Taking terahertz-based electron acceleration to the next level

Probing molecular dynamics, which take place in femtoseconds, requires extremely fast and sustained particle acceleration capacity. A relatively new development, terahertz-based electron acceleration, may well supersede conventional particle accelerators to drive a new generation of compact ultrafast electron and x-ray sources. But electron gain has thus far been limited by short interaction distance and low terahertz (THz) energy.

Zhang et al. introduced a double vacuum channel phase-shifter that locally modulates THz phase velocity in the dielectrically-loaded waveguide, allowing electrons to sweep back and forth during acceleration for extended interaction lengths.

“For efficient electron acceleration, the electron bunch should stay in the accelerating phase of the THz wave over the entire THz-electron interaction region,” said coauthor Franz X. Kärtner. “However, with the increase of the electron speed, dephasing happens, which is one of the key challenges in electron acceleration driven by shortwave THz radiation.”

The novel phase-shifter design, which is simple and relatively easy to fabricate with small reflective losses, features a double vacuum channel to enable increases in both phase and group velocities of the THz wave. It can also be adapted to synchronous acceleration that suppresses not only dephasing but the “walk-off” between the electron bunch and temporal envelope of the THz pulse, which limits interaction duration.

“The work addresses a difficult problem of transitioning from non-relativistic to relativistic beams, an as-yet unsolved challenge for THz-driven electron acceleration,” said Kärtner. “But the potential here may help pave the way for compact MeV electron-beam generation, and is highly relevant to table-top acceleration devices and electron sources for probing material beyond current temporal resolution limits.”

Source: “Long range terahertz driven electron acceleration using phase shifters,” by Dongfang Zhang, Yushan Zeng, Moein Fakhari, Xie He, Nicholas H. Matlis, and Franz X. Kärtner, Applied Physics Reviews (2022). The article can be accessed at https://doi.org/10.1063/5.0096685 .