Generation of high-power, tunable terahertz radiation from laser interaction with a relativistic electron beam

We propose a method based on the slice energy spread modulation to generate strong subpicosecond density bunching in high-intensity relativistic electron beams. A laser pulse with periodic intensity envelope is used to modulate the slice energy spread of the electron beam, which can then be converted into density modulation after a dispersive section. It is found that the double-horn slice energy distribution of the electron beam induced by the laser modulation is very effective to increase the density bunching. Since the modulation is performed on a relativistic electron beam, the process does not suffer from strong space charge force or coupling between phase spaces, so that it is straightforward to preserve the beam quality for terahertz (THz) radiation and other applications. We show in both theory and simulations that the tunable radiation from the beam can cover the frequency range of 1–10 THz with high power and narrow-band spectra.

Figure 1

Bunching factor evolution versus $k_1{R}_{56}\overline{\sigma }$ with $A=0.9$ and different slice energy distributions: (solid line) Gaussian distribution, (dashed line) Gaussian distribution but the bunching factor is calculated by Eq. (7) and (dotted line) double-horn distribution from laser heater.

Figure 2

Double-horn slice energy distribution of the electron beam when the laser spot size is much larger than the beam size in the laser heater. The detailed parameters can be found in Table 1.

Figure 3

Longitudinal phase spaces and the corresponding density profile with the largest bunching factor in the two different energy distributions with the same modulation depth $A=0.9$. The energy is scaled by the rms energy spread and the time is scaled by the period of the bunching. (a) and (b) are Gaussian energy distribution and (c) and (d) are double-horn distribution.

Figure 4

Schematic layout of the proposed beam line for tunable intense narrow-band THz radiation generation.

Figure 5

Laser pulse train distribution. The dashed line denotes the full sinusoidal modulation profile for comparison.

Figure 6

Bunching factor at 2 THz and peak current of the electron beam for different $R_{56}$ of the chicane. The laser power is fixed at 1 GW and zero energy chirp is added onto the beam.

Figure 7

Phase space of the electron beam with largest bunching factor. $R_{56}=-29\mathrm{mm}$ and laser power 1 GW.

Figure 8

Bunching factor and the FWHM of the bunching spectra for different radiation frequency by matching the $R_{56}$ and chirp energy. The laser power is fixed at 1 GW.

Figure 9

Bunching factor and the FWHM of the bunching spectra for different radiation frequency by matching the laser power and chirp energy. The $R_{56}$ is fixed at $-29\mathrm{mm}$.

Figure 10

(top) Bunching factor for different radiation frequency with and without the X-band cavity. (middle) The requirements for $R_{56}$ and chirp energy. (bottom) The requirement for energy gain of the X-band cavity.

Figure 11

THz pulse energy around 2 THz frequency in 5% bandwidth emitted from 10-ps beam for different beam charge.