Evolution of toroidal Alfvén eigenmode instability in Tokamak Fusion Test Reactor

Abstract

The nonlinear behavior of the Toroidal Alfvén Eigenmode (TAE) driven unstable by energetic ions in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 1, 1560 (1994)] is studied. The evolution of instabilities can take on several scenarios: a single mode or several modes can be driven unstable at the same time, the spectrum can be steady or pulsating and there can be negligible or anomalous loss associated with the instability. This paper presents a comparison between experimental results and recently developed nonlinear theory.Many features observed in experiment are compatible with the consequences of the nonlinear theory. Examples include the structure of the saturated pulse that emerges from the onset of instability of a single mode, and the decrease, but persistence of, TAE signals when the applied rf power is reduced or shut off.

High harmonic ion cyclotron heating in DIII-D: Beam ion absorption and sawtooth stabilization

“…Gradual saturation near the marginal stability point is readily achieved in a weakly driven system [55], but more complicated phenomena occur in strongly driven systems. (It should be noted, however, that, although there is a general tendency in the TFTR data for more peaks to appear in the spectrum with increasing RF power [48], complicated spectra are sometimes observed near the threshold for sawtooth stabilization [47,56]. )…”

“…Presumably, the Alfvén modes cause anomalous beam ion transport in the plasma core. The redistributed beam ions continue to produce neutrons, but the central beam ion density saturates near the marginal stability point [48]. With the central beam ion density fixed, the power transfer from the energetic ions to the electrons saturates, which results in saturation of T e (0).…”

Alpha particle physics experiments in the Tokamak Fusion Test Reactor

“…the effect of TAE modes on the internal redistribution of alphas in TFTR. Some progress has been made in the theory of this interaction [28,29,[183][184][185][186][187], and in interpreting the TAE-induced transport of NBI or RF tail ions due to TAE modes [188][189][190][191]. The results of such studies would help to clarify the degree to which collective alpha effects will affect the confinement of reactor-grade tokamak plasmas.…”

Critical nonlinear phenomena for kinetic instabilities near threshold

“…we infer that the ratio of perturbed to equilibrium magnetic field is roughly and the RF heating time is 0.2 s; results that are consistent with the experiment . 24 This correlation indicates that the collisional relaxation process can indeed be an important ingredient in the long time evolution of a weak TAE instability.…”

“…We now address the question of stability for the constant amplitude solutions (23) and (24). In order to make the analysis more compact, we use the transformation A = a ( t ) exp( -it tan $) (25) and also combine Eqs.…”