Figure 1

A composite spectrogram shows the many RSAEs and TAEs excited during the current ramp phase of discharge #14211 and captured by the ECEI diagnostic. Many RSAEs, or Alfvén cascades, are excited simultaneously at rational crossings of $q_{\min }$. The vertical dashed lines identify the passing of $q_{\min }=4$ at $t\sim 525\mathrm{ms}$ and $q_{\min }=3$ at $t\sim 800\mathrm{ms}$.Reuse & Permissions

Figure 2

Eigenmode structures for three RSAEs excited near $t=550\mathrm{ms}$ are captured with ECEI and compared to simulated mode structures obtained with NOVA. The excitation frequencies and simulation times from NOVA are given in the figure and found to be in good agreement with those of the corresponding modes observed experimentally, which were (a) $f=70.8\mathrm{kHz}$, $t=551.1\mathrm{ms}$, (b) $f=67.3\mathrm{kHz}$, $t=554.2\mathrm{ms}$, and (c) $f=73.2\mathrm{kHz}$, $t=570.6\mathrm{ms}$. The color bar given is applicable to the measured data from ECEI, while simulated modes are plotted on a normalized scale. The modes in (a) and (c) show a “fundamental” radial mode structure with a single maximum in the radial mode envelope. The mode shown in (b) exhibits a radial null in the fluctuation amplitude and is identified as the first radial harmonic of the $n=3$ mode shown in (a).Reuse & Permissions

Figure 3

Simulations using TAEFL illustrate that mode structures are strongly influenced by the fast-ion population. In (a), a structure with no shearing is obtained by retaining only the conventional ideal MHD symmetry. Nonperturbative coupling to the experimentally measured fast-ion pressure profile results in the $n=4$ RSAE predicted at 44.6 kHz by TAEFL and shown in (b). By systematically varying the fast-ion density and, correspondingly, the eigenmode drive, the zero growth rate frequency (ideal MHD frequency) is estimated to be 41.6 kHz.Reuse & Permissions

Figure 4

Imaging of RSAEs near $t=725\mathrm{ms}$ reveals shearing of the Alfvén eigenmode structure which cannot be described by the ideal MHD approximation. In the (a) $n=3$ and (b) $n=4$ modes shown, the fluctuation phase reveals an outward spiraling, or poloidal shearing, of the mode in the ion diamagnetic drift direction. This behavior is well represented in the nonperturbative code TAEFL, where the effect of fast-ion dynamics is included in the 2D eigenmode structure. A discrepancy in mode frequency arises from the omission of compressibility in the simulation model and is found to be consistent with the geodesic acoustic shift of the Alfvén continuum.Reuse & Permissions