SELF-RESONANT PLASMA WAKE-FIELD EXCITATION BY A LASER-PULSE WITH A STEEP LEADING-EDGE FOR PARTICLE-ACCELERATION

The self-modulational instability of a relatively long laser pulse wit h a power close to or less than the critical power for relativistic se lf-focusing in plasma is considered. Strong wake-field excitation occu rs as the result of a correlated transverse and longitudinal evolution of the pulse. The dependence of the magnitude of the plasma wave on t he duration of a flat-top pulse is investigated. The power necessary t o reach 20% electron density modulation behind the laser pulse is show n to decrease as the pulse duration increases, while the phase velocit y of the plasma wave remains close to the group velocity of the laser pulse. This provides an opportunity to operate a laser wake-field acce ralator in the self-modulated regime at a subcritical laser power, at least twice less than the critical one, and to obtain a sufficiently l arge accelerating gradient (> 20 GV/m) in a region which is longer tha n that required for the acceleration of an ultrarelativistic particle.