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Controlled Stabilization of Alfv�n Eigenmodes in DIII-D and Validation of Theory and Simulations

  • Author(s): Tang, Shawn
  • Advisor(s): Carter, Troy A
  • Crocker, Neal A
  • et al.
Abstract

Understanding the interaction between wave excitation and damping is essential in the advancement of research on magnetized plasmas in space, laboratory, and astrophysical settings. Alfv en waves can be excited across all of these settings, and in fusion research plasmas such as tokamak plasmas, they are present due to energetic particles from neutral beam injection and fusion reactions. The interplay between wave-particle interaction and damping processes is at the core of understanding wave excitation. \

In the work presented in this dissertation, an experiment was designed to investigate high frequency Alfv en waves excited through Doppler-shifted cyclotron resonance with energetic particles from neutral beam injection in the DIII-D tokamak. These waves, compressional (CAE) and global (GAE) Alfv en eigenmodes, typically have frequencies close to the ion cyclotron frequency $f_{ci}$; the frequency and amplitude of these modes was measured with the Ion Cyclotron Emission (ICE) diagnostic. The experiment utilized the unique capability of the DIII-D neutral beams to separately control the energy and injection rate. A parametric scan across many magnetic fields and beam geometries was performed to study the dependencies of these modes on various plasma parameters.\

An energetic ion density threshold was observed during a discharge in which the voltage of an off-axis co-injecting beam was held constant while the current was ramped down by $40\%$. During this discharge, a spectrum of high frequency AEs at $f=0.58f_{ci}$ was stabilized via a controlled energetic ion density ramp for the first time in a fusion research plasma. This observation demonstrates an important property of resonant AEs: that the growth rate of these waves is set by the balance between fast-ion drive and damping processes. The controlled stabilization of this mode also validates previous simulations done on high frequency AEs in which an instability threshold was observed by varying the beam density without changing the shape of the distribution. \

The scaling of the amplitude of this wave with the beam injection rate was found to be consistent with predictions for single mode collisional saturation near marginal stability. Analytic theory found that for the observed beam injection rate threshold, the mode was near marginal stability throughout the entire beam ramp. This is notably different from previous simulations of CAEs/GAEs that were in the collisionless regime and often far from marginal stability. \

Modeling codes such as TRANSP and ORB\_GC were used to model and analyze the fast-ion distribution for this discharge. This analysis found that the modes were likely excited by a high energy subset of the fast-ion population with strong gradients in parallel velocity space. Resonance analysis of this subset of the fast-ion population, in conjunction with considerations from dispersion relations, shows that the mode is likely a shear-polarized GAE. This marks the first identification of a GAE excited through Doppler-shifted cyclotron resonance with sub-Alfv enic energetic ions, a first in fusion research plasmas.

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