Capabilities of coherent spontaneous terahertz undulator radiation from short electron bunches

Density modulation of picosecond or sub-picosecond quasi-plane electron bunches formed in laser-driven photo-injectors cannot lead to excitation of non-linear plasma waves (unlike it is asserted in [1,2]). Nevertheless, though Coulomb repulsion increases axial scales, a density periodicity in such beams is nearly preserved. As a result, relatively long modulated quasi-plane bunches can emit higher energy and narrower spectrum than short single bunches with the same charge. Significantly higher effect can be obtained due to stabilization of a single bunch size in a combined undulator and strong guiding magnetic fields due to Negative Mass Instability (NMI).