Experimental study of reversed shear Alfvén eigenmodes during the current ramp in the Alcator C-Mod tokamak

Experiments conducted in the Alcator C-Mod tokamak have explored the physics of reversed shear Alfvén eigenmodes (RSAEs) during the current ramp. The frequency evolution of the RSAEs during the current ramp provides a constraint on the evolution of q_min, a result which is important in transport modeling and for comparison with other diagnostics which directly measure the magnetic field line structure. Additionally, a scaling of the RSAE minimum frequency with the sound speed is used to derive bounds on the adiabatic index, a measure of the plasma compressibility. This scaling places the adiabatic index at 1.40±0.15 and supports the kinetic calculation of separate electron and ion compressibilities with an ion adiabatic index close to 7/4.