Alignment and absolute wavelength calibration of imaging Bragg spectrometers

In the present and the next generation of fusion devices, imaging Braggspectrometers are keydiagnostics tomeasureplasma parameters in the hot core, especially iontemperature andplasma rotation. The latter quantities are routinely obtained using the Doppler-width and -shift of the emittedspectral lines, respectively. Line shiftmeasurements require absolute accuracies Δλ/λ of about 10 ppm, where λ-is the observed wavelength. For ITER and the present fusion devices,spectral lines of He-and H-like argon, iron, and krypton as well as Ne-like tungsten are foreseen for themeasurements. For these lines, K α lines can be found, some in higher order, which fit into the narrow energy window of thespectrometers. For arbitrary wavelength settings, K α lines are also used tomeasure the miscut of the spherical crystals; afterwards thespectrometers can be set according to the geometrical imaging properties using coordinatemeasurement machines. For thespectrometersmeasuring Ly α lines of H-like ions,fluorescence targets can providein situ localizedcalibration lines on thespectra. Thefluorescence targets are used best in transmission and are excited by the thermalx-rayradiation of theplasma. An analytic theory offluorescence is worked out.

Source: Review of Scientific Instruments - September 26, 2016 Category: Physics Authors: G. Bertschinger, O. Marchuk and R. Barnsley Source Type: research