2D surface temperature measurement of plasma facing components with modulated active pyrometry.

In nuclear fusion devices, such as Tore Supra, the plasma facing components (PFC) are in carbon. Such components are exposed to very high heat flux and the surface temperature measurement is mandatory for the safety of the device and also for efficient plasma scenario development. Besides this measurement is essential to evaluate these heat fluxes for a better knowledge of the physics of plasma-wall interaction, it is also required to monitor the fatigue of PFCs. Infrared system (IR) is used to manage to measure surface temperature in real time. For carbon PFCs, the emissivity is high and known (ɛ ∼ 0.8), therefore the contribution of the reflected flux from environment and collected by the IR cameras can be neglected. However, the future tokamaks such as WEST and ITER will be equipped with PFCs in metal (W and Be/W, respectively) with low and variable emissivities (ɛ ∼ 0.1-0.4). Consequently, the reflected flux will contribute significantly in the collected flux by IR camera. The modulated active pyrometry, using a bicolor camera, proposed in this paper allows a 2D surface temperature measurement independently of the reflected fluxes and the emissivity. Experimental results with Tungsten sample are reported and compared with simultaneous measurement performed with classical pyrometry (monochromatic and bichromatic) with and without reflective flux demonstrating the efficiency of this method for surface temperature measurement independently of the reflected flux and the emissivity.

Modeling of the ITER-like wide-angle infrared thermography view of JET.

Infrared (IR) thermography systems are mandatory to ensure safe plasma operation in fusion devices. However, IR measurements are made much more complicated in metallic environment because of the spurious contributions of the reflected fluxes. This paper presents a full predictive photonic simulation able to assess accurately the surface temperature measurement with classical IR thermography from a given plasma scenario and by taking into account the optical properties of PFCs materials. This simulation has been carried out the ITER-like wide angle infrared camera view of JET in comparing with experimental data. The consequences and the effects of the low emissivity and the bidirectional reflectivity distribution function used in the model for the metallic PFCs on the contribution of the reflected flux in the analysis are discussed.

Experimental study of the ion critical-gradient length and stiffness level and the impact of rotation in the JET tokamak.

Experiments were carried out in the JET tokamak to determine the critical ion temperature inverse gradient length (R/LTi=R|nablaTi|/Ti) for the onset of ion temperature gradient modes and the stiffness of Ti profiles with respect to deviations from the critical value. Threshold and stiffness have been compared with linear and nonlinear predictions of the gyrokinetic code GS2. Plasmas with higher values of toroidal rotation show a significant increase in R/LTi, which is found to be mainly due to a decrease of the stiffness level. This finding has implications on the extrapolation to future machines of present day results on the role of rotation on confinement.

Particle pinch with fully noninductive lower hybrid current drive in Tore Supra.

Recently, plasmas exceeding 4 min have been obtained with lower hybrid current drive (LHCD) in Tore Supra. These LHCD plasmas extend for over 80 times the resistive current diffusion time with zero loop voltage. Under such unique conditions the neoclassical particle pinch driven by the toroidal electric field vanishes. Nevertheless, the density profile remains peaked for more than 4 min. For the first time, the existence of an inward particle pinch in steady-state plasma without toroidal electric field, much larger than the value predicted by the collisional neoclassical theory, is experimentally demonstrated.

Application of the pulsed photothermal effect to fast surface temperature measurements.

It is well known that the accuracy of surface temperature measurements by optical means is limited because of the uncertainties that are associated with the emissivity and the reflected fluxes. The application of the photothermal effect produced by a chopped laser beam for surface temperature measurements has proved to be a valuable tool to avoid the errors that are due to the reflected fluxes. In this paper we show that a pulsed laser may also be used for the same purpose. Since the measurement is quite rapid, this technique allows measurements to be made on moving surfaces. We present a careful analysis of the role of the experimental parameters and also give typical results.

Noncontact surface temperature measurement by means of a modulated photothermal effect.

The main problems when measuring surface temperature by means of radiometry (i.e., optical pyrometry) are the unknown emissivity and radiation reflected by the sample. The latter problem becomes critical when the sample is placed in hot surroundings, such as furnaces or combustion chambers; indeed, the reflected flux may then become larger than the emitted flux. In this paper we describe a novel technique, based on the photothermal effect, which allows the surface temperature to be measured without error due to reflected fluxes. The influence of the parameters of the experimental setup are discussed. Experimental data obtained with a sample placed inside a furnace are reported in the (300-1150 K) range. The experimental results show the efficiency of the technique which proves to be a general solution to extend the domain of application of optical pyrometry.