Concept of a new bolometer based on soft x-ray–UV/IR conversion with discrete lines of sight

We present the concept of a new bolometer based on soft x-ray-UV to IR conversion with a high-Z foil and individual Lines of Sight (LOS). A solid block collimator is used with drilled channels defining the LOS. Each LOS is equipped with a separate metallic circular carbonized foil of 2 μm thickness, and the edge of the foil is cooled to maintain a constant temperature. Laboratory experiments with carbonized gold foils have shown that it is possible to obtain a calibration curve for the IR response as a function of incoming UV power in stationary mode. The IR signal is found to be proportional to the incoming UV signal. Levels as low as 50 μW of UV incoming power have been detected in unfavorable conditions (in air, at room temperature) with both an IR camera and industrial low-cost thermopiles. In addition, there is no crosstalk between two adjacent detectors. An analytical model confirms that the IR power is proportional to the X-UV input power and gives the physical dependences of the proportionality factor in the stationary mode. The time-dependent solution allows us to get the radiative power as a function of the time varying voltage measured at the output of the thermopiles. Using a platinum foil and the parameters of an industrial low-cost thermopile, we find that levels as low as five μW should be measured. A prototype is under construction to be tested in the WEST tokamak. It consists of two lines of sight observing the low divertor and the center of the plasma. The interest of this system is that it can be rendered compatible with ITER or DEMO. Another point is that, due to its simplicity, the cost of the diagnostic can be reduced to a fraction of the price of standard metallic resistive bolometers used in tokamaks to measure the radiated power [Mast et al., Rev. Sci. Instrum. 62, 744 (1991)].