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Transformed frequency (kHz)

By R. G. L. Vann, R. O. Dendy, S. D. Pinches, S. E. Sharapov and Jet-efda Contributors


Figure 1(a) shows the power spectrum of a family of toroidal Alfvén eigenmodes (TAEs) in JET shot 40332, previously considered in Ref. [1]. Each TAE begins life as a single mode which later develops sidebands. The TAEs are excited via a combination of NBI and ICRH heating. The different TAEs have toroidal mode numbers in the range 5–12 and frequencies that are Doppler-shifted due to plasma rotation. In the model analysis below, we concentrate on a single excited mode. To analyse the experimentally observed structures, the slow frequency drift of the modes (due to a slow change in the properties of the background plasma) is compensated for by a nonlinear mapping, the result of which is shown in Fig. 1(b). This enables the power spectrum to be compared at different times on the same frequency scale. Figure 2 shows two cuts through the transformed power spectrum Fig. 1(b) at t = 12.40s and t = 12.45s. We note that each of the dominant peaks in panel 2(a) has developed sidebands in panel 2(b). Figure 1: (a) Experimental observation of TAEs in JET plasma 40332 undergoing frequency splitting. The slow frequency drift is due to slow variation of macroscopic plasma parame-ters. (b) Transformed power spectrum of JET plasma 40332. Frequency has been nonlinearly stretched to eliminate the slow frequency drift. This transformation allows cross sections corre-sponding to different times (as in Fig. 2) to be compared on the same frequency axis

Year: 2016
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