J. Gaspar

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.

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.

Successive identification of surface heat flux and thermophysical properties of plasma facing components inside the JET tokamak: numerical feasibility

We present here the numerical feasibility of using 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, the spatial and time variation of the thermal resistance of an unknown deposited surface carbon layer (SCL). The Conjugate Gradient Method (CGM) and the adjoint state were applied to perform these two identifications.

Wall surface temperature calculation in the SolEdge2D-EIRENE transport code

A thermal wall model is developed for the SolEdge2D-EIRENE edge transport code for calculating the surface temperature of the actively-cooled vessel components in interaction with the plasma. This is a first step towards a self-consistent evaluation of the recycling of particles, which depends on the wall surface temperature. The proposed thermal model is built to match both steady-state temperature and time constant of actively-cooled plasma facing components. A benchmark between this model and the Finite Element Modelling code CAST3M is performed in the case of an ITER-like monoblock. An example of application is presented for a SolEdge2D-EIRENE simulation of a medium-power discharge in the WEST tokamak, showing the steady-state wall temperature distribution and the temperature cycling due to an imposed Edge Localised Mode-like event.

Heat Flux estimation in WEST divertor with embedded thermocouples

The present paper deals with the surface heat flux estimation with embedded thermocouples (TC) in a Plasma Facing Component (PFC) of the WEST Tokamak. A 2D nonlinear unsteady calculation combined with the Conjugate Gradient Method (CGM) and the adjoint state is achieved in order to estimate the time evolution of the heat flux amplitude and decay length λq . The method is applied on different synthetic measurements in order to evaluate the accuracy of the method. The synthetic measurements are generated with realistic values of λq and magnitudes as those expected for ITER.

Heat source estimation in low diffusive materials

In this work, a method to estimate unsteady 2D heat sources is developed. The sources are estimated from Infra-Red (IR) temperature mapping on the front face of thermally thin material. A comparison of the estimation accuracy as well as the evaluation of time cost is also carried out between a direct method (coupled with filtering techniques) and an iterative method (of conjugate gradient type). The present work was realised on experimental data.