Global Alfven Waves Research Articles

Excitation of Global Alfvén Waves by Low RF Power on TCABR

Recent results of Global Alfven wave (GAW) excited by external antenna and fed by low radio-frequency (RF) power (≤1kW) in tokamak TCABR are presented. The goal of this work is to develop a diagnostic tool based on the excitation of GAW using low power RF generators when the waves can be excited without perturbing the basic plasma parameters in TCABR. This method named MHD diagnostics has already been developed on other tokamaks for toroidicity induced Alfven eigenmodes as well for GAW. Two magnetic probes are used for measurements of the magnetic field perturbations in two regimes of excitation. In the first one, the fixed RF frequency is combined with gas-puffing induced density rise and relaxation (1-2)×1013/cm3 in order to meet the GAW resonance, while in the second excitation method RF frequency sweeps 2-4 MHz are applied to the relatively stable plasma density. The plasma discharges are accompanied by saw-tooth (ST) oscillations registered by Soft-X Ray diagnostics that helps in an identification of the GAW resonances due to ST density oscillations. It is found that the fixed frequency GAW resonance appears during density rise, as well during slow density reducing after stop of gas puffing. The effective mass number Aeff=1.36 is found.

Report on recent results obtained in TCABR

Recent results of experimental work and theoretical modeling carried out in the TCABR tokamak are reported on characterization of MHD instabilities, improved diagnostics of rotation of the plasma column, excitation of Alven global modes, identification of GAMs, and the effect of rotation on their behavior. Detailed measurements of edge electrostatic perturbations and of magnetic island evolution and rotation indicate that the edge turbulence is substantially affected by the islands growth, leading to a strong modulation of the edge particle losses at the same frequency of the MHD activity. Measurements with spatial resolution also show that the growth of the MHD activity is due to nonlinear coupling of magnetic islands with different poloidal mode numbers, which increases the impurity influx. A new system of data acquisition and processing of the TCABR plasma rotation diagnostic was implemented. The system is based upon a single monochromator coupled with six photomultipliers tubes and allows one toroidal and two poloidal simultaneous rotation measurements. The excitation of Global Alfven Waves - GAW has been investigated, using a new type of radio frequency amplifier. The GAW resonances are searched either by a pre-programmed density variation, at fixed generator frequency, or through three RF frequency sweeps from 2 to 4.5 MHz, at stationary density. GAW resonances have been found and their somewhat new characteristics are presented. The investigation of the effect of poloidal and toroidal rotation on the characteristics of the geodesic acoustic mode has been investigated, both theoretically and experimentally. It is found that the assumption of isothermal flux surfaces gives rise to a third branch of this mode. Detailed predictions coupled with experimental measurements are currently being carried out to investigate this question.

Global Alfven Wave Heating of the Magnetosphere of Young Stars

Excitation of a global Alfvén wave (GAW) is proposed as a viable mechanism to explain plasma heating in the magnetosphere of young stars. The wave and basic plasma parameters are compatible with the requirement that the dissipation length of GAWs be comparable to the distance between the shocked region at the star's surface and the truncation region in the accretion disk. A two-fluid magnetohydrodynamic plasma model is used in the analysis. A current-carrying filament along magnetic field lines acts as a waveguide for the GAWs. The current in the filament is driven by plasma waves along the magnetic field lines and/or by plasma crossing magnetic field lines in the truncated region of the disk of the accreting plasma. The conversion of a small fraction of the kinetic energy into GAW energy is sufficient to heat the plasma filament to observed temperatures.

Magnetohydrodynamic Modes in a Current-Carrying Inhomogeneous Plasma

Magnetohydrodynamic (MHD) modes with m = ±1 ( m : azimuthal mode number) in a current-carrying inhomegeneous plasma are numerically investigated on the basis of wave equations derived from MHD equations, which include effects of finite ion-cyclotron frequency and ion-neutral collisions. In addition to MHD surface waves, kink instability, global Alfven eigenmode (GAE) and shear Alfven wave appear when plasma current flows in a plasma column with inhomogeneous density.

Coronal heating in flare stars: Resonant MHD absorption?

We report on the occurrence of periodicity in X-rays from certain flare stars. Statistically significant periods in the range 70-200 s are present in Einstein data for EQ Vir, Proxima Cen, and AD Leo. Periodicities are also present (although with lower statistical significance) in ROSAT data for AD Leo, and in EXOSAT data for EQ Vir and AD Leo. In UV Cet, we also find periodicity in Einstein data, but with lower statistical significance than for the above three stars: however, the UV Cet X-ray period is close to a period discovered by Gary et al. (1982) in microwaves in the close companion L726-8A. We argue that the properties of magnetic loops in flare stars are such that the variations in a single magnetic loop may be detectable in integrated flux. The period which we find in the X-ray flux from Proxima Cen is consistent with a global Alfven wave resonance in a large loop whose properties were derived some years ago by Haisch (1983) using an entirely different line of reasoning from what we use here. The periods, amplitudes, and durations of the variations which we have found in X-ray emission are consistent with the hypothesis that resonant absorption of MHD waves is occurring at certain times in coronal loops in the above four stars.

Gyrokinetic construction of wave equations at low frequencies in tokamaks

Using the gyrokinetic formalism, a conductivity tensor operator is constructed for low frequency perturbations. This operator includes the cold Alfven waves as well as kinetic Alfven waves. This operator should be appropriate for studies of toroidicity-induced Alfven waves as well as antenna coupling of global Alfven waves. In particular, terms normally missing in the usual magnetohydrodynamic description, as well as terms that are usually discarded in descriptions of Alfven wave heating, are retained in this formalism and are identified.

Coronal loop heating by Alfvén waves

The excitation and dissipation of global and surface Alfven waves and their conversion into kinetic Alfven waves have been analyzed for solar coronal loops using a cylindrical model of a magnetized plasma. Also the optimal conditions for coronal loop heating regimes with density of dissipated power ≈103 erg cm−3 s−1 by the new scheme named combined Alfven wave resonance are found. Combined Alfven wave heating regime appears when the global Alfven wave is immersed into the Alfven continuum with the condition of not-so-sharp distribution of axial current.

Stability of global Alfven waves (TAE, EAE) in JET tritium discharges

The interaction of alpha -particles in JET tritium discharges with global Alfven waves via inverse Landau damping is analysed. It is found that alpha -particle driven eigenmodes were stable in the PTE1 and should also be stable in a future 50:50 deuterium-tritium mix discharge aiming at QDT identical to 1 provided the same ion density as in the discharges with the best deuterium-deuterium performance is maintained.

Damping of global Alfven waves in tokamaks due to resonant absorption

The MHD spectrum of circular cross-section tokamak plasmas with small aspect ratio is studied for low mode numbers. Particular attention is given to the continuous part of the ideal MHD spectrum of such plasmas. Poloidal mode coupling in finite aspect ratio tokamaks yields gaps in the Alfven continuum. Global Alfven modes are found with a frequency inside these gaps. By interaction with the continuum branches the global Alfven modes experience damping via phase-mixing. This damping is computed in resistive MHD. It is shown that for asymptotically small resistivity the damping is finite and independent of eta .

On plasma processes in the solar corona

Abstract This paper reviews theoretical research on the solar corona performed in Brazil. Three coronal processes are discussed: (1) coronal heating by surface Alfven waves, (2) coronal heating by global Alfven waves, and (3) generation of type III radio emission. The surface Alfven waves can power, directly, at least 10% of the required coronal flux density; in the case of mode conversion into kinetic Alfven waves the bulk of energy flux density can be provided. The global waves can power at least 20% of the required coronal energy flux density. It is shown that the electromagnetic oscillating two-stream instability driven by intense Langmuir waves, can be a likely generation mechanism of type −III radio bursts.