Experimental observation of the electron beam focusing effect induced by plasma currents with opposite directions.

We report on the experimental observation of the focusing effect of a 50MeV accelerator electron beam in a gas-discharge plasma target. The plasma is generated by igniting an electric discharge in two collinear quartz tubes, with the currents up to 1.5kA flowing in opposite directions in either of the two tubes. In such plasma current configuration, the electron beam is defocused in the first discharge tube and focused with a stronger force in the second one. With symmetric plasma currents, asymmetric effects are, however, induced on the beam transport process and the beam radius is reduced by a factor of 2.6 compared to the case of plasma discharge off. Experimental results are supported by two-dimensional particle-in-cell simulations.

Permeability enhancement of the KcsA channel under radiation of a terahertz wave.

Potassium ion channels are essential elements in cellular electrical excitability and help maintain a resting potential in nonexcitable cells. Their universality is based on a unique combination of strong selectivity for K^{+} ions and near-diffusion-limited permeation efficiency. Understanding how the channel regulates the ion conduction would be instructive to the treatment of ion channelopathies. In this work, by means of molecular dynamics simulations, we demonstrate the significantly enhanced permeation of KcsA channel in reaction to an external terahertz wave, due to the effective response of the K^{+} ions in the selectivity filter regions of the channel. Compared to the case without external terahertz wave, a fourfold increase in the ion current through the channel is found.

Collective energy-spectrum broadening of a proton beam in a gas-discharge plasma.

An accurate understanding of ion-beam transport in plasmas is crucial for applications in inertial fusion energy and high-energy-density physics. We present an experimental measurement on the energy spectrum of a proton beam at 270 keV propagating through a gas-discharge hydrogen plasma. We observe the energies of the beam protons changing as a function of the plasma density and spectrum broadening due to a collective beam-plasma interaction. Supported by linear theory and three-dimensional particle-in-cell simulations, we attribute this energy modulation to a two-stream instability excitation and further saturation by beam ion trapping in the wave. The widths of the energy spectrum from both experiment and simulation agree with the theory.

Enhanced collective stopping and drift of electron beams in fusion plasmas with heavy-ion species.

The transport and energy deposition of relativistic electron beams in transversely nonuniform plasmas are investigated with two-dimensional electromagnetic particle-in-cell simulations. For the beam with radius much larger than plasma skin depth, the current filamentation instability excited by the electron beam can be observed, which breaks the beam into filaments and leads to the formation of strong magnetic fields consequently. The effects of plasma ion species are significant and asymmetric transverse magnetic fields are formed in plasmas with heavy-ion species due to the asymmetric neutralization of beam space charge by plasma ions. The asymmetric transverse magnetic fields contribute to the directional drift of beam electrons to lower plasma density regions, which may accelerate the filaments merger process and lead to highly localized beam-energy deposition in plasmas.

Dynamic polarization and energy dissipation for charged particles moving in magnetized two-component plasmas.

Energy losses of test particles in magnetized two-component plasmas are investigated within the framework of the linearized Vlasov-Poisson theory, taking into account the dynamic polarization effects of both the plasma ions and electrons. General expressions of the potential and stopping power are obtained and calculations are performed for protons in a magnetized hydrogen plasma. The influences of the magnetic field, the angle between the proton velocity and magnetic field, and certain plasma parameters on the stopping power are studied. Numerical results show that for low particle velocities and strong magnetic field the dynamic polarization effects of the plasma ions become obvious and contribute mainly to the stopping power.

Time evolution and energy deposition for ion clusters injected into magnetized two-component plasmas.

A two-dimensional particle-in-cell simulation model is proposed to study the time evolution and energy deposition for ion clusters injected into magnetized two-component plasmas. The injection of an isolated ion cluster is studied in the case of weak and strong magnetic fields. For strong magnetic fields, the ions tend to deposit their energy smoothly along the trajectory of the cluster, due to the confinement by the strong magnetic fields. However, in the case of weak magnetic fields, a large amount of energy is deposited by the ions near the initial cluster injection position, where the cluster density is expected to be largest. We attribute these to the influences of interference effects between the cluster ions, which have close relations to the distances between the ions. Furthermore, the influences of various magnetic fields, injection angles, and injection velocities on the time evolution and energy deposition of a beam pulse, which contains several similar ion clusters, are investigated in detail. The influences of different magnetic fields on the beam pulse show similar to that of a single ion cluster. For increasing injection angles, the beam velocity perpendicular to the magnetic field increases, leading to increasing oscillations in the beam trajectory and energy deposition profile. Besides, the amount of energy that transferred from the beam pulse to the plasma increases as the beam injection velocity approaches the electron thermal velocity.

Wake effect and stopping power for a charged ion moving in magnetized two-component plasmas: two-dimensional particle-in-cell simulation.

A two-dimensional particle-in-cell (PIC) model is proposed to study the wake field and stopping power induced by a nonrelativistic charged particle moving perpendicular to the external magnetic field in two-component plasmas. The effects of the magnetic field on the wake potential and the stopping due to the polarization of both the plasma ions and electrons are discussed. The velocity fields of plasma ions and electrons are investigated, respectively, in the weak and strong magnetic field cases. Our simulation results show that in the case of weak magnetic field and high ion velocity, the wakes exhibit typical V-shaped cone structures and the opening cone angles decrease with the increasing ion velocity. As the magnetic field becomes strong, the wakes lose their typical V-shaped structures and become highly asymmetrical. Similar results can be obtained in the case of low ion velocity and strong magnetic field. In addition, stopping power is calculated and compared with previous one-dimensional and full three-dimensional PIC results.