Heat load to a tantalum–tungsten twin-test-limiter and the effect to high-Z core plasma concentration of TEXTOR-94

The high-Z impurity concentration of tungsten (W) and tantalum (Ta) released from a twin-test-limiter half made of W and the other half made of Ta was investigated by inserting the twin-test-limiter into the edge plasmas of the TEXTOR-94 tokamak. To study the spectra of W and Ta in the extreme ultraviolet (XUV) range, these high-Z materials were introduced into the plasma by a laser blow-off technique. The quasi-continuum spectrum near 5 nm by pure Ta shifted about 0.2 nm to the longer wavelength side than that by pure W. The intensities were about the same for both material for the identical discharge parameters. The increment in temperature for the Ta limiter facing the ion drift side was about 1.4 times higher than that of W for identical plasma exposure conditions. The larger increase in temperature can be quantitatively explained by a smaller thermal conductivity of Ta. In the test-limiter experiment, the position of the peak in the XUV spectrum was unaffected by the orientation of the limiter, which corresponded to an equal influence of the particle emissions from both sides of the limiter.