Nano-scale modulation generation

Generating extremely short structures inside an electron beam is important for producing coherent radiation, driving wakefields, and studying ultrafast beam dynamics. However, directly shaping a beam at nanometer or sub-micrometer longitudinal scales is very challenging.

This research uses emittance exchange, or EEX, to convert a transverse beam pattern into a longitudinal structure. Since transverse patterns are easier to create and control, EEX provides a practical way to generate very fine longitudinal modulation.

Our work explores how carefully designed transverse masks, beamline optics, and EEX configurations can produce nano-scale or sub-micron bunch structures with high fidelity.

Our method begins by imprinting a transverse modulation on the electron beam using a TEM grid. After passing through the grid, the beam is broken into a periodic transverse pattern with an initial modulation period of a few to several tens of micrometers. This provides a well-controlled starting structure that can later be transformed into the longitudinal direction.

Next, the beam passes through a demagnifying beamline, which reduces the transverse modulation period to the few-micrometer scale. The beam envelope evolution along this section is carefully designed so that the modulation is compressed while maintaining beam quality. This step is important because it pushes the initial pattern to a much finer scale before the phase-space exchange.

The reduced transverse modulation is then sent through an asymmetric emittance-exchange (EEX) beamline. In this section, the beam’s transverse and longitudinal phase spaces are exchanged, converting the transverse modulation into a longitudinal one. At the same time, the asymmetric layout further demagnifies the modulation, allowing the final longitudinal structure to reach the few-hundred-nanometer scale. This asymmetric EEX configuration is also part of our proposed strategy for mitigating coherent synchrotron radiation (CSR) effects, which can otherwise degrade fine beam structures.

The final result is a beam with a clearly modulated longitudinal phase space, as shown in the zoomed-in image. The corresponding bunching factor confirms that the modulation is strong and well defined at the target frequency. Together, these results show that a transverse pattern created at the micrometer scale can be transformed into a much finer longitudinal structure, providing a practical path toward nano-scale beam modulation for advanced accelerator and radiation applications.