Energy enhancement and spectrum narrowing in terahertz electron sources due to negative mass instability

Simulations of coherent spontaneous undulator radiation in a waveguide demonstrate that the use of negative mass instability (NMI) for retaining longitudinal sizes of dense electron bunches, which are formed in laser-driven photoinjectors, allows one to increase power capabilities of a terahertz radiation source by many times. The NMI is realized in an undulator with combined helical and over-resonance uniform longitudinal magnetic fields due to nonisochronous longitudinal oscillations of electrons, whose frequencies increase/decrease with increasing/decreasing particle energy. In such conditions, an effective longitudinal size of the bunches can be preserved at long distance even at an extremely high electron density. Correspondingly, an energy extraction efficiency of more than 20% is revealed at a narrow frequency radiation spectrum, suggesting realization of a compact and powerful THz source.

Figure 1

Scheme of the radiation section: 1—magnetic system creating a transverse helical undulator and strong uniform longitudinal fields; 2—oversized metal circular waveguide; 3—electron bunch; 4—helical stationary electron trajectory.

Figure 2

Bunch density at the undulator entrance and after each 50 periods along the beam line: charges 0.5 nC (a), 1.5 nC (b), and 2.5 nC (c) for the positive guiding magnetic field, and 0.5 nC for the “negative” field (d). The time is given in the pictures relative to the “center of mass” of the bunch; presence of a bunch tail shifts the dominant peak of the bunch distribution out of the center.

Figure 3

NMI for three macrocharges of 0.5 nC each, moving in the undulator. Fast spreading with the negative guiding field (a) and close motion of two particles presenting a compact “cluster” while the third particle moves away of them presenting a spreading “tail” in a strong positive guiding field (b).

Figure 4

Bunching factor at the central radiation frequency of 1.5 THz as a function of number of undulator periods for various bunch charges.

Figure 5

Spectral radiation density $\mathrm{d}W/\mathrm{d}f$ for charges 0.5 nC (a), 1.5 nC (b), and 2.5 nC (c) obtained after 40 undulator periods. Normalized spectral density $(1/q_b)\mathrm{d}W/\mathrm{d}f$ of the $T{E}_{12}$ waveguide mode (d) is given for comparison.

Figure 6

Energy flux (a) and average power (b) along the beam line.

Figure 7

Charge dependence of radiation energy flux (a) and energy efficiency (b).

Figure 8

Frequency shift of the radiation during the motion of the bunch.