arXiv subjects

Yanzeng Zhang

Publications and source records attributed to Yanzeng Zhang.

26 records · Page 2Linked to original sources

arXiv2020

Anomalous edge plasma transport, neutrals, and divertor plasma detachment

An impact of neutrals on anomalous edge plasma transport and zonal flow (ZF) is considered. As an example, it is assumed that edge plasma turbulence is driven by the resistive drift wave (RDW) instability. It is found that the actual effect of neutrals is not related to a suppression of the instability \textit{per se}, but due to an impact on the ZF. Particularly, it is shown that, whereas the neutrals make very little impact on the linear growth rate of the RDW instability, they can largely reduce the zonal flow generation in the nonlinear stage, which results in an enhancement of the overall anomalous plasma transport. Even though only RDW instability is considered, it seems that such an impact of neutrals on anomalous edge plasma transport has a very generic feature. It is conceivable that such neutral induced enhancement of anomalous plasma transport is observed experimentally in a detached divertor regime, which is accompanied by a strong increase of neutral density.

physics.plasm-ph↗

arXiv2020

Different responses of the Rayleigh-Taylor type and resistive drift wave instabilities to the velocity shear

The effects of velocity shear on the unstable modes driven by the effective gravity (Rayleigh-Taylor and interchange) and resistive drift wave instabilities for inhomogeneous equilibrium fluid/plasma density are analyzed for the localized eigenmode problems. It is shown that the effect of the velocity shear drastically depends on the type of instability. Whereas the velocity shear can significantly suppress both Rayleigh-Taylor and interchange instabilities, it has only a week impact on the growth rate of the resistive drift wave. This is directly related to the physical nature of these instabilities. For the Rayleigh-Taylor and interchange instabilities, the shear flow tilts the eddies of the stream functions, while for the resistive drift wave instability the shear flow simply shifts the eddies in the radial direction with no tilting. However, for large velocity shear, the eigenmode solutions for resistive drift waves cease to exist.

physics.plasm-ph↗

arXiv2019

Electron dynamics in counter-propagating laser waves

The electron dynamics in counter-propagating laser waves is investigated by employing a novel approach, where the new Hamiltonian is time-independent when the perturbative laser wave is absent. The physical picture of stochastic electron dynamics is clearly revealed and the threshold values of the amplitude of the perturbative laser field for triggering stochastic electron acceleration are derived for different laser polarization directions and initial electron momentum. It demonstrates that the dephasing rate (new Hamiltonian) between the electron and the dominant laser can be randomly reduced if the amplitude of the perturbative laser is above the threshold such that the electron could be accelerated by the dominant laser well beyond the ponderomotive energy scaling. The impact of a superluminal phase velocity is examined, which slightly changes the stochastic region in Hamiltonian space if the superluminal phase velocity is under a threshold value but significantly decreases the maximum electron kinetic energy. All the analytic considerations are confirmed by the numerical simulations.

physics.plasm-ph↗

arXiv2019

Electron dynamics in laser and quasi-static transverse electric and longitudinal magnetic fields

By deriving the 3/2 dimensional Hamiltonian equations for electrons in the intense laser radiation and quasi-static transverse electric and longitudinal magnetic fields, the electron heating mechanisms are examined both for low harmonic resonance of electron frequency in the static fields with the laser frequency and for high harmonic resonances where the overlapping of broadened resonances causes the stochastic heating. For both cases, the maximum electron kinetic energies, well beyond the ponderomotive scaling, depend only on a small parameter combining the laser amplitude, electrostatic field strength and the conserved dephasing rate.

physics.plasm-ph↗

arXiv2019

A new approach to stochastic acceleration of electrons in colliding laser fields

The mechanism of stochastic electron acceleration in colliding laser waves is investigated by employing proper canonical variables and effective time, such that the new Hamiltonian becomes time independent when the perturbative (weaker) laser wave is absent. The performed analytical analysis clearly reveals the physical picture of stochastic electron dynamics. It shows that when the amplitude of the perturbative laser field exceeds some critical value, stochastic electron acceleration occurs within some electron energy range. The conditions, at which the maximum electron energy gained under stochastic acceleration is greatly exceeding the ponderomotive energy scaling based on the amplitude of the dominant laser, are derived.

physics.plasm-ph↗

arXiv2018

Stochastic electron heating in the laser and quasi-static electric and magnetic fields

The dynamics of relativistic electrons in the intense laser radiation and quasi-static electromagnetic fields both along and across to the laser propagating direction are studied in the 3/2 dimensional Hamiltonian framework. It is shown that the unperturbed oscillations of the relativistic electron in these electric fields could exhibit a long tail of harmonics which makes an onset of stochastic electron motion be a primary candidate for electron heating. The Poincaré mappings describing the electron motions in the laser and electric fields only are derived from which the criterions for instability are obtained. It follows that for both transverse and longitudinal electric fields, there exist upper limits of the stochastic electron energy depending on the laser intensity and electric field strength. Specifically, these maximum stochastic energies are enhanced by a strong laser intensity but weak electric field. Such stochastic heating would be reduced by the superluminal phase velocity in both cases. The impacts of the magnetic fields on the electron dynamics are different for these two cases and discussed qualitatively. These analytic results are confirmed by the numerical simulations of solving the 3/2D Hamiltonian equations directly.

physics.plasm-ph↗

arXiv2018

Radiation friction force effects on electron dynamics in ultra-intensity laser pulse

The electron dynamics in the ultra-high intensity laser pulse with radiation friction force in theLandau-Lifshitz form are studied. It is demonstrated that widely used approximation, where onlythe term dominating the dissipation of electron kinetic energy is retained in the expression for theradiation friction, is incorrect for the case of diverging electron trajectories. As a matter of fact, forlarge friction force effects, all components of the radiation friction force in the Landau-Lifshitz formhave the same order in the equation of electron motion, being equally important for both electrontrajectory and thus energy gain in the case of diverging electron trajectories (e.g. determined bythe superposition of few electromagnetic waves).

physics.plasm-ph↗

arXiv2018

Electron heating in the laser and static electric and magnetic fields

A 2D slab approximation of the interactions of electrons with intense linearly polarized laser radiation and static electric and magnetic fields is widely used for both numerical simulations and simplified semi-analytical models. It is shown that in this case electron dynamics can be conveniently described in the framework of 3/2 dimensional Hamiltonian approach. The electron acceleration beyond a standard ponderomotive scaling, caused by the synergistic effects of the laser and static electro-magnetic fields, is due to an onset of stochastic electron motion.

physics.plasm-ph↗