β suppression of Alfvén cascade modes in the National Spherical Torus Experiment

Alfvén cascade modes have been found in low density, low β plasmas on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. An extension of the theory of cascade modes which includes the coupling to geodesic acoustic modes [Breizman et al., Phys. Plasmas 12, 112506 (2005)] is shown to imply their absence for typical spherical tokamak ratios of electron thermal to magnetic energy, β. A scan in electron β confirmed a threshold for suppression of cascade modes in good agreement with theoretical predictions.