Frequency splitting in JET: theory and observation

Figure 1(a) shows the power spectrum of a family of toroidal Alfven 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).