arXiv subjects

Yevgen Kazakov

Publications and source records attributed to Yevgen Kazakov.

5 recordsLinked to original sources

arXiv2024

Measurement of zero-frequency fluctuations generated by coupling between Alfven modes in the JET tokamak

We report the first experimental detection of a zero-frequency fluctuation that is pumped by an Alfv\`en mode in a magnetically confined plasma. Core-localized bidirectional Alfv\`en modes of frequency inside the toroidicity-induced gap (and its harmonics) exhibit three-wave coupling interactions with a zero-frequency fluctuation. The observation of the zero-frequency fluctuation is consistent with theoretical and numerical predictions of zonal modes pumped by Alfv\'en modes, and is correlated with an increase in the deep core ion temperature, temperature gradient, and confinement factor $H_{89,P}$. Despite the energetic particle transport induced by the Alfv\`en eigenmodes, the generation of a zero-frequency fluctuation that can suppress the turbulence leads to an overall improvement of confinement.

physics.plasm-ph↗

arXiv2023

Stable Deuterium-Tritium burning plasmas with improved confinement in the presence of energetic-ion instabilities

Providing stable and clean energy sources is a necessity for the increasing demands of humanity. Energy produced by fusion reactions, in particular in tokamaks, is a promising path towards that goal. However, there is little experience with plasmas under conditions close to those expected in future fusion reactors, because it requires the fusion of Deuterium (D) and Tritium (T), while most of the experiments are currently performed in pure D. After more than 20 years, the Joint European Torus (JET) has carried out new D-T experiments with the aim of exploring the unique characteristics of burning D-T plasmas, such as the presence of highly energetic ions. A new stable, high confinement and impurity-free D-T regime, with strong reduction of energy losses with respect to D, has been found. Multiscale physics mechanisms critically determine the thermal confinement and the fusion power yield. These crucial achievements importantly contribute to the establishment of fusion energy generation as an alternative to fossil fuels.

physics.plasm-ph↗

arXiv2022

Representation and modeling of charged particle distributions in tokamaks

Experimental diagnostics, analysis tools and simulations represent particle distributions in various forms and coordinates. Algorithms to manage these data are needed on platforms like the ITER Integrated Modelling & Analysis Suite (IMAS), performing tasks such as archiving, modeling, conversion and visualization. A method that accomplishes some of the required tasks for distributions of charged particles with arbitrarily large magnetic drifts in axisymmetric tokamak geometry is described here. Given a magnetic configuration, we first construct a database of guiding center orbits, which serves as a basis for representing particle distributions. The orbit database contains the geometric information needed to perform conversions between arbitrary coordinates, modeling tasks, and resonance analyses. Using that database, an imported or newly modeled distribution is mapped to an exact equilibrium, where the dimensionality is reduced to three constants of motion (CoM). The orbit weight is uniquely given when the input is a true distribution: one that measures the number of physical particles per unit of phase space volume. Less ideal inputs, such as distributions estimated without drifts, or models of particle sources, can also be processed. As an application example, we reconstruct the drift-induced features of a distribution of fusion-born alpha particles in a large tokamak, given only a birth profile, which is not a function of the alpha's CoM. Repeated back-and-forth transformations between CoM space and energy-pitch-cylinder coordinates are performed for verification and as a proof of principle for IMAS.

physics.plasm-ph↗

arXiv2022

Energy-selective confinement of fusion-born alpha particles during internal relaxations in a tokamak plasma

Long-pulse operation of a self-sustained fusion reactor using toroidal magnetic containment requires control over the content of alpha particles produced by D-T fusion reactions. On the one hand, MeV-class alpha particles must stay confined to heat the plasma. On the other hand, decelerated helium ash must be expelled before diluting the fusion fuel. Our kinetic-magnetohydrodynamic hybrid simulations of a large tokamak plasma confirm the existence of a parameter window where such energy-selective confinement can be accomplished by exploiting internal relaxation events known as `sawtooth crashes'. The physical picture -- consisting of a synergy between magnetic geometry, optimal crash duration and rapid particle motion -- is completed by clarifying the role played by magnetic drifts. Besides causing asymmetry between co- and counter-going particle populations, magnetic drifts determine the size of the confinement window by dictating where and how much `reconnection' occurs in particle orbit topology.

physics.plasm-ph↗

arXiv2014

Kinetic description of wave induced plasma flow in the radio frequency domain

A model for ICRH induced flows in the presence of a strong magnetic field is presented. These flows are the finite temperature counterpart of flows existing in cold plasmas described e.g. in [D. Van Eester et al., Plasma Phys. Control. Fusion 55 (2013) 025002] and thus do not rely on the waves being damped. The kinetic corrections offer insight in what happens at cyclotron resonances. Authors commonly either rely on the confining magnetic field $\vec{B}_o$-field to be strong, or the electric field $\vec{E}$-field to be rapidly varying but are not accounting for both when writing down the solution of the equation of motion on the slow time scale. In this paper, the equation of motion is solved for constant $B_o$ to keep the discussion as simple as possible. The simultaneous presence of $\vec{B}_o$ and the $\vec{E}$-field inhomogeneity causes drifts perpendicular to the $\vec{B}_o$ and to other slow time scale accelerations, the Ponderomotive acceleration being one of them. Because of the first and having tokamak applications in mind, these flows - although small in magnitude - cause drifts that enter in competition with transport induced flows.

physics.plasm-ph↗