A new mechanism is reported that increases electron energy gain from a laser beam of ultrarelativistic intensity in underdense plasma. The increase occurs when the laser produces an ion channel that confines accelerated electrons. The frequency of electron oscillations across the channel is strongly modulated by the laser beam, which causes parametric amplification of the oscillations and enhances the electron energy gain. This mechanism has a threshold determined by a product of beam intensity and ion density.
It is shown that electrons with momenta exceeding the 'free electron' limit of meca 2 0 /2 can be produced when a laser pulse and a longitudinal electric field interact with an electron via a nonwakefield mechanism. The mechanism consists of two stages: the reduction of the electron dephasing rate γ−px/mec by an accelerating region of electric field and electron acceleration by the laser via the Lorentz force. This mechanism can, in principle, produce electrons that have longtudinal momenta that is a significant multiple of meca 2 0 /2. 2D PIC simulations of a relatively simple laser-plasma interaction indicate that the generation of super-ponderomotive electrons is strongly affected by this 'anti-dephasing' mechanism.
Some plasma propulsion concepts rely on a strong magnetic field to guide the plasma flow through the thruster nozzle. The question then arises of how the magnetically confined plasma can detach from the spacecraft. This work presents a magnetohydrodynamic ͑MHD͒ detachment scenario in which the plasma flow stretches the magnetic field lines to infinity. Detachment takes place after the energy density of the expanding magnetic field drops below the kinetic energy density of the plasma. As plasma flows along the magnetic field lines, the originally sub-Alfvénic flow becomes super-Alfvénic; this transition is similar to what occurs in the solar wind. In order to describe the detachment quantitatively, the ideal MHD equations have been solved for a cold plasma flow in a slowly diverging nozzle. The solution exhibits a well-behaved transition from sub-to super-Alfvénic flow inside the nozzle and a rarefaction wave at the edge of the outgoing flow. It is shown that efficient detachment is feasible if the nozzle is sufficiently long.
We examine a regime in which a linearly-polarized laser pulse with relativistic intensity irradiates a subcritical plasma for much longer than the characteristic electron response time. A steady-state channel is formed in the plasma in this case with quasi-static transverse and longitudinal electric fields. These relatively weak fields significantly alter the electron dynamics. The longitudinal electric field reduces the longitudinal dephasing between the electron and the wave, leading to an enhancement of the electron energy gain from the pulse. The energy gain in this regime is ultimately limited by the superluminosity of the wave fronts induced by the plasma in the channel. The transverse electric field alters the oscillations of the transverse electron velocity, allowing it to remain anti-parallel to laser electric field and leading to a significant energy gain. The energy enhancement is accompanied by development of significant oscillations perpendicular to the plane of the driven motion, making trajectories of energetic electrons three-dimensional. Proper electron injection into the laser beam can further boost the electron energy gain.
Helicon discharges produce plasmas with a density gradient across the confining magnetic field. Such plasmas can create a radial potential well for nonaxisymmetric whistlers, allowing radially localized helicon ͑RLH͒ waves. This work presents new evidence that RLH waves play a significant role in helicon plasma sources. An experimentally measured plasma density profile in an argon helicon discharge is used to calculate the rf field structure. The calculations are performed using a two-dimensional field solver under the assumption that the density profile is axisymmetric. It is found that RLH waves with an azimuthal wave number m = 1 form a standing wave structure in the axial direction and that the frequency of the RLH eigenmode is close to the driving frequency of the rf antenna. The calculated resonant power absorption, associated with the RLH eigenmode, accounts for most of the rf power deposited into the plasma in the experiment.
The ongoing development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) involves basic physics analysis of its three major components: helicon plasma source, ion cyclotronresonance heating (ICRH) module, and magnetic nozzle. This paper presents an overview of recent theoretical efforts associated with the project. It includes: 1) a first-principle model for helicon plasma source, 2) a nonlinear theory for the deposition of rf-power at the ion cyclotron frequency into plasma flow, and 3) a discussion of the plasma detachment mechanism relevant to VASIMR.
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