Researchers at DESY’s LUX facility have demonstrated the first active energy compression of a laser–plasma electron beam, reducing its energy spread and jitter to below one-thousandth. This breakthrough achieves beam quality comparable to conventional accelerators, paving the way for compact plasma-driven light sources and storage-ring injectors. The article was published in the journal Nature.
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Background
Laser-plasma accelerators have seen rapid progress over the past two decades, thanks to advances in both laser technology and plasma physics. These compact systems can now generate electron energies in the GeV range within just centimeter-scale channels. However, their spectral precision remains a challenge. The inherently stochastic injection process, along with plasma instabilities and shot-to-shot variations, leads to broad energy spreads, typically a few percent, and significant energy jitter. These limitations make it difficult to integrate laser-plasma accelerators with conventional optical systems, especially in applications like free-electron lasers (FELs), which require extremely narrow energy spreads and high stability, on the order of permille levels, for coherent light generation.
Optical systems are critical for managing the electron beam’s phase space. Magnetic and electromagnetic elements are used to control both energy and longitudinal beam profiles. One key tool is the magnetic chicane, a type of dispersive optic that introduces a controlled energy–position correlation, or "chirp," across the electron bunch. This chirp effectively stretches the bunch and encodes an energy gradient along its length. Following this, RF electromagnetic cavities are used to remove the chirp, compressing the energy distribution and reducing jitter.
Together, these optical techniques replicate the sophisticated beam conditioning strategies found in modern RF accelerators, but they are tailored to accommodate the ultrashort, femtosecond-scale characteristics of laser-plasma-generated beams.
The Current Study
The experimental setup centered around the LUX laser-plasma accelerator, which generates the electron beams under investigation. The laser architecture employs a Ti:sapphire system delivering 2.2 J pulses with a 35-fsec pulse duration at a 1 Hz repetition rate, focused tightly into a 5-mm-long plasma channel. The plasma density profile is optimized with gas-doped regions to enhance injection and stability, using a downramp-assisted ionization mechanism to produce electron bunches with approximately 41 pC charge and initial energy of around 257 MeV, with an energy spread of about 1.8% and energy jitter of approximately 3.5%.
A symmetric four-dipole magnet array creates a dispersive beamline with a fixed R56 parameter (~100 mm). This optical element introduces a longitudinal spread by spreading electrons of different energies spatially, effectively stretching the pulse. The chicane’s optical design ensures that electrons with varying energy traverse different path lengths (an optical property akin to chromatic dispersion in optics), thereby creating a correlation between energy and longitudinal position (phase space tilt).
Acting as an electromagnetic optical element, this RF cavity operates in the S-band at 10 cm wavelength, producing a time-varying electric field that interacts with the extended bunch. Its phase is tuned such that the positive gradient cancels the induced energy–position correlation, effectively restoring a narrow energy bandwidth.
Precise optical measurements of the bunch’s longitudinal profile and energy distribution before and after the optical elements are conducted via spectrometers, and phase space diagnostics, ensuring that the optical manipulations achieve the targeted spectral conditioning.
Results and Discussion
The implementation of active energy compression yielded unprecedented results in spectral quality. Initially, the laser-plasma electron beam possessed a 1.8% energy spread, which effectively reduced the energy spread to 0.097% and the energy jitter to 0.048%, with ~50% of shots achieving sub-permille spread and a best case of 0.068%. This significant reduction was verified with spectrometric measurements demonstrating a high degree of control over the energy distribution, confirming the optical design's effectiveness.
The spectral phase space correction resulted in enhanced temporal coherence, enabling the generation of electron bunches with durations on the order of a few femtoseconds, an optical feat considering the initial micrometre-scale bunch lengths and plasma jitter. The beam's transverse properties, such as normalized emittance (~10 μm), remained stable throughout the process, ensuring that high-brightness properties essential for optical coupling were preserved.
By actively controlling the beam’s energy spectral phase, this method significantly reduced shot-to-shot fluctuations, resulting in a more stable and reproducible beam, an essential step toward precision optical experiments. Notably, the approach also showed that spectral quality could be preserved, or even improved, at electron energies beyond 250 MeV. This makes the system well-suited for integration with infrared and optical radiation generation chambers, expanding its potential for high-resolution, coherent light applications.
Conclusion
This work marks a significant step forward in combining optical and electromagnetic techniques for conditioning electron beams in laser-plasma accelerators. It shows that active optical control, similar in concept to pulse shaping in photonics, can effectively tackle the fundamental spectral limitations of plasma-based electron sources.
More broadly, the study highlights the powerful impact that optical principles can have when applied to accelerator physics. By bridging disciplines, it pushes the boundaries of what's possible with compact, high-performance electron sources driven by laser-plasma technology. The use of optical phase-space manipulation for beam conditioning isn't just an incremental improvement, it represents a meaningful advancement in the quest for precise, ultrafast control of both electron and photon beams.
Journal Reference
Winkler P., Trunk M., et al. (2025). Active energy compression of a laser-plasma electron beam. Nature 640, 907–910. DOI: 10.1038/s41586-025-08772-y, https://www.nature.com/articles/s41586-025-08772-y