Interactions In Magnetized Plasmas Research Articles

Investigation of multi-component magnetized plasma interaction with the carbon surface

The proper understanding of magnetized plasma wall-interaction is important in various plasma applications such as surface modifications, sputtering and etching. A multi-component plasma interacting with carbon surface in presence of an oblique magnetic field has been investigated using three-fluids model. In the present case, the plasma is assumed to consist of two species of positive ions (H+ and He+) and electrons. The governing equations are solved numerically for given initial and boundary conditions. The results show that the magnitude of sheath potential increases with the increase in electron temperature and it affects velocity of both ion species, which is crucial in the understanding of plasma wall-transition mechanism. The reflection coefficients of lighter and heavier ions for the carbon surface decrease with the increase in ion velocity and in reduced energy; however the lighter ions are reflected more than the heavier ions. Hence, the absorption coefficients of both the ions increase for the increase in ion velocity and reduced energy as well. Furthermore, the reflection coefficient of hydrogen ions is about 20% more in magnitude than that of helium ions for the carbon surface.

Plasmaspheric hiss waves generate a reversed energy spectrum of radiation belt electrons

Highly energetic electrons are trapped in the magnetic field of Earth’s radiation belts. The physical mechanisms driving the dynamics of the Van Allen belts can be understood from the electron’s energy spectrum, which is believed to be steeply falling with increasing energy. This view has been prevalent for the past 60 years since the energy spectra were first measured. Here, we report the observation of a reversed energy spectrum with abundant high-energy and fewer low-energy electrons spanning from hundreds of keV to around two MeV in electron energy in data collected with NASA’s Van Allen Probes. We find that this spectrum dominates inside the plasmasphere—a dense cold plasma region co-rotating with the Earth. Using two-dimensional Fokker–Planck diffusion simulations with a time-dependent, data-driven model of hiss waves in the plasmasphere, we demonstrate that the formation of the reversed spectrum is explained by the scattering of hiss waves. The results have important implications for understanding the distributions of charged particles and wave–particle interactions in magnetized plasmas throughout the solar system and beyond. Observations reveal that electrons in Earth’s outer radiation belt possess a spectrum that partially rises with increasing energy, contrary to common beliefs. Plasma hiss waves scattered off electrons are found to be the origin of this phenomenon.

Improving particle beam acceleration in plasmas

The dynamics of wave-particle interactions in magnetized plasmas restricts the wave amplitude to moderate values for particle beam acceleration from rest energy. We analyze how a perturbing invariant robust barrier modifies the phase space of the system and enlarges the wave amplitude interval for particle acceleration. For low values of the wave amplitude, the acceleration becomes effective for particles with initial energy close to the rest energy. For higher values of the wave amplitude, the robust barrier controls chaos in the system and restores the acceleration process. We also determine the best position for the perturbing barrier in phase space in order to increase the final energy of the particles.

B2.5-Eirene modeling of radial transport in the MAGPIE linear plasma device

Radial transport in helicon heated hydrogen plasmas in the MAGnetized Plasma Interaction Experiment (MAGPIE) is studied with the B2.5-Eirene (SOLPS5.0) code. Radial distributions of plasma density, temperature and ambipolar potential are computed for several magnetic field configurations and compared to double Langmuir probe measurements. Evidence for an unmagnetized ion population is seen in the requirement for a convective pinch term in the continuity equation in order to fit the centrally peaked density profile data. The measured slightly hollow electron temperature profiles are reproduced with combinations of on-axis and edge heating which can be interpreted as helicon and Trivelpiece–Gould wave absorption, respectively. Pressure gradient driven radial charged particle diffusion is chosen to describe the diffusive particle flux since the hollowness of the temperature profiles assists the establishment of on-axis density peaking.

Laser-pulse compression using magnetized plasmas.

Proposals to reach the next generation of laser intensities through Raman or Brillouin backscattering have centered on optical frequencies. Higher frequencies are beyond the range of such methods mainly due to the wave damping that accompanies the higher-density plasmas necessary for compressing higher frequency lasers. However, we find that an external magnetic field transverse to the direction of laser propagation can reduce the required plasma density. Using parametric interactions in magnetized plasmas to mediate pulse compression, both reduces the wave damping and alleviates instabilities, thereby enabling higher frequency or lower intensity pumps to produce pulses at higher intensities and longer durations. In addition to these theoretical advantages, our method in which strong uniform magnetic fields lessen the need for high-density uniform plasmas also lessens key engineering challenges or at least exchanges them for different challenges.

Formation of collisionless shocks in magnetized plasma interaction with kinetic-scale obstacles

We investigate the formation of collisionless magnetized shocks triggered by the interaction between magnetized plasma flows and miniature-sized (order of plasma kinetic-scales) magnetic obstacles resorting to massively parallel, full particle-in-cell simulations, including the electron kinetics. The critical obstacle size to generate a compressed plasma region ahead of these objects is determined by independently varying the magnitude of the dipolar magnetic moment and the plasma magnetization. We find that the effective size of the obstacle depends on the relative orientation between the dipolar and plasma internal magnetic fields, and we show that this may be critical to form a shock in small-scale structures. We study the microphysics of the magnetopause in different magnetic field configurations in 2D and compare the results with full 3D simulations. Finally, we evaluate the parameter range where such miniature magnetized shocks can be explored in laboratory experiments.

Research on electron acceleration in interaction of intense laser and magnetized plasma by ponderomotive force

Electron acceleration in intense laser and magnetized plasma interaction by ponderomotive force has been researched. The results are shown that, under the influence of ponderomotive force, electron give rise to transverse oscillation and obtain high energy gain, moreover, the resonance by self-generated magnetic filed enforce ponderomotive force and further accelerate electron, lead to higher energy gain. At last, electron energy trend to a steady value, and the high energy electron would be automatic ejected by the ponderomotive force. The present scheme, it can avoid making use of additional extractor of acquiring high energy electron, which may provide useful physical support for novel design of table accelerator.

Initial damage processes for diamond film exposure to hydrogen plasma

Diamond is considered to be a possible alternative to other carbon based materials as a plasma facing material in nuclear fusion devices due to its high thermal conductivity and resistance to chemical erosion. In this work CVD diamond films were exposed to hydrogen plasma in the MAGnetized Plasma Interaction Experiment (MAGPIE): a linear plasma device at the Australian National University which simulates plasma conditions relevant to nuclear fusion. Various negative sample stage biases of magnitude less than 500V were applied to control the energies of impinging ions. Characterisation results from SEM, Raman spectroscopy and ERDA are presented. No measureable quantity of hydrogen retention was observed, this is either due to no incorporation of hydrogen into the diamond structure or due to initial incorporation as a hydrocarbon followed by subsequent etching back into the plasma. A model is presented for the initial stages of diamond erosion in fusion relevant hydrogen plasma that involves chemical erosion of non-diamond material from the surface by hydrogen radicals and damage to the subsurface region from energetic hydrogen ions. These results show that the initial damage processes in this plasma regime are comparable to previous studies of the fundamental processes as reported for less extreme plasma such as in the development of diamond films.

Design and characterization of the Magnetized Plasma Interaction Experiment (MAGPIE): a new source for plasma–material interaction studies

The Magnetized Plasma Interaction Experiment (MAGPIE) is a versatile helicon source plasma device operating in a magnetic hill configuration designed to support a broad range of research activity and is the first stage of the Materials Diagnostic Facility at the Australian National University. Various material targets can be introduced to study a range of plasma–material interaction phenomena.Initially, with up to 2.1 kW of RF at 13.56 MHz, argon (1018–1019 m−3) and hydrogen (up to 1019 m−3 at 20 kW) plasma with electron temperature ∼3–5 eV was produced in magnetic fields up to ∼0.19 T. For high mirror ratio we observe the formation of a bright blue core in argon above a threshold RF power of 0.8 kW. Magnetic probe measurements show a clear m = +1 wave field, with wavelength smaller than or comparable to the antenna length above and below this threshold, respectively. Spectroscopic studies indicate ion temperatures <1 eV, azimuthal flow speeds of ∼1 km s−1 and axial flow near the ion sound speed.

Wave modeling in a cylindrical non-uniform helicon discharge

A radio frequency field solver based on Maxwell's equations and a cold plasma dielectric tensor is employed to describe wave phenomena observed in a cylindrical non-uniform helicon discharge. The experiment is carried out on a recently built linear plasma-material interaction machine: The magnetized plasma interaction experiment [Blackwell et al., Plasma Sources Sci. Technol. (submitted)], in which both plasma density and static magnetic field are functions of axial position. The field strength increases by a factor of 15 from source to target plate, and the plasma density and electron temperature are radially non-uniform. With an enhancement factor of 9.5 to the electron-ion Coulomb collision frequency, a 12% reduction in the antenna radius, and the same other conditions as employed in the experiment, the solver produces axial and radial profiles of wave amplitude and phase that are consistent with measurements. A numerical study on the effects of axial gradient in plasma density and static magnetic field on wave propagations is performed, revealing that the helicon wave has weaker attenuation away from the antenna in a focused field compared to a uniform field. This may be consistent with observations of increased ionization efficiency and plasma production in a non-uniform field. We find that the relationship between plasma density, static magnetic field strength, and axial wavelength agrees well with a simple theory developed previously. A numerical scan of the enhancement factor to the electron-ion Coulomb collision frequency from 1 to 15 shows that the wave amplitude is lowered and the power deposited into the core plasma decreases as the enhancement factor increases, possibly due to the stronger edge heating for higher collision frequencies.

Second harmonic generation in laser magnetized–plasma interaction

When an intense laser pulse interacts with homogeneous plasma embedded in a transverse magnetic field, transverse current density oscillating with frequency twice that of the laser field is set up. This leads to generation of second harmonic radiation with significant conversion efficiency.

Radiation from high-intensity ultrashort-laser-pulse and gas-jet magnetized plasma interaction.

Using a gas-jet flow, via the interaction between an ultrashort high-intensity laser pulse and plasma in the presence of a perpendicular external dc magnetic field, the short pulse radiation from a magnetized plasma wakefield has been observed. Different nozzles are used in order to generate different densities and gas profiles. The neutral density of the gas-jet flow measured with a Mach-Zehnder interferometer is found to be proportional to back pressure of the gas jet in the range of 1 to 8 atm. Strength of the applied dc magnetic field varies from 0 to 8 kG at the interaction region. The frequency of the emitted radiation with the pulse width of 200 ps (detection limit) is in the millimeter wave range. Polarization and spatial distributions of the experimental data are measured to be in good agreement with the theory based on the V(p)xB radiation scheme, where V(p) is the phase velocity of the electron plasma wave and B is the steady magnetic field intensity. Characteristics of the radiation are extensively studied as a function of plasma density and magnetic field strength. These experiments should contribute to the development of a new kind of millimeter wavelength radiation source that is tunable in frequency, pulse duration, and intensity.

Properties of transit-time interactions in magnetized plasmas: Analytic and numerical results

The recently developed perturbation theory of transit-time interactions between particles and coherent wave packets in magnetized plasmas is applied to particular field structures. Limits of validity are determined by comparison with test-particle simulations, showing that the theory is accurate everywhere except near certain well-determined resonances, for wave fields exceeding a characteristic threshold, and for particles below a particular velocity. The properties of transit-time interactions in magnetized plasmas are investigated in detail to determine their dependence on the fields and parameters of the particle motion. Resonant particle scattering is found to occur at low particle velocities when the frequency of the coherent wave packet is an integer multiple of the gyrofrequency. Two different types of resonant transit-time dissipation are also observed: one arises from transient cyclotron acceleration in the localized wave packet, the other from beating between the gyration of the particles and the oscillation of the wave packet field. Both effects involve an interplay between the field geometry and resonant oscillations.

Transit-time interactions in magnetized plasmas

The theory of transit-time interactions between particles and coherent wave packets is generalized to include the effects of ambient and induced magnetic fields. Using a Born approximation, the wave–particle energy transfer is calculated analytically to second order in the perturbing fields, which is sufficient to treat both particle scattering and wave damping. The results possess the correct unmagnetized limit, and can be approximated in a simple form in the small gyroradius regime.

Nonlinear aspects of collective, electromagnetic interactions in magnetized plasmas with anisotropic protons and isotropic alpha particles

We use computer simulation experiments to investigate the nonlinear behaviour of plasmas with a mixture of anisotropic protons and isotropie alpha particles, embedded in a static magnetic field. Specifically, we study the linearly predicted ‘stop-band’ for the propagation of the proton-produced electromagnetic ion cyclotron waves in conjunction with the energization of the heavier ions by the same waves. For this, three cases are considered: (1) proton + electron plasma; (2) proton + electron + cold alpha particle plasma, and (3) proton + electron + warm alpha particle plasma. Among the main results obtained we mention the following, (a) In the presence of significant relative He2+ concentrations (either cold or warm) all proton-produced left-polarized waves having frequencies above the alpha-particle gyrofrequency are practically suppressed, during the entire nonlinear evolution of the system, indicating that particle–wave–particle interactions are confined to the low-frequency branch of the waves, (b) The ‘remnant’ wave energy, i.e. that part of the wave energy not transferred to the particles, decreases significantly when going from case 1 to case 3. (c) Nevertheless, in all three cases, the initial proton thermal anisotropy relaxes to the same quasi-equilibrium value (≃ 1·5). (d) The cold alpha particles in case 2 are strongly heated by their non-resonant interaction with the proton-produced ion cyclotron electromagnetic waves, (e) In contrast, the initially warm isotropic alpha particles in case 3 are heated by resonant interaction with the proton-produced waves, resulting in an increase in the perpendicular energy and a decrease in the parallel energy. The physical processes involved in the collisionless interaction of these mixed protons and heavier ions (alpha particles) are discussed.

Coherent nonlinear effects on electromagnetic wave-particle interactions

In many particle simulations and space experiments, the knowledge of nonlinear characteristics of both waves and particles in plasmas is very helpful in the data analysis phase. Such knowledge is needed even more in the designing phase of appropriate diagnostics and probings in both simulations arid space experiments. In this tutorial lecture, I will attempt to provide a basic introduction to the fundamental features of coherent nonlinear wave-particle and wave-wave interactions in magnetized plasmas. The present lecture covers only some of the important and basic characteristics in the coherent nonlinear interactions. The main subjects are: (1) trapping dynamics in the electrostatic wave, (2) nonlinear phase trapping and phase bunching in electromagnetic wave both in homogeneous and inhomogeneous plasmas, and (3) coherent three wave interactions.

Parametric Instabilities of Waves in Magnetized Plasmas

AbstractResults from kinetic theory are reformulated in order to generalize previous equations for wave‐wave interactions in magnetized plasmas.

Three‐wave Interactions in Magnetized Plasmas

AbstractThis paper considers the nonlinear interaction between three waves in a warm magnetized twocomponent plasma. A general formalism is developed within a simple macroscopic theory in order to treat waves propagating in arbitrary directions. Some results of previous papers are generalized and discussed.