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Register a new userEffects of beam velocity and density on an ion-beam pulse moving in magnetized plasmas
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The wakefield and stopping power of an ion-beam pulse moving in magnetized plasmas are investigated by particle-in-cell (PIC) simulations. The effects of beam velocity and density on the wake and stopping power are discussed. In the presence of magnetic field, it is found that beside the longitudinal conversed V-shaped wakes, the strong whistler wave are observed when low-density and low-velocity pulses moving in plasmas. The corresponding stopping powers are enhanced due to the drag of these whistler waves. As beam velocities increase, the whistler waves disappear, and only are conversed V-shape wakes observed. The corresponding stopping powers are reduced compared with these in isotropic plasmas. When high-density pulses transport in the magnetized plasmas, the whistler waves are greatly inhibited for low-velocity pulses and disappear for high-velocity pulses. Additionally, the magnetic field reduces the stopping powers for all high-density cases.
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We analyze how the turbulent transport of $mathbf{E}times mathbf{B}$ type in magnetically confined plasmas is affected by intermittent features of turbulence. The latter are captured by the non-Gaussian distribution $P(phi)$ of the turbulent electric potential $phi$. Our analysis is performed at an analytical level and confirmed numerically using two statistical approaches. We have found that the diffusion is inhibited linearly by intermittency, mainly via the kurtosis of the distribution $P(phi)$. The associated susceptibility for this linear process is shown to be dependent on the poloidal velocity $V_p$ and on the correlation time $tau_c$ with a maxima at the time-of-flight $tau_{fl}$. Intermittency does not affect the Kubo number scaling in the strong regime.
Compound ion distributions, fi(v), have been measured by NASAs Magnetospheric Multi-Scale Mission (MMS) and have been found in reconnection simulations. A complex distribution, fi(v), consisting, for example, of essentially disjoint pieces will be called a multi-beam distribution and modeled as a sum of beams, fi(v) = f1(v) + ... +fN(v). Velocity moments of fi(v) are taken beam by beam and summed. Such multi-beam moments of fi(v) have advantages over the customary standard velocity moments of fi(v), forwhich there is only one mean flow velocity. For example, the standard thermal energy momentof a pair of equal and opposite cold particle beams is non-zero even though each beam has zero thermal energy. We therefore call this thermal energy pseudo-thermal. By contrast, a multi-beam moment of two or more beams has no pseudo-thermal energy. We develop three different ways of decomposing into a sum and finding multi-beam moments for both a multi-beam fi(v) measured by MMS in the dayside magnetosphere during reconnection and a multi-beam fi(v) found in a PIC simulation of magnetotail reconnection. The three methods are: A visual method in which the velocity centroid of each beam is estimated and its density determined self-consistently; A k-means method in which particles in a particle-representation of fi(v) are sorted into a minimum energy configuration of N (= k) clusters; A nonlinear least squares method based on a fit to a sum of N kappa functions. Multi-beam energy moments are calculated and compared with standard moments for the thermal energy density, pressure tensor, thermal energy flux (heat plus enthalpy fluxes), bulk kinetic energy density, RAM pressure and bulk kinetic energy flux. Applying this new formalism to real data demonstrates in detail how multi-beam techniques may provide significant insight into the properties of observed space plasmas.
The friction force on a test particle traveling through a plasma that is both strongly coupled and strongly magnetized is studied using molecular dynamics simulations. In addition to the usual stopping power component aligned antiparallel to the velocity, a transverse component that is perpendicular to both the velocity and Lorentz force is observed. This component, which was recently discovered in weakly coupled plasmas, is found to increase in both absolute and relative magnitude in the strongly coupled regime. Strong coupling is also observed to induce a third component of the friction force in the direction of the Lorentz force. These first-principles simulations reveal novel physics associated with collisions in strongly coupled, strongly magnetized, plasmas that are not predicted by existing kinetic theories. The effect is expected to influence macroscopic transport in a number of laboratory experiments and astrophysical plasmas.
Plasma turbulence is studied via direct numerical simulations in a two-dimensional spatial geometry. Using a hybrid Vlasov-Maxwell model, we investigate the possibility of a velocity-space cascade. A novel theory of space plasma turbulence has been recently proposed by Servidio {it et al.} [PRL, {bf 119}, 205101 (2017)], supported by a three-dimensional Hermite decomposition applied to spacecraft measurements, showing that velocity space fluctuations of the ion velocity distribution follow a broad-band, power-law Hermite spectrum $P(m)$, where $m$ is the Hermite index. We numerically explore these mechanisms in a more magnetized regime. We find that (1) the plasma reveals spectral anisotropy in velocity space, due to the presence of an external magnetic field (analogous to spatial anisotropy of fluid and plasma turbulence); (2) the distribution of energy follows the prediction $P(m)sim m^{-2}$, proposed in the above theoretical-observational work; and (3) the velocity-space activity is intermittent in space, being enhanced close to coherent structures such as the reconnecting current sheets produced by turbulence. These results may be relevant to the nonlinear dynamics weakly-collisional plasma in a wide variety of circumstances.
A model of an electron-beam-plasma system is introduced to model the electrical breakdown physics of low-pressure nitrogen irradiated by an intense pulsed electron beam. The rapidly rising beam current induces an electric field which drives a return current in the plasma. The rigid-beam model is a reduction of the problem geometry to cylindrical coordinates and simplifications to Maxwells equations that are driven by a prescribed electron beam current density. The model is convenient for comparing various reductions of the plasma dynamics and plasma chemistry while maintaining a good approximation to the overall magnitude of the beam-created electric field. The usefulness of this model is demonstrated by coupling the rigid-beam model to a fluid plasma model and a simplified nitrogen plasma chemistry. The dynamics of this coupled system are computed for a range of background gas pressures, and the results are compared with experimental measurements. At pressures 1 Torr and above, the simulated line-integrated electron densities are within a factor of two of measurements, and show the same trend with pressure as observed in experiment.
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