Ambipolar acceleration of ions in a magnetic nozzle
A.V. Arefiev, B.N. Breizman
Abstract
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. ©2008 American Institute of Physics
DOI:10.1063/1.2907786
Magnetic nozzle and plasma detachment model for a steady-state flow
B.N. Breizman, M.R. Tushentsov, A.V. Arefiev
Abstract
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. ©2008 American Institute of Physics
DOI:10.1063/1.2903844
"Maximum" entropy production in self-organized plasma boundary layer: A thermodynamic discussion about turbulent heat transport
Z. Yoshida, S.M. Mahajan
Abstract
A thermodynamic model of a plasma boundary layer, characterized by enhanced temperature contrasts and “maximum entropy production,” is proposed. The system shows bifurcation if the heat flux entering through the inner boundary exceeds a critical value. The state with a larger temperature contrast (larger entropy production) sustains a self-organized flow. An inverse cascade of energy is proposed as the underlying physical mechanism for the realization of such a heat engine. ©2008 American Institute of Physics
DOI: 10.1063/1.2890189
Studies of laser wakefield structures and electron acceleration in underdense plasmas
A. Maksimchuk, S. Reed, S. S. Bulanov, V. Chvykov, G. Kalintchenko, T. Matsuoka, C. McGuffey, G. Mourou, N. Naumova, J. Nees, P. Rousseau, V. Yanovsky, K. Krushelnick, N. H. Matlis, S. Kalmykov, G. Shvets, M. C. Downer, C. R. Vane, J. R. Beene, D. Stracener, D. R. Schultz
Abstract
Experiments on electron acceleration and optical diagnostics of laser wakes were performed on the HERCULES facility in a wide range of laser and plasma parameters. Using frequency domain holography we demonstrated single shot visualization of individual plasma waves, produced by 40 TW, 30 fs laser pulses focused to the intensity of 1019 W/cm2 onto a supersonic He gas jet with plasma densities ne<1019 cm−3. These holographic “snapshots” capture the variation in shape of the plasma wave with distance behind the driver, and resolve wave front curvature seen previously only in simulations. High-energy quasimonoenergetic electron beams were generated using plasma density in the range 1.5×1019≤ne≤3.5×1019 cm−3. These experiments demonstrated that the energy, charge, divergence, and pointing stability of the beam can be controlled by changing ne, and that higher electron energies and more stable beams are produced for lower densities. An optimized quasimonoenergetic beam of over 300 MeV and 10 mrad angular divergence is demonstrated at a plasma density of ne≅1.5×1019 cm−3. The resultant relativistic electron beams have been used to perform photo-fission of 238U with a record high reaction yields of ~3×105/J. The results of initial experiments on electron acceleration at 70 TW are discussed. ©2008 American Institute of Physics
DOI:10.1063/1.2856373
Classical Perfect Diamagnetism: Expulsion of Current from the Plasma Interior
S.M. Mahajan
Abstract
The vanishing of generalized helicity is shown to be the necessary and sufficient condition for a perfect conductor to display perfect diamagnetism, considered to be the defining attribute of a conventional superconductor. Although conventional superconductivity is brought about by quantum correlations in classical systems, prepared in the state of zero initial helicity (helicity is a constant of the motion for a perfect conductor), it can mimic the superconductor's behavior. ©2008 The American Physical Society
DOI:10.1103/PhysRevLett.100.075001
Manipulating Electromagnetic Waves in Magnetized Plasmas: Compression, Frequency Shifting, and Release
Yoav Avitzour, G Shvets
Abstract
A new approach to manipulating the duration and frequency of microwave pulses using magnetized plasmas is demonstrated. The plasma accomplishes two functions: (i) slowing down and spatially compressing the incident wave, and (ii) modifying the propagation properties (group velocity and frequency) of the wave in the plasma during a uniform in space adiabatic in time variation of the magnitude and/or direction of the magnetic field. The increase in the group velocity results in the shortening of the temporal pulse duration. Depending on the plasma parameters, the frequency of the outgoing compressed pulse can either change or remain unchanged. Such dynamic manipulation of radiation in plasma opens new avenues for manipulating high power microwave pulses. ©2008 The American Physical Society
DOI:10.1103?PhysRevLett.100.065006
Metamaterials add an extra dimension
G. Shvets
Abstract
In a major departure from their humble origins as ultrathin monolayers, optical metamaterials have now advanced to three-dimensional bulk media exhibiting both electric and magnetic activity. © 2008 Nature Publishing Group
DOI:10.1038/nmat2088
The complex Bloch bands of a 2D plasmonic crystal displaying isotropic negative refraction
M Davanco, Y Urzhumov, G Shvets
Abstract
The propagation characteristics of a subwavelength plasmonic crystal are studied based on its complex Bloch band structure. Photonic crystal bands are generated with an alternative 2D Finite Element Method formulation in which the Bloch wave problem is reduced to a quadratic eigenvalue system for the Bloch wavevector amplitude k. This method constitutes an efficient and convenient alternative to nonlinear search methods normally employed in the calculation of photonic bands when dispersive materials are involved. The method yields complex wavevector Bloch modes that determine the wave-scattering characteristics of finite crystals. This is evidenced in a comparison between the band structure of the square-lattice plasmonic crystal and scattering transfer-functions from a corresponding finite crystal slab. We report on a wave interference effect that leads to transmission resonances similar to Fano resonances, as well as on the isotropy of the crystal’s negative index band. Our results indicate that effective propagation constants obtained from scattering simulations may not always be directly related to individual crystal Bloch bands. © 2007 Optical Society of America
Spectral Gap of Shear Alfvén Waves in a Periodic array of Magnetic Mirrors
Yang Zhang, W.W. Heidbrink, H. Boehmer, R. McWilliams, Guangye Chen, B.N. Breizman, S. Vincena, T. Carter, D. Leneman, W. Gekelman, P. Pribyl, B. Brugman
Abstract
A multiple magnetic mirror array is formed at the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] to study axial periodicity-influenced Alfvén spectra. Shear Alfvén waves (SAW) are launched by antennas inserted in the LAPD plasma and diagnosed by B-dot probes at many axial locations. Alfvén wave spectral gaps and continua are formed similar to wave propagation in other periodic media due to the Bragg effect. The measured width of the propagation gap increases with the modulation amplitude as predicted by the solutions to Mathieu's equation. A two-dimensional finite-difference code modeling SAW in a mirror array configuration shows similar spectral features. Machine end-reflection conditions and damping mechanisms including electron-ion Coulomb collision and electron Landau damping are important for simulation. © 2008 American Institute of Physics
DOI: 10.1063/1.2827518