging due to low emissivity and non-negligible reflected fluxes. The pulsed active pyrometry proposed in this paper allows a punctual surface temperature measurement independently of the emissivity and the reflected fluxes. Experimental results for aluminium and tungsten samples are reported. In addition it is shown that using the local measurement by active pyrometry, an overall 2D standard IR perturbed by a reflected flux can be corrected to recover the full 2D real surface temperature.

Prediction of spatial resolutions of future IR cameras at ITER

Here are presented the predictions of the spatial resolutions of one of the future IR camera that will survey the divertor of the Tokamak ITER. The objective is to associate, in Fourier space, the optical transfer function and the detector transfer function to calculate the total transfer function (TTF) of the virtual IR camera. The modulation transfer function (modulus of TTF) quantifies the ‘imaging’ performances of the virtual camera. Its ‘measuring’ performances are estimated by the simulation of the slit response function experiment. Finally, some results of sharp temperature profile measurements in realistic plasma situations are presented.