Abstract
The spectrum of compressional Alfvén eigenmodes (CAE) is analysed and shown to be discrete in tokamaks with low aspect ratio, such as the National Spherical Torus Experiment (NSTX), as well as in conventional tokamaks, such as DIII-D. The study is focused on recent similarity experiments on NSTX and DIII-D in which sub-cyclotron frequency instabilities of CAEs were observed at similar plasma conditions (W.W. Heidbrink et al 2006 Nucl. Fusion 46 324). The global ideal MHD code NOVA recovers the main properties of these modes predicted by theory and observed in both devices. The discrete spectrum of CAEs is characterized by three quantum mode numbers for each eigenmode, (M, S and n), where M, S and n are poloidal, radial and toroidal mode numbers, respectively. The expected mode frequency splitting corresponding to each of these mode numbers seems to be observed in experiments and is consistent with our numerical analysis. The polarization of the observed magnetic field oscillations in NSTX was measured and is also consistent with the numerical analysis, which helps to identify them as CAE activity. CAE mode structure was obtained and shown to be localized in both radial and poloidal directions with typical radial localization toward the plasma edge and poloidal localization at the low field side of the plasma cross section.
Highlights
The spectrum of compressional Alfven eigenmodes (CAE) driven by a phase space gradient is measured and systematically analysed numerically in the National Spherical Torus Experiment (NSTX) and DIII-D plasmas for the first time
In experiments the Alfven velocity was varied by controlling the magnetic field and plasma density, which resulted in a correlation with the mode frequencies
CAEs were observed only in plasmas heated by a deuterium beam with a power of Pb = 1.5–3 MW
Summary
The spectrum of compressional Alfven eigenmodes (CAE) driven by a phase space gradient is measured and systematically analysed numerically in the National Spherical Torus Experiment (NSTX) and DIII-D plasmas for the first time. Note that the poloidal localization of the CAEs is critical for the modes with frequencies at high harmonics of the thermal ion cyclotron frequency as it allows avoiding the cyclotron damping of these modes on bulk ions [12] It seems to be less critical for the instabilities in NSTX, but appears to be favourable for the CAE instability because injected mostly passing beam ions interact more efficiently with such modes due to outboard radial deviation of their drift trajectories at the low field side (LFS) of the plasma cross section. We use global ideal MHD code NOVA [28, 29], which is able to recover slow, shear and compressional Alfven branches
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