X-ray observations of up-down impurity density asymmetries in Alcator C-Mod plasmas

A five chord, high resolution X-ray spectrometer array has been used to measure vertical brightness profiles of helium-like argon emission from Alcator C-Mod plasmas, out to the last closed flux surface. During standard Alcator C-Mod operation, with the X point and the ion B* Del |B| drift downward, the helium-like argon brightness is a factor of ~8 larger at the top of the plasma than at the bottom, near the plasma edge. In these edge regions, where the electron temperature is low, the upper levels of observed transitions are populated by radiative recombination of hydrogen-like argon. This up-down brightness asymmetry can be explained by a factor of ~8 enhancement of hydrogen-like argon at the top of the machine at r/a=0.9. This implies a vertical impurity drift from inside the plasma, since the hydrogen-like argon is born near the plasma centre. With the ion B* Del |B| drift upward, the enhancement switched to the bottom of the machine, but did not change when the X point was moved up. This asymmetry agrees qualitatively with the predictions of neoclassical parallel impurity transport