Search Results (86 total)

A laser wakefield acceleration study has been performed in the matched, self-guided, blowout regime producing 720±50  MeV quasimonoenergetic electrons with a divergence Δθ_FWHM of 2.85±0.15  mrad using a 10 J, 60 fs 0.8  μm laser. While maintaining a nearly constant plasma density (3×10¹⁸  cm^-3), the energy gain increased from 75 to 720 MeV when the plasma length was increased from 3 to 8 mm. Absolute charge measurements indicate that self-injection of electrons occurs when the laser power P exceeds 3 times the critical power P_cr for relativistic self-focusing and saturates around 100 pC for P/P_cr>5. The results are compared with both analytical scalings and full 3D particle-in-cell simulations.

The acceleration of electrons to ≃0.8  GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5×10¹⁸  cm^-3 by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10z_R), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a₀>6, thus driving a highly nonlinear plasma wave.

Nonlinear electron plasma wave packets are shown to locally damp at the rear of the packet. Resonant particles enter the back of the packet and linearly damp the first few wavelengths, thereby carrying energy away from the back edge and eventually eroding the packet. This process could significantly affect the recurrence and long-time behavior of stimulated Raman scattering because it is predicted that a nonlinear packet will erode away before it travels a speckle length. The effects of a density gradient on the packet’s propagation are also discussed.

Positron trapping and acceleration in a plasma wake using a four-bunch scheme [X. Wang , Phys. Rev. Lett. 101, 124801 (2008)] is numerically investigated through 2D particle-in-cell simulations. This scheme that integrates positron generation, trapping, and acceleration into a single stage is a promising approach for investigating positron acceleration in an electron-beam-driven wake. It consists of a plasma with an embedded thin foil target into which two closely spaced electron beams are shot. The first beam creates a region for accelerating and focusing positrons and the second beam provides positrons to be accelerated. Some of the outstanding issues related to the quality of the accelerated positron beam load are discussed as a function of the beam and plasma parameters. Simulations show that a large number of positrons (10⁷–10⁸) can be trapped when the plasma wake is modestly nonlinear, and the positron-generating foil target must be immersed into the plasma. Beam loading can reduce the energy spread of the positron beam load. The quality of the positron beam load is not very sensitive to the exact bunch spacing between the drive electron bunch and the positron beam load.

Multi-GeV trapped electron bunches in a plasma wakefield accelerator (PWFA) are observed with normalized transverse emittance divided by peak current, ϵ_N,x/I_t, below the level of 0.2  μm/kA. A theoretical model of the trapped electron emittance, developed here, indicates that emittance scales inversely with the square root of the plasma density in the nonlinear “bubble” regime of the PWFA. This model and simulations indicate that the observed values of ϵ_N,x/I_t result from multi-GeV trapped electron bunches with emittances of a few  μm and multi-kA peak currents.

The self-guiding of relativistically intense but ultrashort laser pulses has been experimentally investigated as a function of laser power, plasma density, and plasma length in the blowout regime. The extent of self-guiding, observed by imaging the plasma exit, is shown to be limited by nonlinear pump depletion with observed self-guiding of over tens of Rayleigh lengths. Spectrally resolved images of the plasma exit show evidence consistent with self-guiding in the plasma wake. Minimal losses of the self-guided pulse resulted when the initial spot size was matched to the blowout radius.

A theory that describes how to load negative charge into a nonlinear, three-dimensional plasma wakefield is presented. In this regime, a laser or an electron beam blows out the plasma electrons and creates a nearly spherical ion channel, which is modified by the presence of the beam load. Analytical solutions for the fields and the shape of the ion channel are derived. It is shown that very high beam-loading efficiency can be achieved, while the energy spread of the bunch is conserved. The theoretical results are verified with the particle-in-cell code OSIRIS.

A novel approach for generating and accelerating positron bunches in a plasma wake is proposed and modeled. The system consists of a plasma with an embedded thin foil into which two electron beams are shot. The first beam creates a region for accelerating and focusing positrons and the second beam provides positrons to be accelerated. Monte Carlo and 3D PIC simulations show a large number of positrons (10⁷∼10⁸) are trapped and accelerated to ∼5  GeV over 1 m with relatively narrow energy spread and low emittance.

An ultrarelativistic 28.5 GeV, 700-μm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n_e between ≈10¹³ and ≈5×10¹⁴  cm^-3. Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of ≈3 in the high emittance plane of the beam ≈1  m downstream from the plasma exit. As n_e increases, the formation of a beam halo containing ≈40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of ≈3 and emittance ratio of ≈5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.

Two dimensional particle-in-cell simulations show that laser channeling in millimeter-scale underdense plasmas is a highly nonlinear and dynamic process involving longitudinal plasma buildup, laser hosing, channel bifurcation and self-correction, and electron heating to relativistic temperatures. The channeling speed is much less than the linear group velocity of the laser. The simulations find that low-intensity channeling pulses are preferred to minimize the required laser energy but with an estimated lower bound on the intensity of I≈5×10¹⁸  W/cm² if the channel is to be established within 100 ps. The channel is also shown to significantly increase the transmission of an ignition pulse.

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x rays extending to 50 keV is observed. The measured x rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the Maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy converted into x rays.

The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al.

The extraordinary ability of space-charge waves in plasmas to accelerate charged particles at gradients that are orders of magnitude greater than in current accelerators has been well documented. We develop a phenomenological framework for laser wakefield acceleration (LWFA) in the 3D nonlinear regime, in which the plasma electrons are expelled by the radiation pressure of a short pulse laser, leading to nearly complete blowout. Our theory provides a recipe for designing a LWFA for given laser and plasma parameters and estimates the number and the energy of the accelerated electrons whether self-injected or externally injected. These formulas apply for self-guided as well as externally guided pulses (e.g. by plasma channels). We demonstrate our results by presenting a sample particle-in-cell (PIC) simulation of a 30   fs, 200 TW laser interacting with a 0.75 cm long plasma with density 1.5×10¹⁸  cm^-3 to produce an ultrashort (10 fs) monoenergetic bunch of self-injected electrons at 1.5 GeV with 0.3 nC of charge. For future higher-energy accelerator applications, we propose a parameter space, which is distinct from that described by Gordienko and Pukhov [Phys. Plasmas 12, 043109 (2005)] in that it involves lower plasma densities and wider spot sizes while keeping the intensity relatively constant. We find that this helps increase the output electron beam energy while keeping the efficiency high.

The effect of laser-focusing conditions on the evolution of relativistic plasma waves in laser-wakefield accelerators is studied both experimentally and with particle-in-cell simulations. For short focal-length (w₀<λ_p) interactions, beam breakup prevents stable propagation of the pulse. High field gradients lead to nonlocalized phase injection of electrons, and thus broad energy spread beams. However, for long focal-length geometries (w₀>λ_p), a single optical filament can capture the majority of the laser energy and self-guide over distances comparable to the dephasing length, even for these short pulses (cτ≈λ_p). This allows the wakefield to evolve to the correct shape for the production of the monoenergetic electron bunches, as measured in the experiment.

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36  GV/m, in good agreement with an analytical model and PIC simulations.

A Reply to the Comment by S. V. Bulanov and T. Esirkepov.

Positrons in the energy range of 3–30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1–50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra.

A beam of multi-MeV helium ions has been observed from the interaction of a short-pulse high-intensity laser pulse with underdense helium plasma. The ion beam was found to have a maximum energy for He²⁺ of (40_-8⁺³)  MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10°. 2D particle-in-cell simulations show that the ions are accelerated by a sheath electric field that is produced at the back of the gas target. This electric field is generated by transfer of laser energy to a hot electron beam, which exits the target generating large space-charge fields normal to its boundary.

Beam profile measurements of laser-wakefield accelerated electron bunches reveal that in the monoenergetic regime the electrons are injected and accelerated at the back of the first period of the plasma wave. With pulse durations cτ≥λ_p, we observe an elliptical beam profile with the axis of the ellipse parallel to the axis of the laser polarization. This increase in divergence in the laser polarization direction indicates that the electrons are accelerated within the laser pulse. Reducing the plasma density (decreasing cτ/λ_p) leads to a beam profile with less ellipticity, implying that the self-injection occurs at the rear of the first period of the plasma wave. This also demonstrates that the electron bunches are less than a plasma wavelength long, i.e., have a duration <25  fs. This interpretation is supported by 3D particle-in-cell simulations.

We present a theory for nonlinear, multidimensional plasma waves with phase velocities near the speed of light. It is appropriate for describing plasma waves excited when all electrons are expelled out from a finite region by either the space charge of a short electron beam or the radiation pressure of a short intense laser. It works very well for the first bucket before phase mixing occurs. We separate the plasma response into a cavity or blowout region void of all electrons and a sheath of electrons just beyond the cavity. This simple model permits the derivation of a single equation for the boundary of the cavity. It works particularly well for narrow electron bunches and for short lasers with spot sizes matched to the radius of the cavity. It is also used to describe the structure of both the accelerating and focusing fields in the wake.

We consider how an unmagnetized plasma responds to an incoming flux of energetic electrons. We assume a return current is present and allow for the incoming electrons to have a different transverse temperature than the return current. To analyze this configuration we present a nonrelativistic theory of the current-filamentation or Weibel instability for rigorously current-neutral and nonseparable distribution functions, f₀(p_x,p_y,p_z)≠f_x(p_x)f_y(p_y)f_z(p_z). We find that such distribution functions lead to lower growth rates because of space-charge forces that arise when the forward-going electrons pinch to a lesser degree than the colder, backward-flowing electrons. We verify the growth rate, range of unstable wave numbers, and the formation of the density filaments using particle-in-cell simulations.

The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam’s space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested.

A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with 1.8×10¹⁰ electrons is compressed to 20  μm longitudinally and focused to a transverse spot size of 10  μm at the entrance of a 10 cm long column of lithium vapor with density 2.8×10¹⁷  atoms/cm³. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.

The spectra of energetic electrons produced by a laser interaction with underdense plasma have been measured at intensities >3×10²⁰  W cm^-2. Electron energies in excess of 300 MeV have been observed. Measurements of the transmitted laser spectrum indicate that there is no correlation between the acceleration of electrons and plasma wave production. Particle-in-cell simulations show that the laser ponderomotive force produces an ion channel. The interaction of the laser field with the nonlinear focusing force of the channel leads to electron acceleration. The majority of the electrons never reach the betatron resonance but those which gain the highest energies do so. The acceleration process exhibits a strong sensitivity to initial conditions with particles that start within a fraction of a laser wavelength following completely different trajectories and gaining markedly different energies.