ELEMENTARY PARTICLE PHYSICS

Laser Surfing: Scientists Unlock the Secret to Precise Electron Acceleration

Principal Investigator:
Dr. Daniel Seipt

Affiliation:
Helmholtz Institute Jena,

Local Project ID:
wobble

HPC Platform used:
JUWELS CPU of JSC

Date published:

In a breakthrough that could revolutionize particle accelerators, scientists have discovered how to better control high-energy electron beams using ultra-powerful lasers. This new understanding delves deep into the complex dance between intense laser pulses and the plasma they create, revealing the subtle mechanisms that influence electron beam stability.

Riding the Plasma Wave

Imagine surfing on an ocean wave. Now, picture electrons "surfing" on a wave of plasma created by an incredibly intense laser pulse. This is the essence of laser-wakefield acceleration (LWFA), a cutting-edge technique that promises to shrink particle accelerators from kilometers to just meters in length.

LWFA works by firing an ultra-short, ultra-intense laser pulse into a gas. The laser strips electrons from the gas atoms, creating a plasma. As the laser plows through this plasma, it pushes electrons aside, leaving a wake of positively charged ions. This wake, shaped like a bubble, creates enormous electric fields that can accelerate electrons to very high energies over very short distances.

However, just as surfers can be thrown off course by unpredictable waves, these electron beams can suffer from instabilities that affect their precision. Now, an international team of researchers has shed light on why this happens and how to control it.

The Wobble Problem and the Surprising Role of CEP

Dr. Daniel Seipt from the Helmholtz Institute Jena, who led the study, explains: "We found that the electron beam can develop a 'wobble' as it rides the plasma wave. This wobble can cause the beam to point in slightly different directions from shot to shot, which is a problem for applications requiring high precision."

The team discovered that this wobble is influenced by two key factors: the polarization of the laser light and its carrier-envelope phase (CEP) - essentially, how the peaks of the light waves align with the peak of the laser pulse envelope.

In a surprising twist, the researchers found that the CEP plays a major role in LWFA, even for relatively long laser pulses of 20-30 femtoseconds commonly used for LWFA. "Previously, it was believed that LWFA with these pulse durations would be CEP independent," says Dr. Seipt. "But we've shown that the wobble mechanism makes CEP control important, even for these longer pulses."

This revelation comes from the team's discovery of pulse self-steepening in the plasma. As the laser pulse travels through the plasma, its front edge is etched away, creating a sharp leading edge. This steepened pulse becomes sensitive to the CEP, influencing the electron dynamics in ways previously unrecognized.

The Dance of Bubbles and Electrons

The researchers identified a phenomenon that leads to the electron pointing jitter. Dr. Bifeng Lei, a co-author of the study, elaborates: "The plasma bubble that accelerates the electrons isn't perfectly stable. Its center oscillates, much like a wobbling top. This oscillation can couple with the natural oscillations of the electrons in the bubble, leading to a resonance effect."

This resonance, aptly named the "betatron-wobble-resonance," is the key to understanding why the electron beam's pointing direction can fluctuate from shot to shot. As the electrons accelerate, their oscillation frequency changes, eventually matching the bubble's wobble frequency. When this happens, the electrons can receive a significant "kick," altering their final trajectory.

Surfing in Sync

Using advanced computer simulations run on the JUWELS supercomputer at Forschungszentrum Jülich, the researchers were able to model how electrons behave under different laser conditions.

"Our simulations revealed fascinating dynamics," says Dr. Seipt. He points to two key figures from their study. "Figure 1 shows how the beam's transverse momentum evolves over time. You can clearly see the collective betatron oscillations of the electron beam. And in Figure 2, we see how these oscillations translate into beam pointing jitter. As the CEP varies, the final pointing angle of the beam fluctuates significantly."

But the team didn't stop at numerical simulations. "We were able to develop an analytical model using WKB approximation for the driven oscillator equation," Dr. Seipt explains. "Our model and numerical simulations agree well with experimental data obtained at the JeTi-200 laser at Helmholtz Institute Jena."

The analytical model allows the researchers to predict the size of the beam-pointing jitter based on the amplitude and frequency of the bubble centroid oscillation. It even predicts how the jitter changes with acceleration length and plasma density, matching experimental observations.

Towards Precision Control

The team found that the shape of the laser pulse front plays a crucial role. They also discovered that the process of electron injection into the plasma bubble can be crucial. In some cases, the laser's carrier-envelope phase can directly affect how electrons are injected, adding another layer of complexity to the wobble problem.

"It's like finding the perfect wave," says Dr. Lei. "We now understand how to 'tune' our laser pulses to minimize the wobble and get more consistent electron beams."

Dr. Seipt is enthusiastic about their findings: "Our work provides a comprehensive understanding of how these tiny fluctuations in the laser pulse can lead to significant changes in the electron beam properties. It's a crucial step towards making laser-wakefield accelerators more reliable and precise."

As scientists continue to refine their control over these "laser-surfing" electrons, we may be on the brink of a new era in particle physics. The waves of the future, it seems, will be made of plasma and ridden by electrons, with lasers as the ultimate surfboard - and now we know how to fine-tune that surfboard for the perfect ride.

References

[1] A. Seidel et al, PHYSICAL REVIEW RESEARCH 6, 013056 (2024).

[2] B. Lei et al, PHYSICAL REVIEW E 109, 015204 (2024).