Fig. 1. (a) Schematic layout of tunable intense narrowband THz radiation generation, which consists of a uniform plasma section, a chicane, an undulator, and three quadrupoles. (b) Simulation result of plasma density when the drive beam and the main beam (propagating from left to right) pass through the uniform plasma successively. (c) The Ez field excited by two beams (left figure), and the lineout of the on-axis Ez (x=0μm, ξ) (right figure) illustrating an ideal sawtooth shape.

Fig. 2. Simulated characteristics of electrons after the main beam of 135 MeV is compressed in the chicane. (a) The beam longitudinal phase space for the “full compression” case, with R56=−0.96mm. (b) The corresponding beam current distribution with bunch length of each microbunch reduced to ∼50fs. (c) The bunching factor values at the fundamental and harmonic frequencies. The b value reaches 0.94 at the fundamental frequency and over 0.5 at high harmonic frequencies.

Fig. 3. Radiation energy versus the propagation distance within the undulator. The emitted THz pulse energy can reach an energy as high as 2.4 mJ in a 5 m-long undulator.

Fig. 4. Simulated characteristics of electrons after the main beam of 50 MeV is compressed in the chicane. (a) The beam longitudinal phase space with an obvious curvature, resulting from the nonlinear process of magnetic compression. (b) The corresponding beam current distribution. (c) The corresponding bunching factor values at the fundamental and harmonic frequencies after density modulation. The bunching factor is 0.85 at the fundamental frequency.

Fig. 5. After the compression process in the chicane under the two plasma lengths, (a) the beam phase space when the plasma lengths are 3.7 (red) and 2.7 mm (blue), respectively. The energy spread is lower when the plasma length is reduced. (b) The current distribution comparison. (c) The bunching factor is increased from 0.85 to 0.9 at the fundamental frequency of 1 THz when the plasma length is reduced from 3.7 (red) to 2.7 mm (blue).