Relativistic Bi-Stability in a Plasma Beatwave Accelerator

Beatwave excitation of plasma waves is analyzed in the regime where relativistic corrections to the plasma frequency are important. It is shown that a long beatwave pulse can excite strong plasma waves in its wake even when the beatwave frequency is detuned from the electron plasma frequency. The wake is caused by the dynamic bi-stability of the nonlinear plasma wave if the beatwave amplitude exceeds the analytically calculated threshold. Two possible beatwave drivers are considered: intensity-modulated laser pulse and density-modulated electron beam. It is found that, due to the relativistic bi-stability, so portions of the driver may experience photon blue-shifting (for the laser driver) or electron acceleration (for the beam driver). In the latter case a combined accelerator/injector is envisioned.