Excitation of Alfv\'{e}n Eigenmodes by Low Velocity Beam Ions in the JET and DIII-D Tokamaks

New data on the DIII-D and JET tokamaks reveal a rich variety of Alfv\'{e}nic activity excited by neutral beam ions traveling at only a small fraction of the local Alfv\'{e}n velocity. These observations challenge our detailed understanding of the excitation of Alfv\'{e}nic phenomena and provide a validation platform for testing fundamental theoretical predictions. In addition, precise internal measurements of density and temperature fluctuations reveal new information on the kinetic properties of Alfv\'{e}n eigenmodes that challenge ideal MHD descriptions of these instabilities. Recent experiments on the JET facility with 3.5 T magnetic field and low plasma density demonstrate that Cascade modes are excited by 50 keV beam ions corresponding to only v$_{A}$/6, where v$_{A}$ is the local Alfv\'{e}n velocity. Toroidal Alfv\'{e}n eigenmodes are excited by ions traveling at only v$_{A}$/4, well below the v$_{A}$/3 sideband condition for the primary resonance. Detailed stability analysis reveals a key role played by finite orbit effects and in particular the beam ion anisotropy for these low energy excitations. Similarly, studies on DIII-D with 2.0 T magnetic fields reveal that the direction of injection of neutral beam injection is a critical factor in the excitation of Alfv\'{e}n eigenmodes. As in JET, a key to directional sensitivity is the finite orbit width of the fast ions. New observations are also obtained on the excitation of $n$=0 modes in both JET and DIII-D driven by low energy (50-80 keV) beam ions. Internal measurements reveal much smaller temperature-to-density fluctuation levels for these modes, suggesting that the fluctuations cannot be interpreted as due to the radial displacement of magnetic field lines.