Effect of Externally Applied Perturbation Fields on Alfv\'{e}nic MHD Activity in the NSTX Tokamak

Observations from NSTX demonstrate that externally applied magnetic perturbations (MP) can alter the dynamic of beam driven Alfv\'{e}n modes. Bursting Global Alfv\'{e}n Eigenmodes (GAE, n$=$7-9, 400-700 kHz) respond to pulses of static n$=$3 fields ($\delta $B/B $\sim$ 0.01 at the plasma edge) reducing mode amplitude, bursting period and frequency sweep by a factor of 2-3 [Bortolon et al., Phys. Rev. Letters, Vol. 110 (2013) 265008]. Similar MP attenuate the amplitude of continuous Toroidal Alfv\'{e}n Eigenmodes (TAE, n$=$2-3, 50-90 kHz). Calculations of the perturbed beam-ion distribution function, considering MP from ideal or resistive plasma response, confirm an enhanced fast-ion transport consistent with a reduced drive for the GAE. At the same time, MP can also affect the Alfv\'{e}n stability by altering the structure of Alfv\'{e}n continua through modification of the kinetic profiles or introducing toroidal coupling as result of the broken axisymmetry. Computations of the n$=$2 Alfv\'{e}n continuum for NSTX equilibria with n$=$3 MP show strong modification of the TAE continuum near the plasma edge, where coupling between n$=$2 and n$=$5 continuum modes reduces the gap, providing an additional damping for TAE modes extending in this region.