Generation of Tunable 10-mJ-Level Terahertz Pulses through Nonlinear Plasma Wakefield Modulation

High-density electron-bunch train–based tunable coherent terahertz radiation with a narrow spectral bandwidth is highly essential for many scientific applications. Here we propose a scheme to produce ultrahigh-peak-current (more than 10 kA) electron-bunch trains with tunable picosecond spacing for the generation of such terahertz sources. Passing through a plasma section and interacting with the self-excited nonlinear plasma wake, the beam gains a “sawtooth” energy modulation. By means of magnetic optics, it is then effectively converted into a beam density modulation forming microbunches with a high charge of up to a few nanocoulombs and a bunching factor as high as approximately $0.8$. Tunable narrowband terahertz radiation with energy on the order of 10 mJ can be generated, which we show in both theoretical analyses and simulations via this process. This scheme can be realized in accelerator facilities with repetition rates on the order of kilohertz or even submegahertz, which provides great potential in modern science and related technologies.

Figure 1

(a) Layout of the proposed concept. (b) The beam longitudinal phase space and current distribution at the entrance of the plasma modulator. (c) The longitudinal wakefield $E_z$ excited by the beam in the plasma. (d),(e) The beam longitudinal phase space and current distribution at the entrance and exit of the chicane, respectively.

Figure 2

Three-dimensional particle-in-cell simulation of the beam (propagating to the right) passing through the plasma modulator. (a) The densities of the plasma and the electron beam. The definition of $r_b$ is also illustrated. (b) The $E_z$ field excited by the electron bunch. The line out of the on-axis $E_z$ ($x=0\mu \mathrm{m}$, $\xi$) is shown with the solid green line. The dashed black line shows the slope of $E_z$ at the maximum blowout radius. (c) The transverse wakefield $W_{\perp }=E_x-c{B}_y$ excited by the electron beam. The line out of the off-axis $W_{\perp }$ along the dashed black line ($x=0.3k_p^{-1},\xi$) is shown with the solid green line.

Figure 3

(a) The beam $p_x$-$\xi$ phase space. (b) The beam longitudinal phase space and current profile.

Figure 4

(a) Bunching factor $b$ at the fundamental frequency of 1 THz for different $R_{56}$ values of the chicane. (b) The beam longitudinal phase space for the “full compression” case. (c) The corresponding beam current distribution. (d) The corresponding bunching factors at the fundamental and harmonic frequencies.

Figure 5

(a) The beam longitudinal phase space and the corresponding beam current distribution at the entrance of the undulator. (b) The radiation energy versus the propagation distance within the undulator.