Preliminary observation of tungsten-impurity suppression using on-axis ECRH by X-ray crystal spectroscopy in EAST

Abstract

Impurities degrade tokamak plasma confinement by causing energy loss, diluting fuel concentration, and even terminating discharge in some extreme cases. Previously, the suppression effects of impurity accumulation due to on-axis electron cyclotron resonance heating (ECRH) have been studied on Experimental and Advanced Superconducting Tokamak (EAST) using extreme ultraviolet (EUV) spectroscopy. However, it is difficult to quantify changes in the tungsten (W) impurity profile since W-line emission in the EUV range cannot be easily resolved. X-ray crystal spectroscopy (XCS) is widely used to measure the ion temperature and rotation velocity of plasmas by using line emission in the soft X-ray range. In addition, the XCS can also be used to study the behavior of impurities. An in situ absolute intensity calibration of tangential XCS was conducted by analyzing calculations and measurements of bremsstrahlung radiation. After obtaining the calibration coefficient, the W44+-ion-density profiles were evaluated using Abel inversion operations and the spectral line of W XLV (W44+, 3.9095 Å). Thus, a direct observation of the W44+-impurity concentration suppressed by ECRH was accomplished. Such W44+-density profiles can be used in the future to analyze W transport in combination with impurity transport codes.