Alexey Arefiev

Magnetohydrodynamic scenario of plasma detachment in a magnetic nozzle

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 (MHD) 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-Alfvenic flow becomes super-Alfvenic; 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-Alfvenic 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...

Theoretical components of the VASIMR plasma propulsion concept

The ongoing development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) involves basic physics analysis of its three major components: helicon plasma source, ion cyclotron-resonance heating 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.

Nonlinear physics of laser-irradiated microclusters

A nonlinear theory has been developed to describe electron response and ion acceleration in dense clusters that are smaller in size than the laser wavelength. This work is motivated by high-intensity laser-cluster interaction experiments. The theory reveals that the breakdown of quasineutrality affects the cluster dynamics in a dramatic way: the laser can create a positively charged ion shell that expands due to its own space charge much faster than the central part of the cluster. The developed theory also shows a trend for the electron population to have a two-component distribution function: a cold core that responds to the laser field coherently and a hot halo that undergoes stochastic heating. The hot electrons expand together with the equal number of ions that are accelerated to supersonic velocities in a double layer at the cluster edge. This mechanism produces fast ions with energies much greater than the ponderomotive potential and it suggests that larger deuterium clusters can significantly enhance the neutron yield in future experiments.

Ambipolar acceleration of ions in a magnetic nozzle

This paper describes a magnetic nozzle with a magnetic mirror configuration that transforms a collisionless subsonic plasma flow into a supersonic jet expanding into the vacuum. The nozzle converts electron thermal energy into the ion kinetic energy via an ambipolar electric field. The ambipolar potential in the expanding plume involves a time-dependent rarefaction wave. Travelling through the rarefaction wave, electrons lose some kinetic energy and can become trapped downstream from the mirror throat. This work presents a rigorous adiabatic description of the trapped electron population. It examines the impact of the adiabatic cooling of the trapped electrons on the ambipolar potential and the ensuing ion acceleration. The problem is formulated for an arbitrary incoming electron distribution and then a “water-bag” electron distribution is used to obtain a closed-form analytical solution.

Compact tunable Compton x-ray source from laser-plasma accelerator and plasma mirror

We present an in-depth experimental-computational study of the parameters necessary to optimize a tunable, quasi-monoenergetic, efficient, low-background Compton backscattering (CBS) x-ray source that is based on the self-aligned combination of a laser-plasma accelerator (LPA) and a plasma mirror (PM). The main findings are (1) an LPA driven in the blowout regime by 30 TW, 30 fs laser pulses produce not only a high-quality, tunable, quasi-monoenergetic electron beam, but also a high-quality, relativistically intense (a0 ∼ 1) spent drive pulse that remains stable in profile and intensity over the LPA tuning range. (2) A thin plastic film near the gas jet exit retro-reflects the spent drive pulse efficiently into oncoming electrons to produce CBS x-rays without detectable bremsstrahlung background. Meanwhile, anomalous far-field divergence of the retro-reflected light demonstrates relativistic “denting” of the PM. Exploiting these optimized LPA and PM conditions, we demonstrate quasi-monoenergetic (50% FWHM...

Enhanced Multi-MeV Photon Emission by a Laser-Driven Electron Beam in a Self-Generated Magnetic Field

The rapid development of high brilliance X-ray radiation sources is revolutionizing physics, chemistry, and biology research through their novel applications. Another breakthrough is anticipated with the construction of next-generation laser facilities which will operate at intensities beyond 10 W/cm, leading to higher yield, shorter wavelength radiation sources. We use numerical simulations to demonstrate that a source of collimated multi-MeV photons with conversion efficiency comparable to the one expected for these facilities is achievable at an order of magnitude lower in intensity, within reach of the existing facilities. In the optimal setup, the laser pulse irradiates a bulk solid-density target, heating the target electrons and inducing relativistic transparency. As the pulse then propagates, it generates a beam of energetic electrons which in turn drives a strong azimuthal magnetic field. This field significantly enhances the radiation reaction for the electrons, yielding tens of TW of directed MeV photons for a PW-class laser.

Enhancement of laser-driven electron acceleration in an ion channel

A laser beam with duration longer than the period of plasma oscillations propagating through an underdense plasma produces a steady-state positively charged channel in the electron density. We consider a test electron in the two-dimensional plane channel under the combined action of the laser field and the transverse static electric field of the channel. At ultrarelativistic laser wave amplitude (a≫1), the electron is pushed primarily forward. As the electron gradually dephases from the wave, the field it samples and its relativistic γ-factor strongly oscillate. The natural frequency of electron oscillations across the channel (betatron frequency) depends on γ, which couples the betatron oscillations to the longitudinal motion induced by the wave. We show that the modulation of the natural frequency makes the oscillations unstable. The resulting amplification of the oscillations across the channel reduces the axial dephasing between the electron and the wave, leading to a considerable electron energy enha...

Magnetic nozzle and plasma detachment model for a steady-state flow

Plasma propulsion concepts that employ a guiding magnetic field raise the question of how the magnetically controlled plasma can detach from the spacecraft. This paper presents a detachment scenario relevant to high-power thrusters in which the plasma can stretch the magnetic field lines to infinity, similar to the solar wind. In previous work, the corresponding ideal magnetohydrodynamics equations have been solved analytically for a plasma flow in a slowly diverging nozzle. That solution indicates that efficient detachment is feasible if the nozzle is sufficiently long. In order to extend the previous model beyond the idealizations of analytical theory, a Lagrangian code is developed in this work to simulate steady-state kinetic plasma flows and to evaluate nozzle efficiency. The code is benchmarked against the analytical results and then used to examine situations that are not analytically tractable, including plasma behavior in the recent Detachment Demonstration Experiment at the National Aeronautics and Space Administration.

Harmonic generation in clusters

A model is presented for the nonlinear response of a small cluster, with a size much smaller than the wavelength, at the third harmonic of the laser frequency. The model involves collective modes of a cold electron core confined within a positively charged ion background. The response of the electron core to the laser field is similar to that of a weakly nonlinear oscillator driven by an external force. In particular, there is a resonant enhancement of the third harmonic when the frequency of the applied field is close to one third of the core eigenfrequency. It is shown that density nonuniformity or nonspherical shape of the ion background is necessary for harmonic generation. Particle-in-cell simulations have been performed to model the time evolution of the third harmonic response as the ion density profile changes due to cluster expansion. The simulation results are consistent with the predictions of the cold electron core model. In addition, the code quantifies the role of stochastic electron heating...

Single-pass ion cyclotron resonance absorption

The ion response to the rf-field during single-pass ion-cyclotron resonance heating (ICRH) can be essentially nonlinear. This paper presents a self-consistent theory of the rf-wave propagation and ion motion through the resonance. An important ingredient of the problem is the ion flow along the magnetic field. The flow velocity limits the time the ions spend at the resonance, which in turn limits the ion energy gain. A feature that makes the problem nonlinear is that the flow accelerates under the effect of the ∇B force and rf-pressure. This acceleration can produce a steep decrease in the plasma density at the resonance, resulting in partial reflection of the incident wave.