Fundamental Plasma Physics最新文献

Despite the undoubted importance of having fairly simple analytical expressions for the refractive indices of wave modes in a magnetoactive plasma, such expressions are known only in some particular cases. For electron waves with frequencies much higher than the lower hybrid resonance frequency, such an expression is known only for whistler waves in a dense plasma when the electron plasma frequency significantly exceeds the electron cyclotron frequency. In this Letter, we propose simple operational expressions for the refractive indices of all four electron modes in a magnetoactive plasma, namely, the fast magnetosonic, also called whistler mode, the slow extraordinary mode, the ordinary mode, and the fast extraordinary mode. The form of these expressions does not depend on the value of the ratio of plasma frequency to cyclotron frequency.

Although an accurate description of wave propagation and absorption in plasmas requires complicated full-wave solutions or kinetic simulations, local dispersion analysis can still be helpful to capture the main physics of wave properties. Plasma wave propagation conditions or accessibility informs whether a wave can propagate to a region, which usually depends on the wave frequency, wave vector, the local plasma density, and magnetic field. We demonstrate a warm multi-fluid eigenvalue model and a matrix approach to rapidly calculate plasma wave accessibility diagrams, where thermal effects are also included via an isotropic pressure term. All cold plasma waves, from high-frequency electron cyclotron waves, intermediate-frequency lower hybrid waves, to low-frequency ion cyclotron waves, are presented. By comparing with the kinetic model, it is interesting to find that the warm multi-fluid model, though incapable of reproducing the Bernstein modes, can provide a quick way to determine whether thermal effects are important.

The steady-state confinement, beta limit, and divertor heat load are among the most concerned issues for toroidal confinement of fusion plasmas. In this work, we show that the negative triangularity tokamak has promising prospects to address these issues. We first demonstrate that the negative triangularity tokamak generates the filed line rotation transform more effectively. This brings bright prospects for the advanced steady-state tokamak scenario. Given this, the MHD stability and equilibrium confinement of negative triangularity tokamak are investigated. We point out that the negative triangularity configuration with a broad pressure profile is indeed more unstable for low-$n$ magnetohydrodynamic modes than the positive triangularity case so that the H-mode confinement can hardly be achieved in this configuration, where $n$ is the toroidal mode number. Nevertheless, we found that the negative triangularity configuration with high bootstrap current fraction, high poloidal beta, and peaked pressure profiles can achieve higher normalized beta for low-$n$ modes than the positive triangularity case. In a certain parameter domain, the normalized beta can reach about twice the extended Troyon limit, while the same computation indicates that the positive triangularity configuration is indeed constrained by the Troyon limit. This shows that the negative triangularity tokamaks are not only favorable for divertor design to avoid the edge localized modes but also can have promising prospects for advanced steady-state confinement of fusion plasmas in high beta.

This study assesses the plasma sheath formation on the night side of the Moon when exposed to highly energetic ambient plasma. The calculations indicate that the secondary electron emission (SEE) due to highly energetic plasma electrons leads to the formation of the inverse sheath around the positively charged lunar surface on the night side, where a traditional Debye sheath with a high negative surface potential is anticipated. Analytical formulation of Debye sheath and inverse sheath formation is given considering Maxwellian plasma and secondary electrons and cold ions. For a given SEE yield, a temperature regime is predicted where the inverse sheath is possible.

We show how some different fundamental plasma processes - the ideal kink instability, magnetic reconnection and magnetohydrodynamic oscillations - can be causally linked. This is shown through reviewing a series of models of energy release in twisted magnetic flux ropes in the solar corona, representing confined solar flares. 3D magnetohydrodynamic simulations demonstrate that fragmented current sheets develop during the nonlinear phase of the ideal kink instability, leading to multiple magnetic reconnections and the release of stored magnetic energy. By coupling these simulations with a test particle code, we can predict the development of populations of non-thermal electrons and ions, as observed in solar flares, and produce synthetic observables for comparison with observations. We also show that magnetic oscillations arise in the reconnecting loop, although there is no oscillatory external driver, and these lead to pulsations in the microwave emission similar to observed flare quasi-periodic pulsations. Oscillations and propagating waves also arise from reconnection when two twisted flux ropes merge, which is modelled utilising 2D magnetohydrodynamic simulations.

A comprehensive overview is presented of recent theoretical advancements and observational manifestations of a relatively new type of electrostatic solitary wave (ESW), known as supersoliton or supersolitary wave (SSW). These nonlinear structures are characterized by a distorted pulse-shaped electrostatic potential excitations, deviating from the standard (“sech²”-like) form generally expected from solitonic pulses. In Space plasmas, in particular, e.g. in magnetospheric observations, SSWs may be associated with a characteristic wiggly bipolar electric field waveform. It has been shown that a three-component configuration is essential, as a minimum requirement for SSWs to occur in a plasma.

Various spacecraft missions have recorded evidence of “non-conventional” electrostatic solitary waves (pulses) such as wiggly bipolar pulses, offset bipolar pulses, and monopolar pairs. This review article aims to present the current state of the art in this fascinating new theme, first outlining the basic framework for the modeling of such “exotic” ESWs and then putting forward a correlation between SSW structures with certain non-standard bipolar electric field forms observed in planetary magnetospheres.

Superior lifetime stability for the microplasma device developed by decorating nanodiamonds (nDs) on laser induced graphene (LIG) is reported. Upon overwhelming the difficulty of poor stability in graphene, the nD-LIG displays exceptional lifetime stability of 1770s verified at an applied voltage of 340 V. But, the lifetime stability of LIG is only 718 s at the same applied voltage. Therefore, the nD-LIG with enhanced lifetime stability have pronounced prospective as cathodes in microplasma device applications.

We report on the first implementation of a miniature laser-driven shock tube (LDST) of 5 × 5 mm cross section and 50-mm length for generating and studying strong shock waves (SW) and hypersonic gas flows with M > 10. Operation of the LDST is based on the acceleration of a thin CH-film by ablative plasma pressure produced when the film is irradiated by high-energy UV pulse of the GARPUN KrF laser (100 J & 100-ns). The film serves as a piston that pushes a SW in the gas filling the LDST. An optical system based on a multi-element prism raster provides focusing of KrF laser beam into 7 × 7 mm square spot with 100 J/cm² energy fluence (1 GW/cm² intensity) with inhomogeneity ∼3 % across the LDST aperture. It is expected that the LDST with KrF laser driver can be an effective tool for studying hydrodynamic phenomena, such as hydrodynamic instabilities and transition to a turbulence, hypersonic gas flow around bodies, reflection and cumulation of strong SW.

We show that the wakefield driven by fast electrons inside the nanowire when irradiated with an ultra-short relativistic laser pulse strips atoms to a higher charge state. Using particle-in-cell simulations, we demonstrate that the charge state agrees with the barrier suppression threshold of the wakefield and reaches a higher value via collision. The ionisation of gold nanowires occurs only via collisional-damped wakefield. We found that the collisional ionisation of high-Z nanowires depends on the onset of the z pinch. These results suggest a different ionisation mechanism of the structured target in the subfemtosecond regime.

We consider a visco-resistive magnetohydrodynamic modelling of a steady-state incompressible tokamak plasma with a prescribed toroidal current drive, featuring constant resistivity η and viscosity ν. It is shown that the plasma velocity root-mean-square behaves as $\eta f\left(H\right)$ as long as the inertial term remains negligible, where H stands for the Hartmann number $H\equiv {\left(\eta \nu \right)}^{-1/2}$, and that $f\left(H\right)$ exhibits power-law behaviours in the limits $H\ll 1$ and $H\gg 1$. In the latter limit, we establish that $f\left(H\right)$ scales as $H^{1/4}$, which is consistent with numerical results.