Relativistic Attosecond Electron Pulses from a Photocathode Radio-Frequency Gun

The interaction of light and electrons has been one of the important frontiers for generating ultrashort electron pulses in free-electron lasers, ultrafast science, and dynamical analysis of matter. However, the generation of a relativistic attosecond electron beam remains a challenge. In a photocathode radio-frequency (rf) gun, this work identifies a regime for obtaining an isolated or a train of relativistic attosecond electron pulses. A photoelectron beam is generated from the cathode under the illumination of a driving laser pulse in the electron gun, which is subsequently microbunched to optical wavelength scale by the field of a synchronized radially polarized laser (RPL) pulse that is focused near the cathode surface. The rf field in the gun cavity simultaneously accelerates the electron pulse to several MeV. The rf field causes a velocity differential inside the electron beam as it passes into the region near the focal point of the RPL (microbunching is processed), and the microbunch at the head is substantially faster than that at the tail. Using this knowledge, the spacing between consecutive microbunches, referred to as bunch spacing, may be controlled across a wide range by regulating the velocity difference, which can be accomplished by tuning the phase and amplitude of the rf field. Numerical simulations show that a train of attosecond bunches with tunable bunch spacing can be generated using realistic laser parameters corresponding to current GW-power-level laser systems, and an isolated attosecond pulse can be obtained when the driving laser pulse is about 50 fs. This regime may result in opportunities in ultrafast electron diffraction and microscopy, free-electron lasers, and other applications that require high-energy electrons with the temporal structure of single-cycle light.

Figure 1

(a) The schematic layout and (b) the phase space and current distribution of an electron beam after interaction with a RPL, for two cases: with (right column) and without (left column) a cathode boundary.

Figure 2

Longitudinal phase space of the electron beam at the beginning of electron-laser interaction (a),(d),(g) and after the interaction and rf acceleration (b),(e),(h). Current distribution of the bunched electron beams (c),(f),(i). (a)–(c) $a_0=0.0018$, $E_{01}=60\text{MV/m}$, ${\varphi }_{01}={280}^{\circ }$; (d)–(f) $a_0=0.0015$, $E_{01}=40\text{MV/m}$, ${\varphi }_{01}={280}^{\circ }$; (g)–(i) $a_0=0.0018$, $E_{01}=40\text{MV/m}$, ${\varphi }_{01}={300}^{\circ }$. The beam head is on the right-hand side and the beam tail is on the left-hand side, also for the subsequent figures.

Figure 3

The phase space (a) and current distribution (b) of the electron beam.

Figure 4

Longitudinal phase space (a),(c) and current distribution (b),(d) of the electron beam. (a),(b) Charge: 0.0125 fC; (c),(d) charge: 0.025 fC.

Figure 5

The electron beam distribution at 0.55 m away from the cathode surface without $B_z$ (a) and with $B_z=0.162\mathrm{T}$ (b).

Figure 6

The ratio of bunch spacing to RPL wavelength as a function of rf field amplitude with optimized laser power (${\varphi }_{01}={280}^{\circ }$).

Figure 7

The ratio of bunch spacing to RPL wavelength as a function of rf field phase with optimized laser power ($E_{01}=40\text{MV/m}$).

Figure 8

Simulation results of the transverse emittance ($E_{01}=60\text{MV/m}$, ${\varphi }_{01}={280}^{\circ }$).

Figure 9

Simulation results of the pulse width (FWHM) as a function of the bunch charge (${\varphi }_{01}={280}^{\circ }$).

Figure 10

Simulation result of the current distribution as influenced by the laser energy jitter, phase jitter, and rf amplitude jitter.

Figure 11

Simulation result of the arrival time jitter at 0.55 m as influenced by the phase and rf amplitude fluctuation ($E_{01}=60\text{MV/m}$, ${\varphi }_{01}={280}^{\circ }$).