Undulator radiation of premodulated and nonmodulated electron bunches in the negative mass instability regime

The use of both spatial premodulation and negative mass instability is promising for the energy enhancement and spectrum narrowing of the undulator radiation from dense electron bunches formed in modern laser-driven photoinjectors. Combining these two methods will permit the formation of long-lived trains of short and dense bunches with very large charges, which are able to efficiently emit a much more powerful and narrow-band terahertz radiation than single bunches with the same total charge, or premodulated bunches in the positive mass oscillation regime. The development of radiation instability in the negative mass regime can enable significant quasiperiodic self-modulation of relatively long nonmodulated bunches resulting in their fairly efficient and selective terahertz superradiance.