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 Angstrom t o 3.9944 tf have been observed at TEXTOR-94 with two high-resolution X-ray crystal spectrometers at Bragg angles near 54 degrees. Together, these inst ruments can be used as an X-ray polarimeter. The instruments are very sensi tive to small amounts of argon; thus, a count rate of 4x10(5) photons/s, wh ich corresponds to the maximum count rate capability of the presently used detectors, can be obtained for argon densities of less than 10(-3) times th e electron density, if the central electron temperature is about 1 keV. Up to 8192 spectra per discharge can be recorded allowing for the investigatio n of fast events during MHD activities. The instruments have been used to s tudy the electron and ion behavior during internal sawtooth oscillations, w hich affect the confinement properties in the core of the plasma. The X-ray polarimeter at TEXTOR-94 is a unique diagnostic tool for the meas urement of non-Maxwellian electron-velocity distributions, which can be pro duced in tokamak discharges in the presence of electric fields. First polar ization measurements were performed during the flat-top phase in discharges with both pure ohmic heating and additional neutral-beam heating. These me asurements showed that inhomogeneities in the emissivity profile can produc e effects which look like polarization effects. No polarization was found w hen these source inhomogeneities were taken into account. The polarimeter i s presently upgraded by an imaging X-ray crystal to facilitate polarization measurements. This will be important for the upcoming experiments with ele ctron cyclotron heating at TEXTOR-94.