Observation of tungsten impurity suppression with ECRH by an X-ray Crystal Spectrometer on EAST

TL;DR

This study demonstrates the use of X-ray Crystal Spectroscopy on EAST tokamak to directly observe the suppression of tungsten impurity accumulation by on-axis Electron Cyclotron Resonance Heating, providing clearer impurity behavior insights.

Contribution

It introduces a method for quantifying tungsten impurity profiles using X-ray spectroscopy and calibration, enabling direct observation of impurity suppression by ECRH.

Findings

W44+ impurity concentration is suppressed by ECRH

Calibration of XCS allows accurate tungsten density measurement

Profiles of tungsten impurity can be obtained for future transport analysis

Abstract

Impurity degrades tokamak plasmas confinement by causing energy loss, diluting the fuel concentration, even terminating the discharges in some extreme cases. Previously, the suppression effects of on-axis Electron Cyclotron Resonance Heating (ECRH) on the impurity accumulation have been investigated on EAST by the extreme ultraviolet (EUV) spectroscopy. However, it is difficult to quantify the changes in impurity tungsten (W) profile since the W line emissions in the EUV range could not be easily resolved. The X-ray Crystal Spectroscopy (XCS), that used to provide the ion temperature and the rotation velocity by measuring lines emissions in the soft X-ray range, also can be used to study the behavior of impurity W emissions. To begin with, in-situ absolute intensity calibration for Tangential XCS (TXCS) is conducted by analyzing the measurements of the bremsstrahlung radiation intensity. After obtaining the calibration coefficient, W44+ ion density profiles are evaluated by Abel inversion using the spectral line of W XLV (3.9095 {\AA}). Thus, a direct observation of W44+ impurity concentration suppressed by ECRH is accomplished. The obtained W density profiles can be used to analyze the W transport by combining with the impurity transport codes in the future.