Structure of the wake field in plasma channels

An equation is derived that describes the linear response of an underdense inhomogeneous plasma [omega(0) much greater than omega(p)(r), where omega(0) and omega(p)(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 R-ch. For a wide channel, 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 plasma 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 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 (R-ch/lambda(p0)<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 acceleration of electrons are possible also occurs; but, in contrast to a wide channel, a rapid reconstruction of the wake occurs, so that the amplitude of the axial (accelerating) field in the wake decreases while the radial (focusing) field increases with the distance from the pulse trailing edge. The numerical modeling of the laser pulse (90 fs, 2 TW) guiding and the excitation of plasma waves in a narrow plasma channel is carried out and the possibility of reaching GeV energies of accelerated electrons in an experiment is discussed. (C) 1997 American Institute of Physics.