Relativistic Channeling Research Articles

Modeling the electron acceleration in relativistic channels for space irradiation applications

We study the interaction of an intense short laser pulse (duration > 100 fs) with a helium gas jet (with a pressure from 1–80 bar) by performing two-dimensional particle-in-cell (PIC) simulations. The parameters of the existing setups at the CETAL PW facility are used in PIC simulations. The mechanisms of the relativistic laser channeling such as filamentary bifurcation, long-wavelength hosing, bifurcation due to long-wavelength hosing and refractive bifurcation are shown. We also findthe optimum parameters of the laser pulse and the helium gas jet for which electrons are accelerated in the direct laser acceleration regime. We obtained electrons with energies higher than 100 MeV and broad electron energy spectra features that are very useful for space irradiation simulations.

Isotopic effect in half-wavelength-crystal channeling of relativistic ions

The computer simulations of the angular distributions of low-Z relativistic isotopes channeled in a half-wavelength-crystal (HWC) revealed that at equal beam energy, crystal thickness and its alignment, the HWC channeling is sensitive to a mass number A of the low-Z isotope. That means, besides well-known applications of relativistic channeling for beam deflection and splitting, probably the new one is possible – light isotopes mass filter.

Generation of high-charge electron beam in a subcritical-density plasma through laser pulse self-trapping

To maximize the charge of a high-energy electron beam accelerated by an ultra-intense laser pulse propagating in a subcritical plasma, the pulse length should be longer than both the plasma wavelength and the laser pulse width, which is quite different from the standard bubble regime. In addition, the laser-plasma parameters should be chosen to produce the self-trapping regime of relativistic channeling, where the diffraction divergence is balanced by the relativistic nonlinearity such that the laser beam radius is unchanged during pulse propagation in a plasma over many Rayleigh lengths. The condition for such a self-trapping regime is the same as what was empirically found in several previous simulation studies in the form of the pulse width matching condition. Here, we prove these findings for a subcritical plasma, where the total charge of high-energy electrons reaches the multi-nC level, by optimization in a 3D PIC simulation study and compare the results with an analytic theory of relativistic self-focusing. A very efficient explicitly demonstrated generation of high-charge electron beams opens a way to a high-yield production of gammas, positrons, and photonuclear particles.

Fine features of parametric X-ray radiation by relativistic electrons and ions

In present work within the frame of dynamic theory for parametric X-ray radiation in two-beam approximation we have presented detailed studies on parametric radiation emitted by relativistic both electrons and ions at channeling in crystals that is highly requested at planned experiments. The analysis done has shown that the intensity of radiation at relativistic electron channeling in Si (110) with respect to the conventional parametric radiation intensity has up to 5% uncertainty, while the error of approximate formulas for calculating parametric X-ray radiation maxima does not exceed 1.2%. We have demonstrated that simple expressions for the Fourier components of Si crystal susceptibility χ0 and χgσ could be reduced, as well as the temperature dependence for radiation maxima in Si crystal (diffraction plane (110)) within Debye model. Moreover, for any types of channeled ions it is shown that the parametric X-ray radiation intensity is proportional to z2−b(Z,z)/z with the function b(Z,z) depending on the screening parameter and the ion charge number z=Z−Ze.

Relativistic channeling of a linearly polarized laser pulse in overdense plasma

AbstractWe investigated a dynamic procedure for relativistic channeling by a linearly polarized ultraintense laser pulse in overdense plasma, subsequently determining a phenomenological formula for the channel-digging speed. Channeling of the linearly polarized pulse usually results in a sharp-cut (non-adiabatic) pulse front, since the pulse is continuously reflected on the transparency-opacity interface during the channeling process. Using the novel formula for the channel-digging speed, it was possible to analytically predict where such a sharp-cut occurs longitudinally.

Axial magnetic field generation by intense circularly polarized laser pulses in underdense plasmas

Axial magnetic field generation by intense circularly polarized laser beams in underdense plasmas has been studied with three-dimensional particle-in-cell simulations and by means of theoretical analysis. Comparisons between analytical models and simulation results have identified an inverse Faraday effect as the main mechanism of the magnetic field generation in inhomogeneous plasmas. The source of azimuthal nonlinear currents and of the axial magnetic field depends on the transverse inhomogeneities of the electron density and laser intensity. The fields reach a maximum strength of several tens of megagauss for laser pulses undergoing relativistic self-focusing and channeling in moderately relativistic regime. Ultrarelativistic laser conditions inhibit magnetic field generation by directly reducing a source term and by generating fully evacuated plasma channels.

Relativistic laser channeling in plasmas for fast ignition

We report an experimental observation suggesting plasma channel formation by focusing a relativistic laser pulse into a long-scale-length preformed plasma. The channel direction coincides with the laser axis. Laser light transmittance measurement indicates laser channeling into the high-density plasma with relativistic self-focusing. A three-dimensional particle-in-cell simulation reproduces the plasma channel and reveals that the collimated hot-electron beam is generated along the laser axis in the laser channeling. These findings hold the promising possibility of fast heating a dense fuel plasma with a relativistic laser pulse.

Relativistic laser channeling into high-density plasmas

We experimentally studied relativistic laser propagation in preplasmas at the highest powers ever attempted-0.2 to 0.4petawatt. We demonstrated a single conical-shaped plasma channel formation extending several hundred microns in length from the under dense to over dense plasmas, indicating whole-beam self-focused laser channeling into the high-density plasma. The channel cone was reproduced by a three dimensional particle-in-cell simulation. The confirmation of the relativistic laser channeling into high-density plasmas holds the promise of fast igniting a highly compressed fuel plasma core.

Relativistic channeling by intense laser pulse in overdense plasmas

Channeling in overdense plasma by relativistic laser pulse is investigated. The critical laser power needed to maintain a plasma channel as well as the mode profiles of the electromagnetic fields in the channel cross section are obtained analytically. A scaling law showing that the critical power is proportional to the square of the background plasma density is found.

Relativistic effects on intense laser beam propagation in plasma channels

Propagation characteristics of a radiation beam in a preformed, tapered plasma channel are analyzed by means of an envelope equation for the beam spot size. The model allows for relativistic focusing and ponderomotive channeling, radial and axial density gradients, and is valid for arbitrary intensity. The characteristics of laser beam propagation are shown to be governed by two parameters, the ratio of laser power to the critical power for relativistic focusing, and a dimensionless focusing strength parameter that includes contributions from both relativistic and channel focusing. The envelope equation provides a unified approach for exploring diverse applications such as designing a tapered laser wakefield accelerator or a plasma lens. The model is employed in interpretation of pump–probe laser propagation experiments and an x-ray source experiment. Full-scale simulations of a plasma channel lens are presented and shown to be in excellent agreement with the analytical results.

Strong-field approximation to the relativistic channeling of electrons in the presence of electromagnetic waves

We present a study of the interaction of a relativistically planar channeled electron with an intense electromagnetic field. Using a S-Matrix approach in the Strong Field Approximation, it is shown that the crystal periodicity affects drastically the excitation process, suppressing the possibility of multiphoton absorption except for some particular cases. This selective excitation opens the possibility to control the dynamics of the channeling process by means of an external field. Explicit expressions for the S-matrix N-photon excitation rates together with the corresponding conservation laws are obtained from the relativistic quantum mechanical Dirac equation.

Relativistic focusing and ponderomotive channeling of intense laser beams

The ponderomotive force associated with an intense laser beam expels electrons radially and can lead to cavitation in plasma. Relativistic effects as well as ponderomotive expulsion of electrons modify the refractive index. An envelope equation for the laser spot size is derived, using the source-dependent expansion method with Laguerre-Gaussian eigenfunctions, and reduced to quadrature. The envelope equation is valid for arbitrary laser intensity within the long pulse, quasistatic approximation and neglects instabilities. Solutions of the envelope equation are discussed in terms of an effective potential for the laser spot size. An analytical expression for the effective potential is given. For laser powers exceeding the critical power for relativistic self-focusing the analysis indicates that a significant contraction of the spot size and a corresponding increase in intensity is possible.

Relativistic channeling of highly-charged ions—resonant coherent excitation, charge exchange, and energy loss

Relativistic channeling experiments of highly-charged ions provide valuable information of a variety of atomic states and processes in the unique environment of a crystal. A periodic crystal potential forces an ion trajectory away from an atomic string/plane and depresses electron loss/capture probabilities. It leads to smaller energy loss, trajectory-dependent Stark level mixing due to the crystal field and resonant coherent excitation (atomic level excitation by the periodic crystal field) of highly-charged heavy ions. Recent progress at HIMAC is reported with an emphasis of resonant coherent excitation observed in charge exchange and energy loss of the highly-charged ions.

Large Quasistatic Magnetic Fields Generated by a Relativistically Intense Laser Pulse Propagating in a Preionized Plasma

Two spatially separated toroidal magnetic fields in the megagauss range have been detected with Faraday rotation during and after propagation of a relativistically intense laser pulse through preionized plasmas. Besides a field in the outer region of the plasma oriented as a conventional thermoelectric field, a field with the opposite orientation closely surrounding the propagation axis is observed, in conditions under which relativistic channeling occurs. A 3D particle-in-cell code was used to simulate the interaction under the conditions of the experiment.

Relativistic Channeling of a Picosecond Laser Pulse in a Near-Critical Preformed Plasma

Relativistic self-channeling of a picosecond laser pulse in a preformed plasma near critical density has been observed both experimentally and in 3D particle-in-cell simulations. Optical probing measurements indicate the formation of a single pulsating propagation channel, typically of about 5 \ensuremath{\mu}m in diameter. The computational results reveal the importance in the channel formation of relativistic electrons traveling with the light pulse and of the corresponding self-generated magnetic field.

Corrections to an effective Schrödinger equation for relativistic channeling of particles in solids

We obtain an effective wave equation to describe the motion of a channeling particle at relativistic energies in solids. Beginning with a Klein-Gordon equation, we show that when suitable approximations are made the resulting wave equation is of a Schrödinger type but with certain relativistic corrections that are in addition to those previously derived. The consequences on the energy band structure are illustrated by calculations for channeled positrons in copper.

Buildup of x-ray laser gain by fluctuations in channeled relativistic beam systems.

A theory for the buildup of stimulated x-ray radiation from spontaneous emission in relativistic channeling beam systems is presented. Explicit expressions for the startup operator for amplification is obtained from a first-principles quantum Hamiltonian including channel fluctuations (phonons or plasmons). Treating the radiation field as classical, an analytic solution for the evolution of the radiation intensity is derived and threshold conditions are obtained. The evolution toward stimulated emission shows a slow buildup followed by a rapid transition. Time scales of these two stages are analyzed.

On the quantum theory of radiation by channeling particles

On the basis of the Dirac equation the intensity of the spontaneous γ-radiation by relativistic channeling particles has been calculated. In the ultrarelativistic limit the quantum treatment gives a result close to that of the classic calculation.