Continuously tunable narrowband pulses in the THz gap from laser-modulated electron bunches in a storage ring

This article reports on the generation of narrowband coherent synchrotron radiation from an electron storage ring. For the first time, this kind of radiation was now produced with continuously tunable frequencies in the so-called “THz gap” (between 1.2 and 5.6 THz), whereas previous experiments were limited to below 750 GHz. The experiment was performed at the DELTA storage ring in Dortmund, Germany, employing the interaction of external intensity-modulated laser pulses with an electron bunch, which causes a periodic longitudinal modulation of the charge density on a sub-millimeter scale. Furthermore, a strong influence of third-order dispersion in the laser pulses on the bandwidth and peak intensity of the THz radiation was observed. This effect is discussed in detail based on numerical simulations of the laser pulse generation and laser-electron interaction, and a modification of the laser system for compensating third-order dispersion is proposed.

Figure 1

Sketch of the chirped-pulse-beating setup used for the generation of narrowband laser-induced CSR at the electron storage ring DELTA in Dortmund, Germany, (a) and examples of a recorded laser pulse shape (b) and the corresponding narrowband THz spectrum (c).

Figure 2

THz gain curve for the short-pulse facility at DELTA according to Eq. (18) for turn 0 to 3 (see text for details).

Figure 3

Calculated modulation frequency $f_{\mathrm{m}}$ (a) and bandwidth $\mathrm{\Delta }{f}_{\mathrm{m}}$ (b) as function of the interferometer delay $\tau$ (see text for details).

Figure 4

Narrowband form factors resulting from start-to-end simulations with and without third-order dispersion $D_3$ (see text for details).

Figure 5

Top: narrowband form factors resulting from start-to-end simulations with (dotted) and without third-order dispersion (solid) for different modulation frequencies (see text for details). Bottom: gain curves extracted from the simulated form factors and according to theory (see Fig. 2).

Figure 6

Layout of DELTA and the short-pulse facility comprising a linear accelerator (“Linac”), a booster synchrotron (“BoDo”), the electron storage ring, the laser laboratory, the seeding beamline BL3, the electromagnetic undulator U250, the diagnostics beamline BL4, the THz beamline BL5a, the VUV beamline BL5, and the pump-pulse beamline (see text for details).

Figure 7

Top: measured narrowband THz form factors (solid lines) and corresponding laser modulation frequencies (dashed, see text for details). Bottom: gain curves extracted from the measured (top) and simulated form factors (see Fig. 5) and according to theory (see Fig. 2).

Figure 8

Narrowband 3-THz form factor (also shown in Fig. 7) and fast Fourier transform (FFT) of the corresponding laser pulse autocorrelation trace.