N. E. Andreev

Stimulated processes and self‐modulation of a short intense laser pulse in the laser wake‐field accelerator

The basic equations for self‐consistent pulse evolution taking into account stimulated Raman backward and near‐backward scattering are formulated. These equations are used to study the three‐dimensional (3‐D) axisymmetrical self‐consistent laser pulse evolution analytically and numerically. Special attention is paid to the case of the pulse self‐modulation. The spectra and intensity of backscattered radiation are obtained in both the strong and weak coupling limits. A simple criterion to ignore the action of stimulated Raman backscattering on the pulse evolution is derived. The possibility of using a backscattered radiation spectrum for diagnostics of both the laser‐pulse and generated wake‐field evolution is discussed. Triggering of the laser‐pulse self‐modulation by the relativistic self‐focusing and by a second frequency‐shifted weak‐intensity laser pulse is discussed. Basing on the obtained results, a new configuration of stimulation and maintaining a strong wake‐field excitation is proposed. This con...

Plasma guiding and wakefield generation for second-generation experiments

A design study has been carried out for a second-generation experiment on laser guiding and wakefield excitation in a channel. From simple scaling laws for the wakefield amplitude, dephasing length, the relativistic group velocity factor /spl gamma//sub g/, and energy gain with and without guiding, we find that the parameter regime for a compact single stage GeV accelerator favors laser systems producing short pulses (10 fs/spl les//spl tau//spl les/100 fs), each containing an energy on the order of 100 mJ to a few Js. Taking the dephasing length as the maximum acceleration distance, plasma channels with lengths of 1-10 cm and densities of 10/sup 17/-10/sup 19/ cm/sup -3/ need to be produced; whereas the design study has been primarily concerned with diffraction and channel guiding, dephasing and depletion limits, and linear wakefield theory, aspects of the effect of the plasma wave on the evolution of the laser pulse are discussed. We find that transverse and longitudinal pulse distortions could indeed affect the generated plasma wave phase velocity and amplitude, and hence may limit the achievable energy gains over the one-dimensional (1-D) linear estimates. Some issues for experiments on prototype small accelerators (100 MeV-1 GeV, cm scale) are also discussed.

Structure of the wake field in plasma channels

An equation is derived that describes the linear response of an underdense inhomogeneous plasma [ω0≫ωp(r), where ω0 and ωp(r) are the laser-carrier and plasma frequencies, respectively] during the propagation of a laser pulse along the axis of a plasma channel with a characteristic width Rch. For a wide channel, i.e., when Rch/λp0>1 (where λp0=2πc/ωp0 is the wavelength of the excited plasma wave and ωp0 is the plasma frequency at the channel axis), the structure of the wake field is studied analytically. It is shown that this structure changes with the distance from the trailing edge of the pulse. As a result, at a certain distance behind the pulse, the fraction of the plasma wave period in which the simultaneous focusing and acceleration of electrons are possible increases by a factor of 2. For a narrow channel (Rch/λp0<1), the structure of the wake field is studied numerically and it is shown that, in this case, the doubling of the phase interval of the wave where the simultaneous focusing and accelerat...

Laser wakefield structure in a plasma column created in capillary tubes

The structure of the wakefield is studied in a plasma column, created by a monomode laser pulse propagating in a capillary tube, filled with gas affected by tunneling ionization. Linear analytical considerations as well as self-consistent numerical simulations show that in the central bulk part of a plasma column where the laser intensity exceeds the ionization threshold, the wakefield structure is similar to that of an infinite homogeneous plasma. Near the wall of the capillary tube, where the laser intensity decreases below the ionization threshold and where the plasma density falls to zero, the curvature of the plasma wave phase front increases with the distance from the laser pulse, resulting in small-scale radial electric field which may undergo phase mixing.

Laser-driven plasma waves in capillary tubes

The excitation of plasma waves over a length of up to 8 cm is demonstrated using laser guiding of intense laser pulses through hydrogen-filled glass capillary tubes. The plasma waves are diagnosed by spectral analysis of the transmitted laser radiation. The dependence of the spectral redshift-measured as a function of filling pressure, capillary tube length, and incident laser energy-is in excellent agreement with simulation results. The longitudinal accelerating field inferred from the simulations is in the range of 1-10 GV/m.

Guided propagation of short intense laser pulses and electron acceleration

The dynamics of a guided laser pulse propagation and wakefield generation in plasma channels, ionizing gases and gas filled capillaries is investigated. The efficient generation of a regular wakefield over long distances suitable for laser wakefield accelerators is demonstrated. The different schemes of electron bunch injection and compression to provide monoenergetic electron acceleration to high energies are analysed. A possibility of effective multistage acceleration of short electron bunches with low emittance and energy spread is demonstrated. The analytical results are confirmed by the results of numerical modelling.

Filamentation of ultrashort laser pulses propagating in tenuous plasmas

The filamentation of ultrashort laser pulses (shorter than a plasma period) propagating in tenuous plasmas is studied. In this regime relativistic and ponderomotive nonlinearities tend to cancel each other. Time-dependent residual nonlinear plasma response brings about the dynamical filamentation with the maximum unstable transverse wave number decreasing in the course of laser pulse propagation. Dynamics of a hot spot that seeds the filamentation instability is studied numerically and reveals a good agreement with the analytical results.

Linear theory of resonance self-modulation of an intense laser pulse in homogeneous plasma and plasma channels

The analytical solutions describing the linear stage of the intense-laser-pulse self-modulation, which results in a strong plasma wakefield excitation, are studied in terms of the paraxial approximation. The attention is focused on phase relations that were ignored in the previous studies. It is shown that the value of the phase velocity of the plasma wake wave differs from the pulse group velocity so that under some specific conditions, the relativistic factor corresponding to the phase velocity can be substantially less than that for the group velocity. This may be important for the particle acceleration in the self-modulated laser wakefield accelerator.

Analysis of laser wakefield dynamics in capillary tubes

A general approach to the modifications of the spectrum of a laser pulse interacting with matter is elaborated and used for spectral diagnostics of laser wakefield generation in guiding structures. Analytical predictions of the laser frequency red shift due to the wakefield excited in a capillary waveguide are confirmed by self-consistent modeling results. The role of ionization blue shift, and nonlinear laser pulse and wakefield dynamics on the spectrum modification, is analyzed for recent experiments on plasma wave excitation by an intense laser pulse guided in hydrogen-filled glass capillary tubes up to 8?cm long. The dependence of the spectral frequency shift, measured as a function of filling pressure, capillary tube length and incident laser energy, is in excellent agreement with the simulation results, and the associated longitudinal accelerating field is in the range 1?10?GV?m?1.

Coupling Efficiency of Intense Laser Pulses to Capillary Tubes for Laser Wakefield Acceleration

The simulations of the coupling of an incident laser pulse to a capillary tube with a cone-shape entrance are presented. The examples of the perfect Gaussian laser pulses and the experimentally measured radial intensity distributions are studied in a vacuum capillary, as well as the excitation of the wakefield inside a gas-filled tube.