Tomography of fast ion distribution function under neutral beam injection and ion cyclotron resonance heating on EAST

Abstract

In magnetic confinement fusion devices, velocity-space tomography of fast-ion velocity distribution functions is crucial for investigating fast-ion distribution and transport. In the neutral beam injection (NBI) and ion cyclotron resonance heating (ICRF) synergistic heating experiments in Experimental Advanced Superconducting Tokamak (EAST), high-energy particles with energy exceeding NBI was observed. Simulations of synergistic effect on fast-ion velocity distribution functions given by TRANSP also showed the existence of particle with energy higher than NBI. To investigate the behavior of fast ion distribution and calculate the velocity distribution function under different heating conditions, the first-order Tikhonov regularization tomographic inversion method with higher inversion accuracy by comparing various regularization techniques was introduced. The limitations of the dual-view fast-ion D-alpha (FIDA) diagnostic measurements in velocity space was addressed by incorporating prior information such as null measurement and the known peaks and effectively mitigated the occurrence of artifacts. This method is firstly employed in the case of NBI heating. The NBI peaks was successfully reconstructed at the expected location in velocity space, which shows significant improvement in the inversion results. In order to further validate the synergistic effect of NBI-ICRF heating and study the mechanism of fast ion distribution under synergistic heating, the combination of FIDA and neutron emission spectrometers (NES) was employed to the first-order Tikhonov regularization tomographic inversion method for enhancing the coverage of velocity space, through which the issue of artifacts in the inversion results is significantly improved, and thus the precision of the obtained fast-ion velocity distribution functions is enhanced. Based on the benefit described above, the method of combining NES and FIDA diagnostics was used to calculate fast-ion velocity distribution functions in the NBI and ICRF synergistic heating discharge. The synergistic heating effect is manifested in the fast-ion velocity distribution. The availability of this inversion method in reconstructing fast-ion velocity distribution functions during high-performance operations of NBI-ICRF synergistic heating in the EAST experiment was confirmed. In the nextstep EAST research, high performance discharge will demand more efficiency NBI and ICRF synergistic heating, the work in this paper set the stage for investigating the underlying mechanisms of synergistic heating and the intricate behaviors associated with fast ion distribution and transport.