Generation of coherent THz Cherenkov radiation by electron bunch tilt control

We propose a new method that generates coherent Cherenkov radiation by controlling the tilt of the electron bunch. In this scheme, the wavelength band of coherent radiation depends on the transverse size of the electron bunch. This study demonstrates a coherent Cherenkov radiation (CChR) pulse in the THz region corresponding to the beam size. To generate the CChR, we used a 4.8-MeV electron bunch from a photocathode radio-frequency (rf) gun and controlled the angle through an rf transverse deflecting cavity. Our new method increased the pulse intensity by a factor of 7.3 compared to the case without tilting and the THz pulse characteristics well agreed with the prediction.

Figure 1

Diagram of Cherenkov radiation. When a charged particle travels with a velocity $\beta$ exceeding the velocity of light in a medium $c/n$, Cherenkov radiation is generated in the form of a cone with the particle’s orbit as its axis.

Figure 2

Phase-matching principle using the tilted electron beam. The Cherenkov radiation generated at each point of the electron beam trajectory can be overlapped by matching the Cherenkov angle with the electron beam tilt angle.

Figure 3

Experimental setup.

Figure 4

THz intensities on the tilt angle versus beam shift plane. The QOD signal, indicated by the color bar, represents the THz intensity measured by the QOD, which is normalized by the maximum value. The signal intensity increased at the Cherenkov radiation angle (a) but did not change at the reverse angle (b). The beam shift on the horizontal axis refers to the shift of the electron bunch position.

Figure 5

Comparison of calculated and measured values in the relationship between beam angle and THz intensity. The measured values are the average of six data around the beam $\mathrm{shift}=0\mathrm{mm}$ of each angle taken from Fig. 4.

Figure 6

THz intensity versus charge at various tilt angles. The THz intensity increases proportionally to the square of the charge at $\psi =49.2°$ and to the charge itself at $\mathit{\Psi }=49.2°$ and 90°.

Figure 7

Spot image of the CChR pulse at the focal point captured using a Terahertz camera. Solid and dotted lines are the measured and Gaussian-fitted values, respectively.

Figure 8

Polarization characteristic of the CChR pulse measured using a wire grid pair and a THz camera. The solid curve is the best-fitted curve to the data. Error bars represent the fluctuation of THz intensity with standard errors statistically processed by five data of THz camera image.

Figure 9

Schematic of the electro-optic (EO)-sampling measurements. The pellicle beam splitter performs coaxial injection of the coherent Cherenkov radiation pulse (CChR) and the femtosecond laser generated by the Yb fiber laser system into the EO crystal.

Figure 10

(a) Electric field waveform of a CChR pulse obtained by EO sampling and (b) its Fourier transform, calculated with a beam size of $400\mathrm{\mu }\mathrm{m}$ and a bunch length of 2.5 ps.