Stable laser-pulse propagation in plasma channels for GeV electron acceleration

Electron acceleration by a radially-polarized laser pulse in a plasma micro-channel

Encouraged by recent advances in radially-polarized laser technology, simulations have been performed of electron acceleration by a tightly-focused, ultra-short pulse in a parabolic plasma micro-channel. Milli-joule laser pulses, generated at kHz repetition rates, are shown to produce electron bunches of MeV energy, pC charge, low emittance and low divergence. The pivotal role played by the channel length in controlling the process is demonstrated, and the roles of direct and wakefield acceleration are distinguished.

Plasma-based accelerator with magnetic compression

Electron dephasing is a major gain-inhibiting effect in plasma-based accelerators. A novel method is proposed to overcome dephasing, in which the modulation of a modest [~O(10 kG)], axial, uniform magnetic field in the acceleration channel leads to densification of the plasma through magnetic compression, enabling direct, time-resolved control of the plasma wave properties. The methodology is broadly applicable and can be optimized to improve the leading acceleration approaches, including plasma beat wave, plasma wakefield, and laser wakefield acceleration. The advantages of magnetic compression are compared to other proposed techniques to overcome dephasing.

Raman scattering of intense, short laser pulses in modulated plasmas

We examine the exponentiation of the Raman forward scattering instability in modulated plasma channels computationally and analytically. An evolution equation for the complex phases of the Raman scattered waves treating the spatial localization and discrete nature of the channel modes is derived. Simulations with WAKE [P. Mora and T. M. Antonsen Jr., Phys. Plasmas 4, 217 (1997)] verify the theory in the linear growth regime and provide insight into the nonlinear stage of the instability when cascading and pump depletion play a role. We find that the exponentiation in modulated channels depends on two factors: the increase in coupling due to the increased plasma wavenumber in the high-density regions of the channel and a decreased coupling due to the reduced longitudinal spatial coherence. For the parameters considered, simulations show that the finite extent of the pump pulse is more significant in determining the exponentiation than phase mixing due to the transverse variation of the channel. Both the theory and simulations confirm that modulated channels allow for the stable guiding of longer pulses than nonmodulated channels.

Control of laser-wakefield acceleration by the plasma-density profile

We show that both the maximum energy gain and the accelerated beam quality can be efficiently controlled by the plasma-density profile. Choosing a proper density gradient one can uplift the dephasing limitation and keep the phase synchronism between the bunch of relativistic particles and the plasma wave over extended distances. Putting electrons into the n th wake period behind the driving laser pulse, the maximum energy gain is increased by the factor, which is proportional to n, over that in the case of uniform plasma. Layered plasma is suggested to keep the resonant condition for laser-wakefield excitation. The acceleration is limited then by laser depletion rather than by dephasing. Further, we show that the natural energy spread of the particle bunch acquired at the acceleration stage can be effectively removed by a matched deceleration stage, where a larger plasma density is used.

Laser acceleration of electrons to giga-electron-volt energies using highly charged ions

The recent proposal to use highly charged ions as sources of electrons for laser acceleration [S. X. Hu and A. F. Starace, Phys. Rev. Lett. 88, 245003 (2002)] is investigated here in detail by means of three-dimensional, relativistic Monte Carlo simulations for a variety of system parameters, such as laser pulse duration, ionic charge state, and laser focusing spot size. Realistic laser focusing effects--e.g., the existence of longitudinal laser field components-are taken into account. Results of spatial averaging over the laser focus are also presented. These numerical simulations show that the proposed scheme for laser acceleration of electrons from highly charged ions is feasible with current or near-future experimental conditions and that electrons with GeV energies can be obtained in such experiments.

Inverse free electron lasers and laser wakefield acceleration driven by CO2 lasers

The staged electron laser acceleration (STELLA) experiment demonstrated staging between two laser-driven devices, high trapping efficiency of microbunches within the accelerating field and narrow energy spread during laser acceleration. These are important for practical laser-driven accelerators. STELLA used inverse free electron lasers, which were chosen primarily for convenience. Nevertheless, the STELLA approach can be applied to other laser acceleration methods, in particular, laser-driven plasma accelerators. STELLA is now conducting experiments on laser wakefield acceleration (LWFA). Two novel LWFA approaches are being investigated. In the first one, called pseudo-resonant LWFA, a laser pulse enters a low-density plasma where nonlinear laser/plasma interactions cause the laser pulse shape to steepen, thereby creating strong wakefields. A witness e-beam pulse probes the wakefields. The second one, called seeded self-modulated LWFA, involves sending a seed e-beam pulse into the plasma to initiate wakefield formation. These wakefields are amplified by a laser pulse following shortly after the seed pulse. A second e-beam pulse (witness) follows the seed pulse to probe the wakefields. These LWFA experiments will also be the first ones driven by a CO(2) laser beam.

Parametric instability in the formation of plasma waveguides

Plasma waveguides generated by focusing a moderate intensity laser into neutral gas with an axicon lens can be unstable to the generation of axial modulations in the channel parameters. A model is proposed in which the modulations are due to the nonlinear coupling between the axicon field and a scattered mode in the evolving channel. Good agreement is found with experimental measurements of these modulations.

Ultrashort laser pulses and electromagnetic pulse generation in air and on dielectric surfaces

Intense, ultrashort laser pulses propagating in the atmosphere have been observed to emit sub-THz electromagnetic pulses (EMPS). The purpose of this paper is to analyze EMP generation from the interaction of ultrashort laser pulses with air and with dielectric surfaces and to determine the efficiency of conversion of laser energy to EMP energy. In our self-consistent model the laser pulse partially ionizes the medium, forms a plasma filament, and through the ponderomotive forces associated with the laser pulse, drives plasma currents which are the source of the EMP. The propagating laser pulse evolves under the influence of diffraction, Kerr focusing, plasma defocusing, and energy depletion due to electron collisions and ionization. Collective effects and recombination processes are also included in the model. The duration of the EMP in air, at a fixed point, is found to be a few hundred femtoseconds, i.e., on the order of the laser pulse duration plus the electron collision time. For steady state laser pulse propagation the flux of EMP energy is nonradiative and axially directed. Radiative EMP energy is present only for nonsteady state or transient laser pulse propagation. The analysis also considers the generation of EMP on the surface of a dielectric on which an ultrashort laser pulse is incident. For typical laser parameters, the power and energy conversion efficiency from laser radiation to EMP radiation in both air and from dielectric surfaces is found to be extremely small, < 10(-8). Results of full-scale, self-consistent, numerical simulations of atmospheric and dielectric surface EMP generation are presented. A recent experiment on atmospheric EMP generation is also simulated.

Propagation of intense short laser pulses in the atmosphere

The propagation of short, intense laser pulses in the atmosphere is investigated theoretically and numerically. A set of three-dimensional (3D), nonlinear propagation equations is derived, which includes the effects of dispersion, nonlinear self-focusing, stimulated molecular Raman scattering, multiphoton and tunneling ionization, energy depletion due to ionization, relativistic focusing, and ponderomotively excited plasma wakefields. The instantaneous frequency spread along a laser pulse in air, which develops due to various nonlinear effects, is analyzed and discussed. Coupled equations for the power, spot size, and electron density are derived for an intense ionizing laser pulse. From these equations we obtain an equilibrium for a single optical-plasma filament, which involves a balancing between diffraction, nonlinear self-focusing, and plasma defocusing. The equilibrium is shown to require a specific distribution of power along the filament. It is found that in the presence of ionization a self-guided optical filament is not realizable. A method for generating a remote spark in the atmosphere is proposed, which utilizes the dispersive and nonlinear properties of air to cause a low-intensity chirped laser pulse to compress both longitudinally and transversely. For optimally chosen parameters, we find that the transverse and longitudinal focal lengths can be made to coincide, resulting in rapid intensity increase, ionization, and white light generation in a localized region far from the source. Coupled equations for the laser spot size and pulse duration are derived, which can describe the focusing and compression process in the low-intensity regime. More general examples involving beam focusing, compression, ionization, and white light generation near the focal region are studied by numerically solving the full set of 3D, nonlinear propagation equations.

Raman forward scattering and self-modulation of laser pulses in tapered plasma channels

The propagation of intense laser pulses with durations longer than the plasma period through tapered plasma channels is investigated theoretically and numerically. General propagation equations are presented and reduced partial differential equations that separately describe the forward Raman (FR) and self-modulation (SM) instabilities in a nonuniform plasma are derived. Local dispersion relations for FR and SM instabilities are used to analyze the detuning process arising from a longitudinal density gradient. Full-scale numerical fluid simulations indicate parameters that favorably excite either the FR or SM instability. The suppression of the FR instability and the enhancement of the SM instability in a tapered channel in which the density increases longitudinally is demonstrated. For a pulse undergoing a self-modulation instability, calculations show that the phase velocity of the wakefield in an untapered channel can be significantly slower than the pulse group velocity. Simulations indicate that this wake slippage can be forestalled through the use of a tapered channel.

GeV electrons from ultraintense laser interaction with highly charged ions

Ultraintense laser interactions with highly charged ions are investigated using three-dimensional Monte Carlo simulations. Results show that ultraenergetic GeV electrons may be produced for highly charged ions chosen so that their electrons remain bound during the rise time of the laser pulse, and so that the electrons are ionized when the laser is near its maximum amplitude, which satisfies the best injection condition for subsequent laser acceleration.