Compressional Alfven Eigenmodes Research Articles

Non-linear modulation of short wavelength compressional Alfvén eigenmodes

Most Alfvénic activity in the frequency range between toroidal Alfvén eigenmodes and roughly one half of the ion cyclotron frequency on National Spherical Torus eXperiment [Ono et al., Nucl. Fusion 40, 557 (2000)], that is, approximately 0.3 MHz up to ≈1.2 MHz, are modes propagating counter to the neutral beam ions. These have been modeled as Compressional and Global Alfvén Eigenmodes (CAE and GAE) and are excited through a Doppler-shifted cyclotron resonance with the beam ions. There is also a class of co-propagating modes at higher frequency than the counter-propagating CAE and GAE. These modes have been identified as CAE, and are seen mostly in the company of a low frequency, n = 1 kink-like mode. In this paper, we present measurements of the spectrum of these high frequency CAE (hfCAE) and their mode structure. We compare those measurements to a simple model of CAE and present a predator-prey type model of the curious non-linear coupling of the hfCAE and the low frequency kink-like mode.

Compressional Alfvén and ion-ion hybrid modes in the deuterium-tritium plasma of a spherical tokamak power plant

A discrete spectrum of compressional Alfven eigenmodes and ion-ion hybrid eigenmodes is found to exist above the tritium ion cyclotron frequency in the deuterium-tritium (D-T) plasma of a spherical tokamak power plant (STPP) [H. R. Wilson et al., 19th IAEA Fusion Energy Conference (IAEA, Lyon, France, 2002), No. FT/1-4]. An equilibrium magnetic well formed in a STPP, as a result of plasma diamagnetism, causes all externally launched electromagnetic waves to propagate from the side of high total magnetic field, and it is this well that forms discrete spectra in the ion cyclotron resonance heating frequency range near the plasma core. The eigenmodes, as well as the position of the mode conversion layers, are obtained in a one-dimensional magnetic field geometry [S. C. Cowley et al., Phys. Fluids B 3, 2066 (1991)], relevant for the STPP, which will necessarily operate at high β. The possibility of using the discrete spectrum probed with an external antenna, or driven by energetic ions, for measuring the D-T ...

Alfven cyclotron instability and ion cyclotron emission

Two-dimensional solutions of compressional Alfven eigenmodes (CAEs) are studied in the cold plasma approximation. For finite inverse aspect ratio tokamak plasmas the two-dimensional eigenmode envelope is localized at the low magnetic field side with the radial and poloidal localization on the order of a/ square root m and a/4 square root m, respectively, where m is the dominant poloidal mode number. Charged fusion product driven Alfven cyclotron instability (ACI) of the compressional Alfven eigenmodes provides the explanation for the ion cyclotron emission (ICE) spectrum observed in tokamak experiments. The ACI is excited by fast charged fusion products via Doppler shifted cyclotron wave-particle resonances. The ion cyclotron and electron Landau dampings and fast particle instability drive are calculated perturbatively for deuterium-deuterium (DD) and deuterium-tritium (DT) plasmas. Near the plasma edge at the low field side the velocity distribution function of charged fusion products is localized in both pitch angle and velocity. The poloidal localization of the eigenmode enhances the ACI growth rates by a factor of square root m in comparison with the previous results without poloidal envelope. The thermal ion cyclotron damping determines that only modes with eigenfrequencies at multiples of the edge cyclotron frequency of the background ions can be easily excited and form an ICE spectrum similar to the experimental observations. Theoretical understanding is given for the results of TFTR DD and DT experiments with valpha 0/vA approximately=1 and JET experiments with valpha 0/vA>1