STRUCTURE OF THE WAKE-FIELD IN PLASMA CHANNELS

An equation is derived that describes the linear response of an underd ense inhomogeneous plasma [omega(0) much greater than omega(p)(r), whe re omega(0) and omega(p)(r) are the laser-carrier and plasma frequenci es, respectively] during the propagation of a laser pulse along the ax is of a plasma channel with a characteristic width R-ch. For a wide ch annel, i.e., when R-ch/lambda(p0)>1 (where lambda(p0)=2 pi c/omega(p0) is the wavelength of the excited plasma wave and omega(p0) is the pla sma frequency at the channel axis), the structure of the wake held 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 certa in distance behind the pulse, the fraction of the plasma wave period i n which the simultaneous focusing and acceleration of electrons are po ssible increases by a factor of 2. For a narrow channel (R-ch/lambda(p 0)<1), the structure of the wake field is studied numerically and it i s shown that, in this case, the doubling of the phase interval of the wave where the simultaneous focusing and acceleration of electrons are possible also occurs; but, in contrast to a wide channel, a rapid rec onstruction of the wake occurs, so that the amplitude of the axial (ac celerating) field in the wake decreases while the radial (focusing) fi eld increases with the distance from the pulse trailing edge. The nume rical modeling of the laser pulse (90 fs, 2 TW) guiding and the excita tion of plasma waves in a narrow plasma channel is carried out and the possibility of reaching GeV energies of accelerated electrons in an e xperiment is discussed. (C) 1997 American Institute of Physics.