Results of a fully relativistic three-dimensional (3-D) single particle code, supported by a theoretical model, on direct laser acceleration of electrons in radial electric and azimuthal magnetic static fields are presented. The ponderomotive force and the longitudinal components of the laser field are taken into account in the code. The electron motion in the static fields is similar to the motion in a magnetic wiggler. At resonance, when the bounce frequency of the wiggling motion is within a few percent of the Doppler shifted laser frequency, the amplitude of transverse oscillation shows a rapid increase accompanied by a fast rise in energy and parallel momentum. For this situation, a theoretical model of energy exchange between the electrons and the laser provides reasonable estimate of energy gain. The single particle code is used in Monte Carlo simulations to study the energy distribution and angular spread of the accelerated electrons in a self-focused high intensity laser pulse interaction.

A scheme of resonant terahertz radiation generation by the optical rectification of a picosecond laser pulse in rippled density magnetized plasma is examined. The x-mode laser pulse, propagating perpendicular to dc magnetic field, exerts a quasistatic ponderomotive force on electrons, imparting them a drift with finite transverse component. The drift velocity beats with the density ripple to produce a current, resonantly driving terahertz radiation at frequency comparable to the inverse pulse duration. The terahertz power scales as the square of density ripple amplitude and rises with magnetic field strength. In the case when magnetic field is azimuthal with appropriate r-dependence, the terahertz power has a ring shape intensity distribution. At laser intensity of ∼3×1015 W/cm2, in a 0.01% critical density plasma with 30 kG magnetic field and 30% density ripple, one may have power conversion efficiency of 0.04%.

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The steady state self-focusing of a Gaussian electromagnetic beam in a magnetoplasma has been studied. On a short time scale, a non-linearity in the dielectric constant of a plasma appears due to the ponderomotive force. This force in the case of the extraordinary mode has opposite signs for ~o > OJc and oJ < ~oo, where oJo is the electron cyclotron frequency. The self-focusing due to this effect is predicted at frequencies except for o~ c/2 < o, < ~o c. The focusing of the ordinary mode is adversely affected by the magnetic field. On a larger time scale, the non-uniform heating of electrons by the beam and the resulting redistribution of the electron density is a source of non-linearity. This non-local non-linearity is several orders of magnitude higher than the ponderomotive non-linearity. We predict self-focusing of the extraordinary mode only above the gyroresonance (oJ > oJc), while the ordinary mode can be focused at all frequencies. IntroductionIt is well known that when a plane electromagnetic wave propagates through a plasma, a non-linearity (i.e. a non-linear relation between current density and the electric vector) is introduced on account of the electron-energy-dependent collision frequency [1][2][3]. However, when an electromagnetic beam having a non-uniform intensity distribution along its wavefront (e.g. Gaussian) propagates through a plasma an additional mechanism of non-linearity [4] appears; a temperature distribution (due to the non-uniform heating of the electrons) is also set up along the wavefront. This temperature distribution along the wavefront leads to an electron density distribution along the wavefront and thereby an effective dielectric constant distribution in the plasma, which in turn leads to self-focusing. When the collision frequency is much less than the wave frequency it is the later non-local mechanism which is dominant.Another non-linear mechanism, which is important for collisionless plasmas and leads to self-focusing is the ponderomotive force on the electrons acting in an inhomogeneous electromagnetic field (such as that present in a Gaussian beam). The ponderomotive force arises due to (i) the drift of electrons in an inhomogeneous field (namely, the force = --m(v 9 V) v) and (ii) the interaction of electron drift velocity v with the magnetic

An intense short laser pulse or a millimetre wave propagating through a plasma channel may act as a wiggler for the generation of shorter wavelengths. When a relativistic electron beam is launched into the channel from the opposite direction, the laser radiation is Compton/Raman backscattered to produce coherent radiation at shorter wavelengths. The scheme, however, requires a superior beam quality with energy spread less than 1% in the Raman regime.