Meter-Scale Terahertz-Driven Acceleration of a Relativistic Beam

Abstract

Terahertz (THz) radiation promises breakthrough advances in compact advanced accelerators due to the gigavolts-per-meter fields achievable, but the challenge of maintaining overlap and synchronism between beams and short laser-generated THz pulses has so far limited interactions to the few-millimeter scale. We implement a novel scheme for simultaneous group and phase velocity matching of nearly single-cycle THz radiation with a relativistic electron beam for meter-scale inverse free-electron laser interaction in a magnetic undulator, resulting in energy modulations of up to 150 keV using modest THz pulse energies (≤1 μJ). Using this scheme, we demonstrate for the first time the use of a laser-based THz source for bunch-length compression and time-stamping of a relativistic electron beam.

Femtosecond Relativistic Electron Beam with Reduced Timing Jitter from THz Driven Beam Compression

“…Ultrashort electron beams with small timing jitter with respect to external lasers are of fundamental interest in accelerator and ultrafast science communities. For instance, such beams are essential for laser and THz-driven accelerators ( [1][2][3][4]) where the beam energy spread and beam energy stability largely depend on the electron bunch length and injection timing jitter, respectively. For MeV ultrafast electron diffraction (UED [5][6][7][8][9][10][11][12]) where ultrashort electron beams with a few MeV energy are used to probe the atomic structure changes following the excitation of a pump laser, the temporal resolution is primarily limited by the electron bunch length and timing jitter.…”

Terahertz Streaking of Few-Femtosecond Relativistic Electron Beams

“…However, it requires very intense THz source in order to apply this scheme to MeV electron beam. It has been demonstrated recently that a MeV electron beam can also interact with a THz pulse effectively through the inverse FEL mechanism for beam acceleration and manipulation, but this scheme requires a dedicated undulator and careful matching of the phase and group velocity of the THz pulse with the electron beam [39]. It should also be noted that few femtosecond relativistic electron beam with intrinsically small timing jitter has also been produced in a laser wakefield accelerator where the accelerating gradient is several orders of magnitude higher than that achieved in rf guns, but the beam quality and stability still need significant improvements in order to apply the beam for UED applications [40].…”

“…Benefiting from the narrow slit, the shot to shot fluctuation of the beam centroid divergence is found to be about 7.6 µrad, corresponding to an uncertainty of 1.5 fs in beam arrival time determination. It should be mentioned that in principle one may rotate the slit and THz polarization by 90 degrees to imprint energy modulation in the electron beam and determine the beam arrival time by monitoring the energy change of the electron beam imprinted by the THz pulse, similar to that used in [39]. However, the accuracy will be much lower because one can't benefit from the narrow slit (the slit does not reduce the beam energy fluctuation) and the buncher cavity increases the beam energy fluctuation (the buncher cavity does not increase the beam centroid divergence fluctuation).…”

Terahertz-based subfemtosecond metrology of relativistic electron beams

“…From isolated gas atoms, photoelectrons ionized by XUV to hard X-ray pulses can be streaked by laser-generated THz radiation into an energy spectrum, which allows characterization of the temporal structure and timing jitter of those ionizing pulses with femtosecond resolution [21][22]. Besides extracting eV-levelenergy photoelectrons from nanotips and atoms through field emission, THz acceleration [23][24][25] and manipulation [26][27] of in-vacuum free electrons with significantly higher kinetic energies will open up a new era for beam physics and ultrafast science. In a few recent demonstrations, sub-relativistic, <100 keV kinetic energy electron beams from DC sources were compressed using laser-generated THz fields to tens of femtoseconds, with their timing jitter stabilized to a few femtoseconds as characterized by THz streaking [28][29].…”

“…Utilizing such dramatically improved temporal resolution, one can explore the intriguing opportunity of bright electron-beam based spatiotemporal mapping of the THz electromagnetic fields in metamaterial devices [44] and optically excited wakefields in plasma and dielectric structures with nanometer and sub-femtosecond resolutions. With stronger THz radiation [32] and more efficient interaction structures [25], THz metrology can be extended to GeV-kinetic-energy electron beams in FEL drivers.…”