Physics – Plasma Physics
Scientific paper
Nov 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aps..dppji1002c&link_type=abstract
American Physical Society, 49th Annual Meeting of the Division of Plasma Physics, November 12-16, 2007, abstract #JI1.002
Physics
Plasma Physics
Scientific paper
Beam ions and/or fusion alphas are expected to excite Alfv'en Cascade (AC) modes (i.e. reversed-shear Alfv'en eigenmodes) in ITER reversed-shear advanced scenarios. The National Spherical Torus eXperiment (NSTX), where fast-ions with comparable v/vAlfv'en (˜ 2 -- 4) excite ACs, is an ideal device in which to observe ACs and their impact. Its wide range of βe (ratio of electron to magnetic pressure) enables tests of AC theory up to, and beyond, a critical βe where suppression is predicted. A value for critical βe, ˜ [4qmin^2o1+(7/4)(Ti/Te))]-1, may be derived from the theory of Breizman, et al. [Phys. Plasmas 12 (2005) 112506]. Observations of suppression and frequency evolution in NSTX, including onset and saturation, agree well with this theory and calculations by the NOVA-K linear stability code. The dependence of AC frequency on minimum safety factor (qmin) enables a sensitive determination of qmin from the AC spectrum that agrees well with the minimum of the q profile measured using the motional Stark effect. AC structure measurements near critical βe from three fixed frequency (i.e. spatially localized) reflectometers and three tangential interferometers show a structure consistent with predicted localization near the qmin radius. Magnetic measurements indicate shear-wave polarization at qmin. Fast-ion response is monitored with neutral particle analyzers, a fast lost ion probe and neutron detectors. Profile measurements of q, density, electron and ion temperature, and rotation are used by NOVA-K to predict mode structure and frequency, or suppression, for direct comparison with the mode measurements. These novel observations of ACs near critical βe are well explained by theory, allowing us to extrapolate our understanding of this physics with confidence.
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