A new method used to calibrate Fabry-Perot interferometers on tokamaks

Fabry-Perot interferometers are routinely used on the Tore Supra tokamak in order to measure the electron cyclotron emission spectrum especially in the optically thick region (150-300 GHz) for which the intensity is proportional to the electron temperature. In order to give the electron temperature in keV, it is necessary to calibrate the spectral measurements. In practice, one has to determine the calibration curve. The power levels involved in calibration of Fabry-Perot interferometers used on large tokamaks are generally not compatible with classical data processing techniques such as coherent addition or synchronous detection. A new method is presented in this paper able to detect DC signals as low as 0.1 nV. It is based on a proper reduction of the detection frequency bandwidth. The principle of this method is the following: The source signal is chopped and coherently added over N turns of the chopper. The resulting signal is then Fourier analyzed. One finally takes the amplitude at the chopper frequency which allows a much better filtering of the noise. Incident powers less than 1 pW have thus been measured. Till now and probably due to the difficulties detailed above, absolute calibration of Fabry-Perot interferometers has not yet been achieved for applications to plasmas diagnostics in large fusion devices. The method presented here has been first used to achieve this goal. The experimental set up is briefly described and the method detailed. Results on plasma and comparison of the method with more conventional techniques are presented