Temperature Distribution On The Limiter Surface Measured By IR-Camera In HT-7 Tokamak

The major increase in discharge duration and plasma energy in future fusion device will give rise to important plasma-material interaction effects that will critically influence plasma performance and safety operation. Power deposition on the first wall leads to erosion of the material and contamination of plasma. It is important to measure the temperature distribution for inferring the local heat flux deposition on the limiters and limiting the heat load on the leading edge to decrease the erosion especially during long pulse discharges in HT-7 superconducting Tokamak. Thermocouples have been routinely applied in the HT-7 only to give the inside temperature of the tiles. In order to obtain the surface temperature distribution, an infrared (IR) system is proposed by means of a well-calibrated 2-D infrared camera, which has a 320times240 pixels detector, records 7.5-13 mum light emission with 50 Hz acquisition rate and 1.3 mrad spatial resolution. To make temperature measurement correctly, it is necessary to compensate for the effects of a number of different radiation sources: emissivity, distance, ambient, and so on. One of the important issues is emissivity and the calibration of emissivity is important. Temperature measurements were performed under different plasma conditions in particular to focus on long pulse discharges in the HT-7. From the IR images, the highest temperature on the limiter could be seen clearly. During most of long pulse discharges, temperature of hot spot on the belt limiter (graphite coated with 100 mm SiC) was higher than 1000degC, which affected the continuance of the discharge. The correlation between heat deposition pattern and the magnetic configuration and the power of Low Hybrid Wave has been observed. From figure1, it can be seen that when the PLHB (horizontal displacement of plasma) changed a little and the plasma approached to the belt limiter at about 10 s, the temperature of a hot spot on the belt limiter increased to more than - 1000degC rapidly. Figure2 shows that the temperature of a mobile limiter changed with the power of Low Hybrid Wave dramatically. The rapid increasing of local temperature in carbon tiles led to high carbon erosion yield and self-sputtering yield, which caused huge erosion and induced carbon bloom. Then carbon penetrated into the plasma and resulted in immediately plasma collapse. Thermocouples were mounted inside the limiter tiles as a calorimeter for estimating the integral energy flow. The heat flux in a limiter sample(fine graphite coated with 1 mu m W) has a maximum exceeds 7 MW/m². The distance between its surface and the core of the plasma was 287 mm. Using these information improvements have been made to the target power measurements and an overview of the power loading on the first wall.