Characterization of Stable and Unstable Alfvén Eigenmodes in Alcator C-Mod
نویسندگان
چکیده
Experiments designed to characterize both stable and unstable Alfvén eigenmodes are a key part of the Alcator C-Mod physics program. Stable intermediate toroidal mode number (3 ≤ n ≤ 14) Alfvén eigenmodes, which are expected to be the most unstable in ITER, are excited with a set of active MHD antennas and their damping rates are measured as a function of plasma parameters. This is part of an international effort within the ITPA, which includes similar experiments on JET, JT-60U, and MAST. Alcator C-Mod provides results at the same toroidal field and density as ITER in the presence of an ICRF generated fast ion tail, while JET and JT-60U provide results in large high temperature plasmas and MAST provides results on aspect ratio and beta. Unstable toroidal Alfvén eigenmodes (TAEs) are also produced with fast ions generated by hydrogen minority ICRF heating and with fast electrons generated with Lower Hybrid Current Drive (LHCD). The fast ion driven TAEs are observed to be more unstable in steady H-mode discharges than in lower density L-mode discharges with the same ICRF power. The stability properties have been analyzed with the NOVA-K code. The fast electron driven Alfvén eigenmodes with LHCD have only been observed very early in the current rise phase of the discharge at very low densities. These modes, nonetheless, provide a measure of the early evolution of the resonant q and help to constrain the fast electron energy distribution together with hard x ray energy distribution measurements.
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A Comparison of Measured and Calculated Toroidal Alfvén Eigenmode Damping Rates in Alcator C-Mod
Measured damping rates of stable Toroidal Alfvén Eigenmodes (TAEs) have been compared with damping rates calculated with the NOVA-K code for a number of experimental conditions on Alcator C-Mod in an attempt to validate the NOVA-K damping model. Very low amplitude ( / B B θ θ ~ 10 -7 at the wall) stable TAEs are excited in Ohmic plasmas with a pair of active MHD antennas inside C-Mod. By sweepi...
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