A. Ting

Overview of plasma-based accelerator concepts

An overview is given of the physics issues relevant to the plasma wakefield accelerator, the plasma beat-wave accelerator, the laser wakefield accelerator, including the self-modulated regime, and wakefield accelerators driven by multiple electron or laser pulses. Basic properties of linear and nonlinear plasma waves are discussed, as well as the trapping and acceleration of electrons in the plasma wave. Formulas are presented for the accelerating field and the energy gain in the various accelerator configurations. The propagation of the drive electron or laser beams is discussed, including limitations imposed by key instabilities and methods for optically guiding laser pulses. Recent experimental results are summarized.

Self-focusing and guiding of short laser pulses in ionizing gases and plasmas

Several features of intense, short-pulse (/spl lsim/1 ps) laser propagation in gases undergoing ionization and in plasmas are reviewed, discussed, and analyzed. The wave equations for laser pulse propagation in a gas undergoing ionization and in a plasma are derived. The source-dependent expansion method is discussed, which is a general method for solving the paraxial wave equation with nonlinear source terms. In gases, the propagation of high-power (near the critical power) laser pulses is considered including the effects of diffraction, nonlinear self-focusing, ionization, and plasma generation. Self-guided solutions and the stability of these solutions are discussed. In plasmas, optical guiding by relativistic effects, ponderomotive effects, and preformed density channels is considered. The self consistent plasma response is discussed, including plasma wave effects and instabilities such as self-modulation. Recent experiments on the guiding of laser pulses in gases and in plasmas are briefly summarized.

Turbulent relaxation processes in magnetohydrodynamics

Competing processes previously called ‘‘selective decay’’ and ‘‘dynamic alignment’’ are studied numerically for two‐dimensional magnetohydrodynamic turbulence. In selective decay, the energy decays relatively to mean‐square vector potential, and in dynamic alignment, the energy decays relatively to cross helicity. In the former case, the kinetic (fluid) energy decays to zero and the magnetic energy occupies the largest scales allowed by the boundary conditions. In the latter case, the velocity field and magnetic field become aligned and energetically equipartitioned. An extensive study of the initial value problem, with viscous and resistive dissipation, indicates that four distinct regimes of behavior are possible: a magnetically dominated regime, a velocity dominated regime, a dynamic alignment dominated regime, and a transition regime separating the others in parameter space. An analytical variational problem predicts several features seen in the computations, including geometrical alignment of velocit...

Tunable, short pulse hard x‐rays from a compact laser synchrotron source

A compact laser synchrotron source (LSS) is proposed as a means of generating tunable, narrow bandwidth, ultra‐short pulses of hard x rays. The LSS is based on the Thomson backscattering of intense laser radiation from a counterstreaming electron beam. Advances in both compact ultra‐intense solid‐state lasers and high brightness electron accelerators make the LSS an attractive compact source of high brightness pulsed x rays, particularly at photon energies beyond ∼30 keV. The x‐ray wavelength is λ[A]=650 λ0[μm]/Eb2[MeV], where λ0 is the laser wavelength and Eb is the electron beam energy. For Eb=72 MeV and λ0=1 μm, x rays at λ=0.12 A (100 keV) are generated. The spectral flux, brightness, bandwidth, and pulse structure are analyzed. In the absence of filtering, the spectral bandwidth in the LSS is typically ≲1% and is limited by electron beam emittance and energy spread. Two configurations of the LSS are discussed, one providing high peak power and the other moderate average power x rays. Using present da...

Nonlinear analysis of relativistic harmonic generation by intense lasers in plasmas

A linearly polarized, ultraintense laser field induces transverse plasma currents which are highly relativistic and nonlinear, resulting in the generation of coherent harmonic radiation in the forward direction (i.e., copropagating with the incident laser field). A nonlinear cold fluid model, valid for ultrahigh intensities, is formulated and used to analyze relativistic harmonic generation. The plasma density response is included self-consistently and is shown to significantly reduce the current driving the harmonic radiation. Phase detuning severely limits the growth of the harmonic radiation. The effects of diffraction are considered in the mildly relativistic limit. No third-harmonic signal emerges from a uniform plasma of near-infinite extent. A finite third-harmonic signal requires the use of a semi-infinite or finite slab plasma. For an initially uniform plasma, no second-harmonic radiation is generated. Generation of even harmonics requires transverse gradients in the initial plasma density profile. >

Plasma wakefield generation and electron acceleration in a self-modulated laser wakefield accelerator experiment

A self-modulated laser wakefield accelerator (SM-LWFA) experiment was performed at the Naval Research Laboratory. Large amplitude plasma wakefields produced by a sub-picosecond, high intensity laser pulse (7×1018 W/cm2) in an underdense plasma (ne≈1019 cm−3) were measured with a pump–probe coherent Thomson scattering (CTS) technique to last for less than 5 ps, consistent with the decay of large amplitude plasma waves due to the modulational instability. A plasma channel was observed to form in the wake of the pump laser pulse, and its evolution was measured with the pump–probe CTS diagnostic. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) have been measured using an electro-magnetic spectrometer, with the energy spectra and divergence of lower energy (up to 4 MeV) electrons obtained using photographic films. Highly nonlinear plasma waves were also detected using forward Raman scattering diagnostics and were obser...

Incoherent Combining and Atmospheric Propagation of High-Power Fiber Lasers for Directed-Energy Applications

High-power fiber lasers can be incoherently combined to form the basis of a directed high-energy laser system which is highly efficient, compact, robust, low-maintenance and has a long operating lifetime. This approach has a number of advantages over other beam combining methods. We present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers. The experiment combined four fiber lasers using a beam director consisting of individually controlled steering mirrors. Propagation efficiencies of ~90%, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW were demonstrated in moderate atmospheric turbulence. We analyze the propagation of combined single-mode and multimode beams in atmospheric turbulence and find good agreement between theory, simulations and experiments.

Nonlinear wake-field generation and relativistic focusing of intense laser pulses in plasmas

The generation of nonlinear plasma wake fields by an intense, short laser pulse and the relativistic optical guiding of intense laser pulses in plasmas are studied with a nonlinear, self‐consistent model of laser–plasma interactions. Nonlinear steepening and period lengthening of the plasma waves are observed, and expressions are obtained for various nonlinear wake‐field quantities. Relativistic focusing with the self‐consistent plasma response shows that laser pulse fronts and laser pulses shorter than a plasma wavelength, 2πc/ωp, are not relativistically guided and will continuously erode due to diffraction.

Optically guided laser wake‐field acceleration*

The laser wake‐field acceleration concept is studied using a general axisymmetric formulation based on relativistic fluid equations. This formalism is valid for arbitrary laser intensities and allows the laser–plasma interaction to be simulated over long propagation distances (many Rayleigh lengths). Several methods for optically guiding the laser pulse are examined, including relativistic guiding, preformed plasma density channels and tailored pulse profiles. Self‐modulation of the laser, which occurs when the pulse length is long compared to the plasma wavelength and the power exceeds the critical power, is also examined. Simulations of three possible laser wake‐field accelerator (LWFA) configurations are performed and discussed: (i) a channel‐guided LWFA, (ii) a tailored‐pulse LWFA, and (iii) a self‐modulated LWFA.

Channeling of terawatt laser pulses by use of hollow waveguides

Subpicosecond laser pulses at power levels in excess of 1 TW were channeled through hollow microcapillary tubes by use of a combination of grazing-incidence dielectric and plasma-wall reflection mechanisms. Maximum input and output intensities were 10(17) and 10(16) W/cm(2) through 50-microm radius by 3-cm-long glass microcapillary tubes with as few as two waveguide modes being excited. 133-microm radius tubes as long as 13 cm resulted in successful channeling with an extinction coefficient of 0.2 cm(-1) and a plasma-wall reflectivity of 80%.