Measurements of the eigenfunction of reversed shear Alfvén eigenmodes that sweep downward in frequency

Abstract

Reversed shear Alfven eigenmodes (RSAEs) usually sweep upward in frequency when the minimum value of the safety factor q min decreases in time. On rare occasions, RSAEs sweep downward prior to the upward sweep. Electron cyclotron emission measurements show that the radial eigenfunction during the downsweeping phase is similar to the eigenfunction of normal, upsweeping RSAEs. V C 2013 AIP Publishing LLC. [http://dx

Numerical study of Alfvén eigenmodes in the Experimental Advanced Superconducting Tokamak

“…30,43 This provides a way to compare the results of an eigenvalue code with experimental measurements. The radial displacement also relates approximately to the electron temperature fluctuation through…”

“…However, measurements from the electron cyclotron emission radiometer show that the phase of AEs changes across the radius. 18,42,43 The change can be due to various effects, including kinetic and geometric effects. In this paper, as an application of the GTAW code, we calculate the frequency and mode structure of TAEs in a up-down asymmetric EAST equilibrium and demonstrate that up-down asymmetry induces radial phase variation in the eigenfunction.…”

Electron cyclotron heating can drastically alter reversed shear Alfvén eigenmode activity in DIII-D through finite pressure effects

“…This calculated spectrum is very similar to the measured spectrum shown in figure 5(a) and that of the q-evolution model in figure 5(b); note, the approximate experimental time range simulated is ≈ t 370-540 ms. One difference between the TAEFL calculations in figure 19(a) and the experimental spectrum of modes, however, is that the TAEFL calculations predict downsweeping RSAEs over a larger q min range than observed experimentally. The exact reason for this discrepancy is unknown, however, as mentioned previously, the downsweeping RSAE is quite uncommon in DIII-D plasmas [61] and it is an area of active research to understand this effect and resolve the differences. One potential source for the difference in unstable modes predicted by TAEFL and those observed experimentally is the assumption of a Maxwellian fast ion population.…”

“…When one looks at the model prediction for this case (figure 9(b)), f RSAE-min is well below f TAE over the entire current ramp and the predicted frequencies agree extremely well with those observed. Another interesting change in this discharge is the appearance of more downsweeping RSAEs [28,61], a feature which is not as common as the up-sweeping RSAE. Spectra from a set of discharges with similarly unexpected results are shown in figure 10; in these discharges, the same ECH steering geometry and timing as figures 4(b) and (c) was used but the current ramp rate was reduced from 0.8 MA s −1 to 0.44 MA s −1 .…”

Simulation of fast-ion-driven Alfvén eigenmodes on the Experimental Advanced Superconducting Tokamak

“…Measurements from the electron cyclotron emission radiometer show that the phase of AEs usually changes across the radius. [35][36][37] The radial phase variation makes the twodimensional mode structure on the poloidal plane take a twisted structure. The cause of the twist (or radial phase variation) is usually attributed to the non-perturbative kinetic effects of energetic particles (EPs).…”