Collective Plasma Effects Research Articles

Neutralized, intense-ion beams for fusion

A charge- and current-neutralized, intense-ion beam (IIB) will propagate undeflected in a magnetized plasma by the E×B , or diamagnetic, drift for a distance large compared to the beam-ion gyro-radius. This propagation occurs because of collective plasma effects when the beam-energy density is sufficient to sustain the polarization-electric field, or diamagnetic-screening currents. Our principal interest is the E×B drift, with β=Ebeam/Efield<1 . IIBs are characterized by parameters in the range: 0.1–2 MeV ion energy, 1–100 MA m−2 ion-current density, and 17–350 kW average power produced on repetitively-pulsed systems. Multi-MW average-beam power is possible with appropriate modifications to the power supply and ion-source/accelerator. IIBs can be focused geometrically, and/or magnetically, to spot sizes of the order of a few cm’s and their angular divergence is typically, ∼15 mRadians, suitable for long-range propagation in a vacuum beam-line. IIBs lose energy and momentum in the same manner as particles injected by a neutral-beam injector (NBIs), hence could be useful for fusion applications, including: heating, current drive, fueling, profile modifications, etc and such applications have yet to be thoroughly studied. Summarized here are the physical principles accounting for IIB propagation; ion-source designs used to produce the IIB; and pulsed-power methods for energizing the IIB accelerator. This technology base informs scalable metrics for the ion-source/accelerator (excluding the HV power supply and interconnects) used in a conceptual injector that provides the same ion energy and injected power (1 MeV, 17 MW) as NBIs used on ITER. A comparison indicates that the IIB injector would be much smaller in size, lower cost, and have much greater efficiency, while also providing for real-time modulation of the beam energy and intensity and the use of small-diameter injection ports that could minimize fuel contamination and magnetic-field leakage between the IIB injector and tokamak.

Evolution of nanohillocks by fullerene ion-induced localized plasma

Surface nanostructures etch without chemicals; owing to this, their development is a crucial technical process. Surface nanohillocks may be created by irradiating yttrium iron garnet (YIG) with 30-MeV C60 cluster ions. The nanohillock creation mechanism is disputed. In this study, we propose that the formation mechanism is a plasma collective effect of charged particles that depends on localized rogue waves. Rogue waves will explain YIG surface nanohillock creation using a traditional hydrodynamic plasma model. Analytically solving hydrodynamic ion fluid equations and Maxwellian electron distributions yields a non-linear Schrödinger equation. Solving the latter gives us plausible rogue wave domains. Rogue waves concentrate charged ions from the surroundings into a small, confined zone, generating surface nanohillocks. The relevance of different plasma parameters is highlighted in the rogue wave profile.

Linear acceleration emission of pulsar relativistic streaming instability and interacting plasma bunches

Context. Linear acceleration emission is one of the mechanisms that might explain intense coherent emissions of radio pulsars. This mechanism is not well understood, however, because the effects of collective plasma response and nonlinear plasma evolution on the resulting emission power must be taken into account. In addition, details of the radio emission properties of this mechanism are unknown, which limits the observational verification of the emission model. Aims. By including collective and nonlinear plasma effects, we calculate radio emission power properties by the linear acceleration emission mechanism that occurs via the antenna principle for two instabilities in neutron star magnetospheres: (1) the relativistic streaming instability, and (2) interactions of plasma bunches. Methods. We used 1D electrostatic relativistic particle-in-cell simulations to evolve the instabilities self-consistently. From the simulations, the power properties of coherent emission were obtained by novel postprocessing of electric currents. Results. We found that the total radio power by plasma bunch interactions exceeds the power of the streaming instability by eight orders of magnitude. The wave power generated by a plasma bunch interaction can be as large as 2.6 × 1016 W. The number of bunch interactions that are required to explain the typical pulsar power, 1018 − 1022 W, depends on how the coherent emissions of bunches are added up together. Although ∼4 × (101 − 105) simultaneously emitting bunches are necessary for an incoherent addition of their radiation power, ≳6 − 600 bunches can explain the total pulsar power if they add up coherently. The radio spectrum of the plasma bunch is characterized by a flatter profile for low frequencies and by a power-law index up to ≈ − 1.6 ± 0.2 for high frequencies. The plasma bunches simultaneously radiate in a wide range of frequencies, fulfilling no specific relation between emission frequency and height in the magnetosphere. The power of the streaming instability is more narrowband than that of the interacting bunches, with a high-frequency cutoff. In both instabilities, the angular width of the radiation decreases with increasing frequency. In addition, the wave power evolution depends on the pulsar rotation angle, causing microsecond fluctuations in the intensity because it oscillates between positive and negative wave interference as a function of the emission angle.

Creating pair plasmas with observable collective effects

Although existing technology cannot yet directly produce fields at the Schwinger level, experimental facilities can already explore strong-field quantum electrodynamics (QED) phenomena by taking advantage of the Lorentz boost of energetic electron beams. Recent studies show that QED cascades can create electron–positron pairs at sufficiently high density to exhibit collective plasma effects. Signatures of collective pair plasma effects can appear in exquisite detail through plasma-induced frequency upshifts and chirps in the laser spectrum. Maximizing the magnitude of the QED plasma signature demands high pair density and low pair energy, which suits the configuration of colliding an over energy-density electron beam with a 1022– intensity laser pulse. The collision creates pairs that have a large plasma frequency, made even larger as they slow down or reverse direction due to both the radiation reaction and laser pressure. This paper explains at a tutorial level the key properties of the QED cascades and laser frequency upshift, and at the same time finds the minimum parameters that can be used to produce observable QED plasma.

Collective plasma effects of electron–positron pairs in beam-driven QED cascades

Understanding the interplay of strong-field QED and collective plasma effects is important for explaining extreme astrophysical environments like magnetars. It has been shown that QED pair plasma can be produced and observed by passing a relativistic electron beam through an intense laser field. This paper presents in detail multiple sets of 3D QED-particle-in-cell simulations to show the creation of pair plasma in the QED cascade. The beam driven method enables a high pair particle density and also a low particle Lorentz factor, which both play equal roles on exhibiting large collective plasma effects. Finite laser frequency upshift is observed with both ideal parameters (24 PW laser colliding with a 300 GeV electron beam) and with existing technologies (3 PW laser colliding with a 30 GeV electron beam).

Cosmic ray production in superbubbles

ABSTRACT We compute the production of cosmic rays (CRs) in the dynamical superbubble (SB) produced by a cluster of massive stars. Stellar winds, supernova remnants, and turbulence are found to accelerate particles so efficiently that the non-linear feedback of the particles must be taken into account in order to ensure that the energy balance is not violated. High-energy particles do not scatter efficiently on the turbulence and escape quickly after each supernova explosion, which makes both their intensity inside the bubble and injection in the interstellar medium intermittent. On the other hand, the stochastic acceleration of low-energy particles hardens the spectra at GeV energies. Because CRs damp the turbulence cascade, this hardening is less pronounced when non-linearities are taken into account. Nevertheless, spectra with hard components extending up to 1–10 GeV and normalized to an energy density of 1–100 eV cm−3 are found to be typical signatures of CRs produced in SBs. Efficient shock reacceleration within compact clusters is further shown to produce hard, slightly concave spectra, while the presence of a magnetized shell is shown to enhance the confinement of CRs in the bubble and therefore the collective plasma effects acting on them. We eventually estimate the overall contribution of SBs to the Galactic CR content and show typical gamma-ray spectra expected from hadronic interactions in SB shells. In both cases, a qualitative agreement with observations is obtained.

Signature of Collective Plasma Effects in Beam-Driven QED Cascades.

QED cascades play an important role in extreme astrophysical environments like magnetars. They can also be produced by passing a relativistic electron beam through an intense laser field. Signatures of collective pair plasma effects in these QED cascades are shown to appear, in exquisite detail, through plasma-induced frequency upshifts in the laser spectrum. Remarkably, these signatures can be detected even in small plasma volumes moving at relativistic speeds. Strong-field quantum and collective pair plasma effects can thus be explored with existing technology, provided that ultradense electron beams are colocated with multipetawatt lasers.

Magnetic dipole absorption of light at intersubband transitions in quantum wells

We consider theoretically the magnetic dipole mechanism of IR-light absorption at intersubband transitions in wide-gap quantum wells (QWs). In contrast to the electric dipole resonance, the discussed mechanism manifests in the interaction with the s-polarized component of electromagnetic radiation. We demonstrated that (a) the frequencies of magnetic and electric dipole intersubband transitions in the 2D layer are equal in the framework of one-particle approximation; (b) collective plasma effects should lead to a significant frequency shift between the magnetic and electric dipole resonances in a typical QW. Consequently, independent measurements of the magnetic and electric dipole resonant frequencies could allow both the extraction of the QW parameters and the experimental verification of theoretical models of collective fermion interaction. Taking into account the relative weakness of the magnetic dipole absorption, we propose a waveguide scheme for its experimental observation.

Dynamics of millicharged dark matter in supernova remnants

Dark matter could have a small electromagnetic charge, provided the charge-to-mass ratio is much less than that of electrons or protons. This candidate, commonly known as millicharged dark matter (mDM), would form a plasma and interact with the interstellar medium and electromagnetic fields within galaxies. In general, understanding the dynamics of mDM requires consideration of collective plasma effects. It has been proposed that mDM can be accelerated in supernova remnants, forming a dark cosmic ray population that would leave distinct experimental signatures. In this work, we study a microphysical model where mDM is shocked by a supernova remnant and isotropized in the frame of the expanding fluid. We find that for $|q_\chi/m_\chi| \gtrsim 10^{-13} e / {\rm MeV}$, the isotropization length for electromagnetic plasma instabilities is much shorter than the size of the supernova remnant. This is a necessary, though not sufficient, first step for formation of a Fermi-accelerated mDM component, and determining the size of this component requires further study. We discuss additional implications of mDM interactions in supernova remnants.

Electron Acceleration in Direct Laser-Solid Interactions Far Beyond the Ponderomotive Limit

In laser-solid interactions, electrons may be generated and subsequently accelerated to energies of the order-of-magnitude of the ponderomotive limit, with the underlying process dominated by direct laser acceleration. Breaking this limit, realized here by a radially-polarized laser pulse incident upon a wire target, can be associated with several novel effects. Three-dimensional Particle-In-Cell simulations show a relativistic intense laser pulse can extract electrons from the wire and inject them into the accelerating field. Anti-dephasing, resulting from collective plasma effects, are shown here to enhance the accelerated electron energy by two orders of magnitude compared to the ponderomotive limit. It is demonstrated that ultra-short radially polarized pulses produce super-ponderomotive electrons more efficiently than pulses of the linear and circular polarization varieties.

Multi-stage scheme for nonlinear Breit–Wheeler pair-production utilising ultra-intense laser-solid interactions

Multi-petawatt (PW) lasers enable intensities exceeding 1023 W cm−2, at which point quantum electrodynamics (QED) processes, such as electron–positron pair-production via the nonlinear Breit–Wheeler process, will play a significant role in laser-plasma interactions. Using 2D QED-particle-in-cell simulations, we present a two-stage scheme in which nonlinear pair-production is induced via an ultra-intense laser-solid interaction. The first stage is the generation of a γ-ray beam, through the interaction of an ultra-intense laser pulse with a thick target, whose features are found to be strongly dependent on collective plasma effects. This compact, high energy γ-ray beam (characterised by a divergence half-angle ∼10° and average photon energy ∼10 MeV) then interacts with two counter-propagating laser pulses. By varying the laser polarisation and angle of incidence, we show that in the case of two circularly polarised laser pulses propagating at an angle equal to the divergence half-angle of the γ-ray beam, the produced positron distribution is highly anisotropic compared to the case of a standard head-on collision.

Quantum-electrodynamical birefringence vanishing in a thermal relativistic pair plasma.

Quantum electrodynamical (QED) birefringence in a thermal relativistic pair plasma with the presence of the strong crossed field: , is proposed and investigated. We clarify the coupling relationship and competition between the QED effect and the plasma collective effect and find the critical condition that makes the birefringence vanish. In a relative weak electromagnetic field, the birefringence is dominated by the coupling of the QED-effect, the collective effect and the drift effect. In a relative strong electromagnetic field, we obtain the formulations stating the competition between the QED effect and the collective effect and then the critical conditions so that they are canceled with each other and the birefringence vanishes. With our results, a new possible scheme is proposed to estimate the thickness of the magnetosphere in a millisecond pulsar and the plasma density of a pulsar, if the magnetic field is known beforehand.

Dark matter self-interactions via collisionless shocks in cluster mergers

While dark matter self-interactions may solve several problems with structure formation, so far only the effects of two-body scatterings of dark matter particles have been considered. We show that, if a subdominant component of dark matter is charged under an unbroken U(1) gauge group, collective dark plasma effects need to be taken into account to understand its dynamics. Plasma instabilities can lead to collisionless dark matter shocks in galaxy cluster mergers which might have been already observed in the Abell 3827 and 520 clusters. As a concrete model we propose a thermally produced dark pair plasma of vector-like fermions. In this scenario the interacting dark matter component is expected to be separated from the stars and the non-interacting dark matter halos in cluster collisions. In addition, the missing satellite problem is softened, while constraints from all other astrophysical and cosmological observations are avoided.

Characterization of laser-driven electron and photon beams using the Monte Carlo code FLUKA

AbstractWe present a new simulation method to predict the maximum possible yield of X-rays produced by electron beams accelerated by petawatt lasers irradiating thick solid targets. The novelty of the method lies in the simulation of the electron refiluxing inside the target implemented with the Monte Carlo code Fluka. The mechanism uses initial theoretical electron spectra, cold targets and refiluxing electrons forced to re-enter the target iteratively. Collective beam plasma effects are not implemented in the simulation. Considering the maximum X-ray yield obtained for a given target thickness and material, the relationship between the irradiated target mass thickness and the initial electron temperature is determined, as well as the effect of the refiluxing on X-ray yield. The presented study helps to understand which electron temperature should be produced in order to generate a particular X-ray beam. Several applications, including medical and security imaging, could benefit from laser generated X-ray beams, so an understanding of the material and the thickness maximizing the yields or producing particular spectral characteristics is necessary. On the other more immediate hand, if this study is experimentally reproduced at the beginning of an experiment in which there is an interest in laser-driven electron and/or photon beams, it can be used to check that the electron temperature is as expected according to the laser parameters.

Constraining primordial magnetic fields by CMB photon-graviton conversion

We revisit the method of using the photon-graviton conversion mechanism in the presence of the external magnetic field to probe small-scale primordial magnetic fields that may exist between the last scattering surface and present. Specifically, we investigate impacts on the conversion efficiency due to the presence of matter, including the plasma collective effect and the atomic polarizability. In general, these effects tend to reduce the conversion probability. Under this more realistic picture and based on the precision of COBE's measurement of CMB (cosmic microwave background) blackbody spectrum, we find an upper bound for the primordial magnetic field strength, B < 30G, at the time of recombination. Although at present the bound based on the photon-graviton conversion mechanism is not as tight as that obtained by the direct use of CMB temperature anisotropy, it nevertheless provides an important independent constraint on primordial magnetic fields and at epochs in addition to the recombination. The bound can be significantly improved if the CMB blackbody spectrum measurement becomes more precise in future experiments such as PIXIE.

The formation of a collisionless shock

AbstractCollisionless shocks are key processes in astrophysics where the energy dissipation at the shock front is provided by collective plasma effects rather than particle collisions. While numerous simulations and laser-plasma experiments have shown they can result from the encounter of two plasma shells, a first principle theory of the shock formation is still lacking. In this respect, a series of 2D Particle-In-Cells simulations have been performed of two identical cold colliding pair plasmas. The simplicity of this system allows for an accurate analytical tracking of the physics. To start with, the Weibel-filamentation instability is triggered in the overlapping region, which generates a turbulent region after a saturation time τs. The incoming flow then piles-up in this region, building-up the shock density region according to some nonlinear processes, which will be the subject of future works. By evaluating the seed field giving rise to the instability, we derive an analytical expression for τs in good agreement with simulations. In view of the importance of the filamentation instability, we show a static magnetic field can cancel it if and only if it is perfectly aligned with the flow.

Electron acceleration during three-dimensional relaxation of an electron beam-return current plasma system in a magnetic field

We investigate the effects of acceleration during non-linear electron-beam relaxation in magnetized plasma in the case of electron transport in solar flares. The evolution of electron distribution functions is computed using a three-dimensional particle-in-cell electromagnetic code. Analytical estimations under simplified assumptions are made to provide comparisons. We show that, during the non-linear evolution of the beam-plasma system, the accelerated electron population appears. We found that, although the electron beam loses its energy efficiently to the thermal plasma, a noticeable part of the electron population is accelerated. For model cases with initially monoenergetic beams in uniform plasma, we found that the amount of energy in the accelerated electrons above the injected beam-electron energy varies depending the plasma conditions and could be around 10-30% of the initial beam energy. This type of acceleration could be important for the interpretation of non-thermal electron populations in solar flares. Its neglect could lead to the over-estimation of accelerated electron numbers. The results emphasize that collective plasma effects should not be treated simply as an additional energy-loss mechanism, when hard X-ray emission in solar flares is interpreted, notably in the case of RHESSI data.

Plasmon-graviton conversion in a magnetic field in TeV-scale gravity

Kaluza-Klein (KK) gravitons emission rates due to plasmon-graviton conversion in magnetic field are computed within the ADD model of TeV-scale gravity. Plasma is described in the kinetic approach as the system of charged particles and Maxwell field both confined on the brane. Interaction with multidimensional gravity living in the bulk with $n$ compact extra dimensions is introduced within the linearized theory. Plasma collective effects enter through the two-point correlation function of the fluctuations of the energy-momentum tensor. The estimate for magnetic stars is presented leading to the lower limit of the D-dimensional Plank mass.

On description of a collisionless quantum plasma

A plasma is regarded as quantum if its macroscopic properties are significantly affected by the quantum nature of its constituent particles. A proper description is necessary to comprehend when collective quantum plasma effects are important. In this paper, the most commonly used microscopic approaches to describe a collisionless quantum plasma are reviewed, together with their related assumptions and restrictions. In particular, the quantum plasma hydrodynamic approximation is analyzed in detail, and the analytical properties of the linear dielectric response function obtained from quantum plasma kinetic theory are investigated. Special attention is paid to what we consider to be the most important problems that have already appeared in the linear approximation and require further studies.

Black Hole Accretion in Low States: Electron Heating

Plasmas in an accretion flow are heated by magneto-hydrodynamic turbulence generated through the magneto- rotational instability (MRI). The viscous stress driving the accretion is intimately connected to the microscopic processes of turbulence dissipation. We show that, in a few well-observed black hole accretion systems, there is compelling observational evidence of efficient electron heating by turbulence or collective plasma effects in low accretion states, when Coulomb collisions are not efficient enough to establish a thermal equilibrium between electrons and ions at small radii. We consider a Keplerian two-temperature accretion flow with a constant mass accretion rate in the pseudo- Newtonian gravitational potential and take into account the bremsstrahlung, synchrotron, and inverse Comptonization cooling processes. The balance of gravitational energy dissipation and turbulence energy cascade requires that the viscous stress be proportional to the product of the turbulence kinetic energy density and the total turbulence energy density, which may contradict the result of some shearing box simulations that the viscous stress is proportional to the magnetic field energy density. The critical mass accretion rate, below which the two-temperature solution may exist, is determined by the cooling processes and the collisional energy exchanges between electrons and ions and has very weak dependence on the collision-less heating of electrons by turbulence, which becomes more important at lower accretion rates. If the collision-less heating is dominated by the transit-time damping processes, small scale waves propagating obliquely with respect to the large scale magnetic field are prohibited, which may affect the saturate state of the MRI driven turbulence significantly. The plasma also needs to be strongly magnetized with the magnetic field and proton energy densities comparable so that electrons can share more of the dissipated gravitational energy. The heating of relativistic electrons is efficient since the heating rate is proportional to the mean momentum of the particles, and the electron heating may also be enhanced by their resonant scattering with small scale nearly parallel propagating waves.