A 30-fold increase in the energy of a quasi-unipolar terahertz radiation pulse in a remote focus was experimentally demonstrated for the regime of diffuse electron beam generation using a specially designed parabolic concentrator. The pulse of coherent terahertz radiation arose as a result of transition radiation of relativistic electrons accelerated by a femtosecond laser pulse of relativistic intensity (up to 7 10^18 W/cm ^2) as these electrons exited the plasma layer. The plasma layer was created by exposing a 3 m thick ethanol jet to a nanosecond laser pulse with an intensity of 10 10^12 W/cm ^2. Optimisation of the plasma layer parameters (maximum electron concentration and thickness) was achieved by varying the delay between the nanosecond and high-power femtosecond laser pulses.
Samsonov et al. (Mon,) studied this question.