1,720,984 research outputs found
Velocity-space distribution function of fast ions in a sawtoothing plasma
Full text linkThis study explores the influence of sawtooth oscillations on the velocity space distribution of fast ions in tokamak plasma discharges. The relevant Fokker-Planck equation for fast ions is solved analytically. Two distinct effects arising from the temperature drop associated with a sawtooth crash and their impact on the distribution function of fast ions are considered. The first effect involves the modulation of the fusion alpha particle source on the timescale of the sawtooth period, linked to the drop in fusion yield resulting from the sawtooth temperature relaxations. The second effect is tied to the increase of the slowing-down time during the sawtooth ramp, causing particles born later in the sawtooth cycle to experience reduced slowing down compared to those born right after the crash, creating an accumulation-like mechanism at higher energies. In regimes where the sawtooth period is shorter than the fast ion slowing-down time, the combined influence of these effects gives rise to fast ion distribution functions that transiently exhibit positive slopes in velocity space
Fast-ion-driven vertical modes in magnetically confined toroidal plasmas
Full text linkA new type of fast particle instability involving axisymmetric modes in magnetic fusion tokamak plasmas is presented. The relevant dispersion relation involves three roots. One corresponds to a vertical plasma displacement that, in the absence of active feedback stabilization, grows on the wall resistivity time scale. The other two, oscillating close to the poloidal Alfv ́en frequency, are normally damped by wall resistivity. The resonant interaction with fast ions can drive the oscillatory roots unstable. Resonance conditions, stability thresholds and experimental evidence are discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Resonant Axisymmetric Modes
Full text linkAxisymmetric modes in shaped tokamak plasmas are normally associated with vertical displacement events. However, not enough attention has been given to the fact that these modes can be resonant in two different ways. Firstly, for a plasma bounded by a divertor separatrix, a generic n=0 ideal-MHD perturbation, ξ, is singular at the divertor X- point(s), where Beq · ∇ξ = 0, with Beq the equilibrium magnetic field. As a consequence, n=0 perturbations can give rise to current sheets localized along the divertor separatrix. Secondly, a feedback-stabilized n=0 mode tends to acquire an Alfv ́enic oscillation frequency. As a result, a resonant interaction with energetic particle orbits can lead to a new type of fast ion instability
Linear NIMROD simulations of n = 0 modes for straight tokamak configuration and comparison with analytic results
Full text linkComparison between the analytic theory of n = 0 vertical displacement modes in magnetically confined plasmas of fusion interest and numerical simulations using the extended-MHD code NIMROD is presented. Agreement between analytic and numerical results is highly satisfactory. Differences are interpreted to be caused mostly by the different wall shape and by the presence of a halo plasma surrounding the hot plasma adopted in NIMROD. A numerical study of vertical displacement oscillatory modes [Barberis et al., J. Plasma Phys. 88, 905880511 (2022)] is presented. Axisymmetric X-point currents supported by the halo plasma are discussed. The article provides a successful benchmark and a useful starting point for future numerical investigations of n = 0 modes using more realistic tokamak geometry and plasma equilibria
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