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Equations describing drift-Alfv´en eigenmodes and drift-sound eigenmodes in tokamaks and stellarators are derived. They generalize corresponding equations of Kolesnichenko et al (2009 Europhys. Lett. 85 25004), being valid for arbitrary ratio of the diamagnetic drift frequency to the frequency of the geodesic acoustic mode and taking into account the inhomogeneity of the plasma temperature. [ABSTRACT FROM AUTHOR]

The toroidal precession of circulating particles in tokamaks is studied numerically. The dependence of the precession frequency on the magnetic shear, the elongation of the plasma cross-section, and plasma pressure is investigated. It is concluded that the analytical expressions for the precession frequency by Kolesnichenko et al. (2003 Phys. Plasmas10, 1449–1457) represent a reasonable approximation for the limit cases of tokamaks with circular cross-section and shearless tokamaks with elliptical cross-section. The precession frequency was calculated for non-circular tokamaks with magnetic shear. Based on the numerical results, an interpolation formula for the precession frequency is proposed. [ABSTRACT FROM AUTHOR]

Conditions of the existence of the Global Alfvén Eigenmodes (GAE) and Nonconventional Global Alfvén Eigenmodes (NGAE) predicted for stellarators by Ya. I. Kolesnichenko et al. [Phys. Rev. Lett. 94, 165004 (2005)] have been obtained. It is found that they depend on the nature of the rotational transform and that conditions for NGAE can be most easily satisfied in currentless stellarators. It is shown that the plasma compressibility may play an important role for the modes with the frequency about or less than that of the Toroidicity-induced Alfvén Eigenmodes. It is found that features of the Alfvén continuum in the vicinity of the k∥=0 radius (k∥ is the longitudinal wave number) can be very different, depending on a parameter which we refer to as “the sound parameter.” Specific calculations modeling low-frequency Alfvén instabilities in the stellarator Wendelstein 7-AS [A. Weller et al., Phys. Plasmas 8, 931 (2001)] are carried out, which are in reasonable agreement with the observations. It is emphasized that experimental data on low-frequency Alfvénic activity can be used for the reconstruction of the profile of the rotational transform. The mentioned results are obtained with the use of the equations derived in this paper for the GAE/NGAE modes and of the codes COBRAS and BOA-fe. [ABSTRACT FROM AUTHOR]

The Alfvén continuum (AC) in the National Compact Stellarator Experiment (NCSX) [G. H. Neilson et al., in Fusion Energy 2002, 19th Conference Proceedings, Lyon, 2002 (International Atomic Energy Agency, Vienna, 2003), Report IAEA-CN-94/IC-1] is investigated with the AC code COBRA [Ya. I. Kolesnichenko et al., Phys. Plasmas 8, 491 (2001)]. The resonant interaction of Alfvén eigenmodes and the fast ions produced by neutral beam injection is analyzed. Alfvén eigenmodes residing in one of the widest gap of the NCSX AC, the ellipticity-induced gap, are studied with the code BOA-E [V. V. Lutsenko et al., in Fusion Energy 2002, 19th Conference Proceedings, Lyon, 2002 (International Atomic Energy Agency, Vienna, 2003), Report IAEA-CN-94-TH/P3-16]. [ABSTRACT FROM AUTHOR]

Effects of MHD (magnetohydrodynamic) reconnection events on the beam-plasma fusion reactivity and transport of the beam ions are studied. Based on the analysis of fusion reactivity changes induced by MHD events, the conclusion is drawn that the strong drops of the neutron yield during sawtooth crashes observed in the national spherical torus experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] are associated with both a particle redistribution inside the plasma and a loss of the beam ions. Mechanisms of the energetic ion transport during sawtooth crashes are analyzed, in particular, with the use of the resonance adiabatic invariant derived in this paper. A numerical simulation of the particle motion during a sawtooth crash in NSTX is done with the code OFSEF (orbit following in the sawtooth electromagnetic field) [Ya. I. Kolesnichenko et al., Nucl. Fusion 40, 1325 (2000)] extended for a better description of the particle precession. It is shown that the motion of toroidally passing particles in NSTX can become stochastic under the influence of a crash. This stochasticity, as well as the motion along the resonance island, leads to the escape of some particles from the plasma. [ABSTRACT FROM AUTHOR]

The toroidal precession of the circulating particles and semitrapped particles (toroidally passing but poloidally trapped particles) is studied. Expressions for the precession frequency, which are convenient for practical use, are obtained and analyzed. It is found that the key parameters that determine the magnitude and the direction of the precession velocity are the plasma elongation, the magnitudes and profiles of the safety factor and β defined as the ratio of the local plasma pressure to the magnetic field pressure at the magnetic axis. An important role of the "paramagnetic" precession in highly elongated plasmas is revealed. The analysis carried out is based on the obtained expressions for the equilibrium magnetic field strength and the field line curvature. [ABSTRACT FROM AUTHOR]

An equation of shear Alfvén eigenmodes (AE) in optimized stellarators of Wendelstein line (Helias configurations) is derived. The metric tensor coefficients, which are contained in this equation, are calculated analytically. Two numerical codes are developed: the first one, COBRA (COntinuum BRanches of Alfvén waves), is intended for the investigation of the structure of Alfvén continuum; the second, BOA (Branches Of Alfvén modes), solves the eigenvalue problem. The family of possible gaps in Alfvén continuum of a Helias configuration is obtained. It is predicted that there exist gaps which arise due to or are strongly affected by the variation of the shape of the plasma cross section along the large azimuth of the torus. In such gaps, discrete eigenmodes, namely, helicity-induced eigenmodes (HAE[sub 21]) and mirror-induced eigenmodes (MAE) are found. It is shown that plasma inhomogeneity may suppress the AEs with a wide region of localization. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Provides an overview of recent theories and results on the influence of sawtooth oscillations on the superthermal ions in a tokamak plasma. Numerical simulations of the sawtooth-crash-induced redistribution of fast ions; Peculiarities of the particle motion during the crash.

This work generalizes recent results [O. P. Fesenyuk et al., Plasma Phys. Controlled Fusion 54, 085014 (2012)] to plasmas with elongated cross section. It suggests new expressions for the frequencies of the geodesic acoustic mode and Alfvén gap modes in tokamaks, with a large ratio of the plasma pressure to the magnetic field pressure and a large safety factor (q>1, which takes place in discharges with reversed-shear configuration and, especially, in hollow-current discharges). [ABSTRACT FROM AUTHOR]

In tokamaks the crash phase of the sawtooth instability causes fast-ion transport. The DIII-D tokamak is equipped with a suite of core fast-ion diagnostics that can probe different parts of phase space. Over a variety of operating conditions, energetic passing ions are observed to undergo larger redistribution than their trapped counterparts. Passing ions of all energies are redistributed, but only low-energy ([?]40 keV) trapped ions suffer redistribution. The transport process is modeled using a numerical approach to the drift-kinetic equation. The simulation reproduces the characteristic that circulating energetic ions experience the greatest levels of internal transport. An analytic treatment of particle drifts suggests that the difference in observed transport depends on the magnitude of toroidal drift. [ABSTRACT FROM AUTHOR]