Tunable High-Intensity Electron Bunch Train Production Based on Nonlinear Longitudinal Space Charge Oscillation

High-intensity trains of electron bunches with tunable picosecond spacing are produced and measured experimentally with the goal of generating terahertz (THz) radiation. By imposing an initial density modulation on a relativistic electron beam and controlling the charge density over the beam propagation, density spikes of several-hundred-ampere peak current in the temporal profile, which are several times higher than the initial amplitudes, have been observed for the first time. We also demonstrate that the periodic spacing of the bunch train can be varied continuously either by tuning launching phase of a radio-frequency gun or by tuning the compression of a downstream magnetic chicane. Narrow-band coherent THz radiation from the bunch train was also measured with $\mu \mathrm{J}$-level energies and tunable central frequency of the spectrum in the range of $\sim 0.5$ to 1.6 THz. Our results pave the way towards generating mJ-level narrow-band coherent THz radiation and driving high-gradient wakefield-based acceleration.

Figure 1

Schematic layout of the beam line for bunch train generation (not to scale). The inset plots are the longitudinal profile of the laser and electron beams, and the THz spectrum. The deflecting cavity is used to streak a portion of the beam across a luminescent screen to measure the beam’s longitudinal profile.

Figure 2

Measured longitudinal distribution (top) and projected density profile (bottom) of the bunch train with a horizontal slit before the deflector.

Figure 3

(a)–(d) Streak images and temporal profiles for four different plasma oscillation phase advances. (e) Simulated dependence of the final bunching on the phase advance. Bunch head lies to the left.

Figure 4

Normalized autocorrelation function of the CTR THz radiation (top) and the THz spectrum (bottom). The central frequency of the spectrum is $\sim 0.85\mathrm{THz}$ as the launching phase of the gun is 40°.

Figure 5

Central frequency of the THz radiation as a function of the launching phase in the gun.

Figure 6

Central frequency of the THz radiation as a function of the chicane current. The inset shows the $R_{56}$ of the chicane for different exciting currents.

Figure 7

THz radiation energy averaged over 100 shots for different single UV laser FWHMs before stacking. The blue curve is the simulated form factors. The central frequency is fixed at $\sim 0.85\mathrm{THz}$.