Speaker
Description
Alfvén eigenmodes (AE) create a regime in the DIII-D tokamak where zonal flows and currents strongly suppress ion-temperature-gradient (ITG) turbulence. This oral focuses on the differences between AEs in the strongly suppressed ITG regime and “normal" AEs in unsuppressed plasmas. Strongly suppressed cases are compared to closely matched reference shots at several stages: (1) before the relatively gradual forward transition to the suppressed state, (2) early in the suppressed state, (3) deep in the suppressed state, and (4) after the sudden “back" transition to a normal state that has ITG turbulence and lower temperature. During the strong ITG suppression, AE amplitudes increase and radial eigenfunctions shrink; the number of unstable modes also increases. The primary toroidal and poloidal mode numbers remain unchanged in the suppressed state, but changes in poloidal mode composition are observed. The radial phase shift (rotational twist) of the eigenfunction changes from convex to concave, indicating a reversal of energy flow in the suppressed region. The TAEs burst intermittently with similar statistical properties in both normal and suppressed plasmas. The ITG suppressed state is triggered when one or more AEs acquire an electrostatic component to their polarization; other AEs often acquire an electrostatic component later in the suppressed state. Preliminary analysis suggests that polarization changes occur when the ratio of AE power to mode width exceeds a threshold. Speculation about the causes of these effects conclude the talk.
*Work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under Award(s) DE-FC02-04ER54698, DE-SC0020337, DE-FG02-08ER54999, DE-SC0020287, DESC0015878, DE-FG02-97ER54415, DE-SC0014664.