X-ray spectroscopy at the TEXTOR-94 Tokamak

Spectra of the 1s-2p resonance line of helium-like argon, Ar XVII, and the associated satellites in the narrow wavelength range from 3.9494 A to 3.9944 A have been observed at TEXTOR-94 with two high-resolution X-ray crystal spectrometers at Bragg angles near 54 degrees. Together, these instruments can be used as an X-ray polarimeter. The instruments are very sensitive to small amounts of argon; thus, a count rate of 4 x 105 photons/s, which corresponds to the maximum count rate capability of the presently used detectors, can be obtained for argon densities of less than 10-3 times the electron density, if the central electron temperature is about 1 keV. Up to 8192 spectra per discharge can be recorded allowing for the investigation of fast events during MHD activities. The instruments have been used to study the electron and ion behavior during internal sawtooth oscillations, which affect the confinement properties in the core of the plasma. The X-ray polarimeter at TEXTOR-94 is a unique diagnostic tool for the measurement of non-Maxwellian electron-velocity distributions, which can be produced in tokamak discharges in the presence of electric fields. First polarization measurements were performed during the flat-top phase in discharges with both pure ohmic heating and additional neutral-beam heating. These measurements showed that inhomogeneities in the emissivity profile can produce effects which look like polarization effects. No polarization was found when these source inhomogeneities were taken into account. The polarimeter is presently upgraded by an imaging X-ray crystal to facilitate polarization measurements. This will be important for the upcoming experiments with electron cyclotron heating at TEXTOR-94. (orig.)