Scientists aim to create smaller, more powerful particle accelerators by using high-power lasers to fire ultra-fast, high-intensity laser pulses into plasma. This process generates acceleration gradients that are thousands of times stronger than those of traditional radio-frequency accelerators, potentially reducing kilometer-scale facilities to tabletop sizes.

These laser-plasma accelerators, or LPAs, could complement or enhance existing accelerators, including as sources of electrons for free-electron lasers (FELs) developed under the project, which are powerful photon sources that enable the exploration of matter at the atomic level. This reinforces applications in particle accelerators and colliders, medicine, materials science, and many other fields.

However, variations in the relative delay between the different colors or frequencies of the drive laser pulse—known as spectral phase fluctuations—impact the laser pulse shape and intensity, leading to instabilities in the accelerated electron beams, which reduce the performance of LPAs and their ability to drive reliable FELs and other applications.

Researchers from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new, non-invasive diagnostic tool that can directly measure and monitor spectral phase fluctuations in high-power lasers without disrupting laser and accelerator operations. This tool could help characterize and stabilize high-power laser pulses, making LPAs more reliable and practical for applications.

“Instabilities in high-power laser pulses have been a major obstacle in harnessing LPAs for demanding applications like FELs, which require not just powerful electron beams, but ones that are consistent and controllable,” explains Finn Kohrell, a graduate student at the BELLA Center in Berkeley Lab’s Accelerator Technology & Applied Physics (ATAP) Division and lead author of the study.

This work is part of the Lab’s ongoing efforts to develop precise, controllable LPAs. These multidisciplinary, cross-program efforts, which also include recent research using artificial intelligence-based feedback control to stabilize laser pointing in high-power lasers and broad controls to enhance accelerator performance, aim to develop AI controls to enable next-generation accelerators, improved laser and magnet performance, and quantum controls.

Non-invasive monitoring

According to Kohrell, the spectral phase of ultrafast laser pulses is one of the most critical—and elusive—parameters of high-power lasers; it determines the temporal shape of the pulse and governs how the different frequencies of the pulse are synchronized.

“An ideal laser pulse has all its wavelength components arriving at the target nearly simultaneously, but more importantly, in the same way for every shot. If this complex timing fluctuates, even slightly, the shape and peak power of the pulse can vary greatly from one shot to the next, resulting in unstable and unpredictable electron beams from the LPA.”

Spectral phase fluctuations, he notes, can be very subtle, requiring highly sensitive and precise measurement methods. “Achieving this sensitivity without removing too much energy from the pulse—or adding extra noise and distortions—is technically very challenging, especially in the extreme conditions of high-power laser systems.”

Experiments with LPAs, however, depend on the full, unaltered power and precise shape of the pulse hitting the target. Any distortion, energy loss, or phase change caused by the measurement tool would undermine the experiment.

To address this, the researchers aimed to directly measure and monitor the spectral phase fluctuations of a high-power laser system. Working with the BELLA Center’s Hundred Terawatt Undulator (HTU) facility—a world-class, 100-terawatt laser capable of delivering 2.5-joule pulses lasting tens of femtoseconds (millionths of a billionth of a second)—they designed and commissioned a non-disruptive experimental setup, which included leakage optics, chirped mirrors, variable attenuators, and a commercially available tool called the Frequency-Resolved Optical Gating (FROG) for measuring the intensity and spectral phase of the ultrashort laser pulses.

“FROG is like a high-speed camera for laser pulses,” explains Sam Barber, a staff scientist at BELLA, who was the principal investigator for the research and a lead author of the study. “The technique, when combined with our non-perturbative configuration, lets us capture not just the intensity but the entire structure of the pulse in time and frequency, on every shot, without disrupting the experiment.”

To prevent interference with the high-power beam, the team captured a small portion of the laser beam, known as a “leakage” beam—a low-power replica of the main pulse containing all the same temporal and spectral information—behind one of the highly reflective mirrors in the laser system. They then directed this to the FROG.

For each pulse, the FROG produced an image of its characteristics in both the time and frequency domains. A sophisticated retrieval algorithm then analyzed these traces to reconstruct the full spectral amplitude and phase of each pulse. By examining this continuous data stream, the researchers were able to detect subtle fluctuations in the spectral phase over time.

Barber says, “The key innovation was integrating the diagnostics tool into the beamline in a way that provides a detailed, shot-by-shot record while the laser is still running, allowing us to monitor, in real time, the evolution of the spectral phase from pulse to pulse, correlate these fluctuations with the performance of the LPA, and identify the source of the fluctuations.”

Uncovering the source of fluctuations

This new approach enabled the researchers to create a dataset illustrating the evolution of the spectral phase and its impact on the electron beam produced by the LPA, which revealed slow, long-term shifts in the spectral phase, lasting tens of seconds over many shots, rather than the quick, random jitters observed shot by shot.

After comparing datasets taken under different conditions, the team concluded that thermal effects in the multi-pass amplifier crystals—the power-boosting components of high-power lasers that use a charging and repeated pass process to amplify a weak initial pulse into a very powerful one—were much less responsible for the spectral phase fluctuations in the pulses than previously suspected. They found that the primary source of jitter originated in the front end of the laser, including the regenerative amplifier and the early-system laser pulse compressor and stretcher.

Showcasing the observed, significant difference in fluctuations of the pulse duration for two different diagnostic line configurations. Orange: The wedge pair is set to ideally compensate the dispersion on the diagnostic line, pulse duration measured with GRENOUILLE (𝜏𝑚) matches pulse duration of the main laser (𝜏0). Blue: The wedge pair is not set up to ideally compensate for the negative dispersion added by the chirped mirrors. This leads to the measured pulse being stretched with respect to the pulse on target.

“This was important because it showed us that the laser amplifying mechanisms were not contributing to these spectral phase fluctuations, but that the instability originated earlier in the laser chain, helping us to identify where to investigate further,” says Jeroen van Tilborg, a senior scientist and deputy director for experiments at BELLA and one of the authors of the study.

Kohrell says that “with this diagnostic tool, we can observe these fluctuations in real time, which helps us better understand the interaction between the laser pulse and the plasma, enabling us to fine-tune the entire system for optimum performance.”

The researchers plan to conduct independent measurements to verify that their system consistently operates in the so-called “bandwidth-limited regime”—a technical term for the shortest, most efficient pulses possible. They also aim to determine the root causes of the low-power beam instabilities they have observed to eliminate them.

“This work highlights Berkeley Lab’s development of active feedback control to combine the strong acceleration possible in plasmas with precision performance to bring compact, next-generation particle accelerators closer to reality for applications like FELs, high-energy and particle physics colliders, and beyond,” says ATAP Division Director Cameron Geddes.

The research presented here was supported by the U.S. Department of Energy’s Office of Science through the Office of Basic Energy Sciences, the Office of High Energy Physics, as well as through a Cooperative Research and Development Agreement with TAU Systems, Inc.

For more information on ATAP News articles, contact caw@lbl.gov