Fast Particle Finite Orbit Width and Larmor Radius Effects on Low-n Toroidicity-induced Alfvén Eigenmode Excitation
Authors: N.N. Gorelenkov, C.Z. Cheng, and G.Y. Fu
The effects of finite drift orbit width (FOW) and finite Larmor radius (FLR) of fast particles on the stability of low-n toroidicity-induced Alfvén eigenmodes (TAE) are studied. The formulation is based on the solution of the low frequency gyro-kinetic equation [omega is always less than omega(subscript c), where omega(subscript c) is particle cyclotron frequency] by following the particle drift orbit and thus fully retains the FOW effect. A quadratic form has been derived in terms of invariant variables: energy E, magnetic moment mu, and toroidal angular momentum P(subscript psi). The growth rate of TAE is computed perturbatively by numerically averaging over the fast particle drift orbit. These new computational capabilities improve the previous version of NOVA-K code [G.Y. Fu, C.Z. Cheng, and K.L.Wong, Phys. Fluids B 5 (1994) 4040] which includes FOW effects in the growth rate calculation based on a small radial orbit width approximation. The new NOVA-K version has been benchmarked for different regimes of TAE excitation. It is shown that both FOW and FLR effects are typically stabilizing: the TAE growth rate can be reduced by as much as a factor of 2 for the Tokamak Fusion Test Reactor supershots [D.J. Grove and D.M. Meade, Nucl. Fusion 25 (1985) 1167]. However, FOW may be destabilizing for global TAEs, which usually have considerable amplitude in the outer radius region