The spectrum of low‐frequency, m= ±1 oscillations in a cold, collisionless, cylindrical plasma comprises the frequencies of the lowest radial eigenmodes and the surface modes of the fast magnetosonic wave, all the global eigenmodes of the Alfven wave (ion‐cyclotron wave), and the Alfven continuum. The spectrum of a homogeneous, currentless plasma cylinder is obtained from an analytical dispersion relation. This spectrum is compared with numerically obtained spectra of nonuniform plasmas with and without axial current. The connection of the magnetohydrodynamic (MHD) kink and internal kink instabilities with the global eigenmodes of the Alfven wave is evidenced. Among all the modes of the magnetosonic wave which have frequencies in the Alfven continuum, the (m= −1, k<0) ‐surface mode is the best candidate for antenna–plasma coupling because its conversion layer is located well into the plasma interior.

/pdf/spectrum-of-low-frequency-nonaxisymmetric-oscillations-in-a-v9vnyluxx0.pdf

Spectrum of low‐frequency, nonaxisymmetric oscillations in a cold, current‐carrying plasma column

The detailed experimental data obtained on the TCA tokamak [Proceedings of the 11th Symposium on Fusion Technology (CEC, Luxembourg, 1981), Vol. I, p. 601] concerning the antenna loading and wave fields as a function of the Alfven wave spectrum are presented. The plasma density, plasma current, working gas, toroidal magnetic field, and frequency were varied systematically and the detailed results are compiled and discussed. In addition the phasing of the antenna currents was varied systematically, thereby exciting different combinations of modes, including pure traveling waves. The conclusions relevant to the design of an antenna system have been stressed.

The Alfvén wave spectrum as measured on a tokamak

Reference CRPP-ARTICLE-1983-008View record in Web of Science Record created on 2008-04-16, modified on 2017-05-12

Geometrical effects on resonant absorption of MHD-waves

Experimental results are presented which show that the Alfven wave is strongly guided by magnetic fields. The experiment was conducted in a Tokamak plasma using a small dipole loop antenna to generate a localised Alfven ray. The ray was observed, with magnetic probes, to propagate as a localised disturbance along the curved lines of the steady magnetic field without significant refraction due to the effects of finite frequency, resistivity or magnetic field gradients. These results agree with theoretical predictions and demonstrate that a localised Alfven wave may be excited, and may propagate, independently of the fast wave, as expected. The implication of these results for the Alfven wave heating scheme is discussed.

https://iopscience.iop.org/article/10.1088/0741-3335/27/10/004/pdf

Guided propagation of Alfven waves in a toroidal plasma

Results of 3D nonlinear simulations of neutral-beam-driven compressional Alfven eigenmodes (CAEs) in the National Spherical Torus Experiment (NSTX) are presented. Hybrid MHD-particle simulations for the H-mode NSTX discharge (shot 141398) using the HYM code show unstable CAE modes for a range of toroidal mode numbers, n=4−9, and frequencies below the ion cyclotron frequency. It is found that the essential feature of CAEs is their coupling to kinetic Alfven wave (KAW) that occurs on the high-field side at the Alfven resonance location. High-frequency Alfven eigenmodes are frequently observed in beam-heated NSTX plasmas, and have been linked to flattening of the electron temperature profiles at high beam power. Coupling between CAE and KAW suggests an energy channeling mechanism to explain these observations, in which beam-driven CAEs dissipate their energy at the resonance location, therefore significantly modifying the energy deposition profile. Nonlinear simulations demonstrate that CAEs can channel the ...

Nonlinear simulations of beam-driven compressional Alfvén eigenmodes in NSTX

The absorption of magnetoacoustic modes at the spatial Alfven resonance of Tokamak plasmas is investigated within the context of an ideal MHD model which includes finite omega / omega ci effects. It is found that these effects can dramatically modify the pure MHD picture of the resonant absorptiometry and the spectrum of global eigenmodes of the Alfven wave. Implications for the interpretation of recent experimental results are discussed.

Effects of Finite Ion-Cyclotron Frequency on the Absorption of Magnetohydrodynamic Waves at the Spatial Alfven Resonance

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Effects of Finite Ion-Cyclotron Frequency on the Absorption of Magnetohydrodynamic Waves at the Spatial Alfven Resonance