Summary of recent studies of beam-driven BAE and chirping modes in DIII-D

In high beta DIII-D plasmas with intense neutral beam injection, beta induced Alfven eigenmodes (BAE modes) are observed. These instabilities cause concentrated losses of >50% of the fast ions and thus are of concern for future devices. The authors have now observed BAE modes and resultant fast ion loss in full field (2.0 T) discharges where the ratio of parallel velocity to Alfven speed is v||/vA 0.3. In a few discharges, they have also observed a new instability, a 'chirping' mode. These modes have frequencies between 50 and 200 kHz that 'whistle' down a factor of two in a single 2 ms burst. They occur in plasmas with relatively large values of fast ion β((βf) 1%), Alfven speed (v||/vA 0.5) and plasma rotation (frot > 20 kHz). In contrast to the usual Alfven modes, which are fluid modes of the background plasma, the chirping instabilities seem to be beam modes that are nearly stationary in the plasma frame

Weak effect of static, externally imposed, helical fields on fusion product confinement in the DIII-D tokamak

Stationary helical fields with toroidal mode numbers n = 1 and n = 3 are applied to beam heated DIII-D plasmas. Measurements of the 14 MeV neutron emission monitor the confinement of the 1 MeV tritium fusion product. To within approximately 15% uncertainty, static magnetic fields with vacuum amplitudes of δB/B to approximately O(10 ) have no impact on fusion product confinement. -

Simulations of beam ion transport during tearing modes in the DIII-D tokamak

Large coherent MHD modes are observed to reduce the neutral beam current drive efficiency and 2.5 MeV neutron emission in DIII-D by as much as ∼65%. These modes result in large (width w ≲ 20 cm for minor radius a ≈ 60 cm), stationary, single helicity magnetic islands, which might cause anomalous deuterium beam ion losses through orbit stochasticity. An analytic estimate predicts that co-going, passing deuterons with E ≳ 40 keV become stochastic at island widths comparable to those in the experiment. A Hamiltonian guiding centre code is used to follow energetic particle trajectories with the tearing mode modelled as a radially extended, single helicity perturbation. In the simulations, the lost neutral beam current drive and neutron emission are 35% and 40%, respectively, which is consistent with the measured reductions of 40 ± 14% and 40 ± 10%. Several features of the lost particle distribution indicate that orbit stochasticity is the loss mechanism in the simulations and strongly suggest that the same mechanism is responsible for the losses observed in the experiment

What is the "beta-induced Alfvén eigenmode?"

An instability with a lower frequency than the toroidicity-induced Alfvén eigenmode was initially identified as a beta-induced Alfvén eigenmode (BAE). Instabilities with the characteristic spectral features of this "BAE" are observed in a wide variety of tokamak plasmas, including plasmas with negative magnetic shear. These modes are destabilized by circulating beam ions and they transport circulating beam ions from the plasma core. The frequency scalings of these "BAEs" are compared to theoretical predictions for Alfvén modes, kinetic ballooning modes, ion thermal velocity modes, and energetic particle modes. None of these simple theories match the data. © 1999 American Institute of Physics

The toroidicity-induced Alfvén eigenmode structure in DIII-D: Implications of soft x-ray and beam-ion loss data

The internal structure of the toroidicity-induced Alfvén eigenmode (TAE) is studied by comparing soft x-ray profile and beam ion loss data taken during TAE activity in the DIII-D tokamak [W. W. Heidbrink et al., Nucl. Fusion 37, 1411 (1997)] with predictions from theories based on ideal magnetohydrodynamic (MHD), gyrofluid, and gyrokinetic models. The soft x-ray measurements indicate a centrally peaked eigenfunction, a feature which is closest to the gyrokinetic model's prediction. The beam ion losses are simulated using a guiding center code. In the simulations, the TAE eigenfunction calculated using the ideal MHD model acts as a perturbation to the equilibrium field. The predicted beam ion losses are an order of magnitude less than the observed ∼6%-8% losses at the peak experimental amplitude of δBr/B0≃2-5×10-4. © 2001 American Institute of Physics