Convective and Diffusive Energetic Particles Losses Induced by Shear Alfven Waves in the ASDEX Upgrade Tokamak

摘要

Future burning plasma experiments such as ITER,may be subject to the excitation of Alfvén eigen-mode (AE) instabilities by 3.5 MeV fusion born alpha particles as well as fast-ions created by auxiliary heating systems critical for current drive, heating, and momentum input. If allowed to grow unabated, these instabilities have the potential to cause fast-ion redistribution and loss leading to a degradation of the heating and current drive efficiencies as well as to possible serious damage of first wall components. Recently, in ASDEX Upgrade, significant progress has been achieved towards a better understanding of the fast-ion transport across magnetic filed lines in the presence of multiple AEs. In ICRF heated plasmas, strong fast-ion losses in the presence of Alfven Cascades (ACs) and Toroidal Alfven Eigenmodes (TAEs) have been observed with a scintillator based Fast-Ion Loss Detector (FILD) [1]. Fluctuations in the electron temperature profile caused by the ACs and TAEs have been measured with the ECE radiometer at high resolution in space densities (coherence) as a function of p_(pol) and frequency for 50 ms time intervals. The AC and TAE radial structures were obtained by selecting and averaging a certain frequency band of the coherence. The selected AC and TAE frequency bands are given in Fig.1. As Fig.1 shows, their overlapping region (highlighted in yellow) becomes smaller with time. A detailed analysis of the time-resolved fast-ion losses detected with the FILD system during the AE activity allows to identify the loss mechanisms [2]. Fig. 2(a) shows the coherent fast-ion losses due to ACs and TAEs. The raw data of the Fourier-analyzed fast-ion loss signal shown in Fig. 2(a) are presented in Fig. 2(b) to investigate the diffusive and convective character of the losses. The signal consists of a modulated (coherent) signal sitting on an incoherent background whose amplitude varies with time. The coherent component of the fast-ion losses is correlated in frequency and phase with the corresponding magnetic fluctuation, yielding to the spectrogram shown in Fig. 2(a). The incoherent component is dominant, up to 80% of the total losses, in the presence of multiple frequency chirping AEs, t = (1.1-1.3) s, and decreases when the number of modes decreases. However, it should be noted that it is not zero when only one mode is ejecting ions it its amplitude is large enough, t≈a (1.42-1.52) s. During the time window t ≈ (1.52-1.7) s only coherent