The x-ray imaging crystal spectrometer (XICS) is proposed as the principal method of diagnostics for plasma ion temperature and rotation for the China Fusion Engineering Test Reactor (CFETR) for its simplicity in implementation and no reliance on neutral beams. For D–T experiments with the electron temperature as high as 35–40 keV at the core region, highly charged high-Z ions can serve as the diagnostic ions for the XICS. For the CFETR, Xe44+, Xe51+, and W64+ are selected as the impurity ions. Appropriate crystal parameters are selected, as well as the preliminary layout for the spectrometer. We estimated the general performance of the spectrometer, including the emissivity of the impurities, the spatial resolution of the x-ray detector, and the expected count rate of line emissions. For the application in the fusion reactor environment, the effect of neutron irradiation on the crystal is briefly discussed.
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.
Ion Cyclotron Radio Frequencies (ICRFs) have proved to efficiently serve as a toroidal rotation source in tokamak plasmas. Recent experiments on EAST show that the rotation profiles are remarkably modified when the internal inductance (li) changed. The comparisons of the ion and electron temperature profiles among ICRF-heated plasmas suggest that the change in li can remarkably affect the toroidal rotation velocity. The scaling of rotation velocity increments as a function of the change in li also suggests that ICRF would serve as a better rotation source in plasmas with larger li decrements.
The Xe44+ 2.7203Å, line, which is proposed as one of the diagnostic lines for the X-ray imaging crystal spectrometer on ITER, is observed on EAST tokamak together with its several satellite lines. The observations are made under high electron temperature (Te ) conditions (core Te > 5keV). Most of the observed xenon lines are identified by comparing the experiment results with the atmoic simulation results. The first ion temperature measurements made by the Xe spectra on EAST are also reported in this article. These xenon spectra observations contribute to the justification of using xenon as the diagnostic impurity of X-ray crystal spectrometers in future reactor-scale high-temperature plasmas.
The Poloidal and Tangential X-ray imaging Crystal Spectrometers (PXCS and TXCS) were developed on Experimental Advanced Superconducting Tokamak (EAST) to provide spatially and temporally resolved plasmas ion temperature (Ti), electron temperature (Te) and rotation velocity (poloidal-and toroidal-, Vp and Vt) profiles. Each spectrometer consisted of a spherically curved crystal and a CMOS pixelated X-ray Detector. Both spectrometers have recently been upgraded to enhance their measurement capabilities and stabilities. A He-like argon crystal (2d=4.913Å) is deployed on the TXCS and a double-crystal assembly including a He-like argon (2d=4.913Å) crystal and a Ne-like xenon (2d=6.686Å) crystal is deployed on the PXCS. To obtain the optimal spatial resolution, the distance from the crystal to the detector and to the plasma center are modified. Meanwhile, the projection angle of the TXCS sightline to the major radial direction is increased from ~ 22.5º to ~ 29.5º in order to view the plasma with more tangential component. The XCS server is moved from the EAST tokamak hall to an outside lab to avoid harsh electromagnetic environment and thus enhance stability. Finally, the experimental results from the upgraded XCS systems are presented. New spectral lines of Zinc-like, Copperlike and Gallium-like tungsten are identified, which are diffracted by the He-like argon crystal. High-quality He-like argon and Ne-like xenon spectra are observed simultaneously on one detector for measurements of plasmas with wide temperature ranges. Comparison of the Ti-and Vt-profiles measured by TXCS with those measured by charge exchange recombination spectroscopy (CXRS) shows that the results are in well agreement, verifying the reliability of the upgrade of the spectrometers.
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