Strong Magnetic Plasma Research Articles

Laser Amplification in Strongly Magnetized Plasma.

We consider backscattering of laser pulses in strongly magnetized plasma mediated by kinetic magnetohydrodynamic waves. Magnetized low-frequency (MLF) scattering, which can occur when the external magnetic field is neither perpendicular nor parallel to the laser propagation direction, provides an instability growth rate higher than Raman scattering and a frequency downshift comparable to Brillouin scattering. In addition to the high growth rate, which allows smaller plasmas, and the 0.1%-2% frequency downshift, which permits a wide range of pump sources, MLF scattering is an ideal candidate for amplification because the process supports an exceptionally large bandwidth, which particle-in-cell simulations show produces ultrashort durations. Under some conditions, MLF scattering also becomes the dominant spontaneous backscatter instability, with implications for magnetized laser-confinement experiments.

HPIC: A scalable electrostatic Particle-in-Cell for Plasma–Material Interactions

The hPIC code is a fully-kinetic electrostatic Particle-in-Cell application targeted at large-scale simulations of Plasma–Material Interactions. The code can simulate multi-component strongly-magnetized plasmas in a region close to the wall, including the magnetic sheath and presheath, plus the first surface layers which release material impurities. The Poisson solver is based on PETSc conjugate gradient with BoomerAMG algebraic multigrid preconditioners. Scaling tests on the Blue Waters supercomputer have demonstrated excellent weak-scaling (tested up to 65,536 cores) and good strong-scaling up to 262,144 cores. In this paper, we will make an overview of the main features of the code and of the scaling performance as measured on the Blue Waters supercomputer at Illinois.

Asymptotically Preserving Particle-in-Cell Methods for Inhomogeneous Strongly Magnetized Plasmas

We propose a class of Particle-In-Cell (PIC) methods for the Vlasov-Poisson system with a strong and inhomogeneous external magnetic field with fixed direction, where we focus on the motion of particles in the plane orthogonal to the magnetic field (so-called poloidal directions). In this regime, the time step can be subject to stability constraints related to the smallness of Larmor radius and plasma frequency. To avoid this limitation, our approach is based on first and higher-order semi-implicit numerical schemes already validated on dissipative systems [3] and for homogeneous magnetic fields [10]. Thus, when the magnitude of the external magnetic field becomes large, this method provides a consistent PIC discretization of the guiding-center system taking into account variations of the magnetic field. We carry out some theoretical proofs and perform several numerical experiments that establish a solid validation of the method and its underlying concepts.

Influence of magnetic-field-caused modifications of trajectories of plasma electrons on spectral line shapes: Applications to magnetic fusion and white dwarfs

A general framework for calculating shapes of hydrogen (or deuterium) spectral lines in strongly-magnetized plasmas with the allowance for spiraling trajectories of perturbing electrons has been developed. It has been shown that in this situation the first order term Φ(1)(B) of the Dyson expansion of the electron broadening operator does not vanish – in distinction to the case of rectilinear trajectories, where the first non-vanishing term appeared only in the second order. An example of the Lyα line has been used to illustrate the effects of the spiraling trajectories. It has been shown that the shape of each of the two σ-components can become a doublet: in addition to the shifted component, there can appear also an unshifted component. Moreover, the shape of each of the two σ-components can also become a triplet: in addition to the shifted and unshifted component, there can appear also a component shifted to the opposite wing of the line. Both the positions and the intensities of the shifted components depend strongly on the magnitude of Φ(1)(B). The primary effect in the entire spectral line is a significant increase of the ratio of the intensity of the central peak to the intensity of any of the two lateral peaks.As practical examples, magnetic fusion plasmas and plasmas of DA white dwarfs have been considered. It is shown that for both of these, physically very different objects, the allowance for the spiraling trajectories of perturbing electrons can be important.

Radiative decay of keV-mass sterile neutrinos in a strongly magnetized plasma

The radiative decay of sterile neutrinos with typical masses of 10 keV is investigated in the presence of a strong magnetic field and degenerate plasma. Full account is taken of the strongly modified photon dispersion relation relative to vacuum. The limiting cases of relativistic and non-relativistic plasma are analyzed. The decay rate in a strongly magnetized plasma as a function of the electron number density is compared with the un-magnetized case. We find that a strong magnetic field suppresses the catalyzing influence of the plasma on the decay rate.

Kinetic equilibria of relativistic collisionless plasmas in the presence of non-stationary electromagnetic fields

The kinetic description of relativistic plasmas in the presence of time-varying and spatially non-uniform electromagnetic (EM) fields is a fundamental theoretical issue both in astrophysics and plasma physics. This refers, in particular, to the treatment of collisionless and strongly-magnetized plasmas in the presence of intense radiation sources. In this paper, the problem is investigated in the framework of a covariant gyrokinetic treatment for Vlasov–Maxwell equilibria. The existence of a new class of kinetic equilibria is pointed out, which occur for spatially-symmetric systems. These equilibria are shown to exist in the presence of non-uniform background EM fields and curved space-time. In the non-relativistic limit, this feature permits the determination of kinetic equilibria even for plasmas in which particle energy is not conserved due to the occurrence of explicitly time-dependent EM fields. Finally, absolute stability criteria are established which apply in the case of infinitesimal symmetric perturbations that can be either externally or internally produced.

Detailed Atmosphere Modeling for the Neutron Star 1E1207.4−5209: Evidence of Oxygen/Neon Atmosphere

We present a comprehensive investigation of the two broad absorption features observed in the X-ray spectrum of the neutron star 1E1207.4-5209 based on a recent analysis of the 260 ksec XMM-Newton data by Mori et al. 2005. Expanding on our earlier work (Hailey & Mori 2002) we have examined all previously proposed atmospheric models for 1E1207.4-5209. Using our atomic code, which rapidly solves Schrodinger's equation for arbitrary ion in strong magnetic field (Mori & Hailey 2002), we have systematically constructed atmospheric models by calculating polarization-dependent LTE opacities and addressed all the physics relevant to strongly-magnetized plasmas. We have been able to rule out virtually all atmospheric models because they either do not sustain an ionization balance consistent with the claimed atmosphere composition or because they predict line strengths and line widths which are inconsistent with the data. Only Oxygen or Neon atmospheres at B~10^{12} G provide self-consistent atmospheric solutions of appropriate ionization balance and with line widths, strengths and energies consistent with the observations. The observed features are likely composed of several bound-bound transition lines from highly-ionized oxygen/neon and they are broadened primarily by motional Stark effects and magnetic field variation over the line-emitting region. Further considerations of plausible mechanisms for the formation of a mid-Z atmosphere likely rule out Neon atmospheres, and have important implications for the fallback mechanism in supernova ejecta. Future high resolution spectroscopy missions such as Constellation-X will be able to resolve predicted substructure in the absorption features and will measure magnetic field strength and gravitational redshift independently to better than 10% accuracy.

3D‐solitons in a strongly magnetized plasma: Transition from flat to spherical waves of the Zakharov‐Kuznetsov equation

AbstractThe Zakharov‐Kuznetsov equation has been used to describe ion‐acoustic wave propagation in a strong magnetic plasma. An initial‐value problem has been solved for this equation on the basis of a numerical method that uses the fast Fourier transform technique for calculating space derivatives and a fourth order Runge‐Kutta method for the time scheme. Numerical simulations have shown that the disturbed flat solitary waves can break up into spherical ones.

Quantum relativistic theory of the cyclotron radiation in strongly-magnetized plasmas: Fine structure of the spectra and maser effect

Full quantum relativistic treatment of the cyclotron/synchrotron emission and absorption in tenuous plasma with superstrong magnetic fields is developed for the case when the radiation modes are linearly polarized. Spectra of emission, absorption and polarization are investigated both analytically and numerically for the thermal distribution of the radiating particles and for distribution with anisotropic temperature. Quantum relativistic effects lead to a fine structure of the cyclotron harmonics with typical spacing (atB ≪Bc = 4.41 × 1013 G) ∼vΩB(B/Bc), where ν=1,2, ... is the harmonic number, and Ωc is the cyclotron frequency. The fine structure is the most developed for subrelativistic temperatures and magnetic fields at not too small angles\(\vartheta \) between the field and the wave-vector. For essentially non-relativistic temperatures the fine structure can be observed in a wide angle range,\(\cos ^2 \vartheta \gg T_\parallel /mc^2 \), if\(\sqrt {2T_\parallel mc^2 } \cos \vartheta \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{< } T_ \bot \), i.e., for distributions with strong transverse anisotropyT⊥≫T‖ The transverse anisotropy may also lead to the maser amplification of the cyclotron radiation in the narrow frequency ranges corresponding to the fine structure peaks in the emissivity spectra. This occurs for sufficiently high fieldsB≥Bc(T‖/T⊥), and angles\(\vartheta \) not too close to 0 or π/2. These effects can be observed in X-ray and gamma-ray radiation of the objects associated with strongly magnetized neutron stars (particularly of the gamma-ray bursters).

Quantum relativistic theory of the synchrotron/cyclotron radiation in a strongly-magnetized plasma: The case of longitudinal propagation

Full quantum relativistic treatment of the cyclotron/synchrotron emission and absorption in tenuous plasmas with superstrong magnetic field is developed for the case when the radiation wave-vector is parallel to the magnetic field. The emissivities and absorption coefficients for a plasma with arbitrary distribution function of particles are presented in terms of simple sums over the Landau levels. On the basis of these expressions, the negative absorption (maser amplification) is shown to be impossible for the longitudinal propagation in a tenuous plasma. The summation over the Landau levels is performed analytically, and the quantum effects are analysed in detail, for the thermal distribution of plasma particles. A new type of quantum relativistic oscillations is predicted in the emission and absorption spectra for a plasma with anisotropic temperature. The results obtained are useful for an interpretation of the X-ray and gamma-ray observations of the objects associated with strongly magnetized neutron stars (particularly of the gamma-ray bursters).

Radiation from a strongly-magnetized plasma: The case of predominant scattering

On the basis of diffusion approach for normal modes, solutions of the radiative transfer problem are obtained and analysed for an optically thick tenuous plasma with a strong magnetic field. The case is considered when the scattering processes without change of photon frequency ω are dominant. The radiative transfer coefficients as well as spectra, angular dependences and polarization of the outgoing radiation are investigated in detail for a ‘cold’ plasma,kTe≩mc2, |ω−sωB|≫ωkTe/mc2 )1/2|cosϑ|, whereTe is the electron plasma temperature,ωB=eB/mc the electron cyclotron frequency,s=1,2,... the number of cyclotron harmonic and ϑ the angle between the magnetic field and wave vector. The effects of electronpositron vacuum polarization are taken into account and shown to be very significant. Simple analytic solutions are obtained for various limiting cases (small and large vacuum polarization; high, low and close to the cyclotron resonance radiation frequencies; different orientations of the magnetic field, etc). The results obtained are necessary for analysing X-ray and gamma-ray radiation from strongly magnetized neutron stars.

Classical Anomalous Absorption in Strongly Magnetized Plasmas and Effective Shielding Length

The high-frequency conductivity tensor of a plasma in a magnetic field has been evaluated. An anomalous perpendicular conductivity is obtained for a strongly magnetized plasma. Contrarily to the previous prediction, the effective shielding length is found to be the Debye length even when the Debye length is larger than the electron gyroradius. The effective shielding length is further discussed by presenting the generalized Balescu-Lenard equation.