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Phasics SID4 Camera Powers Wavefront Diagnostics at the Petawatt Scale

In October 2025, the ELI Beamlines research center near Prague announced a major milestone: the L4 ATON laser system reached a peak power of 5.1 PW, compressing 786J of energy into a 154 fs pulse at a repetition rate of one shot per minute. This result sets a new energy benchmark for multi-PW lasers and shows that petawatt systems can operate reliably at practical repetition rates.

The L4 ATON system is one of the few high-energy laser platforms that integrate large aperture glass amplifiers, liquid cooling, and adaptive optics into a single, continuously operating system. Its performance enables a new generation of experiments in laser driven particle acceleration, inertial confinement fusion (ICF), and strong field quantum electrodynamics (QED).

Figure 1. ELI Beamlines, Image credit: © ELI Beamlines

Precision Diagnostics with Phasics SID4 Wavefront Sensor

For high-energy laser facilities, every amplifier stage requires careful diagnostic verification to maintain beam quality. Among these tasks, accurate wavefront control throughout the amplification chain is essential for sustaining diffraction limited performance and ensuring reproducible operation.

According to a technical report released by compressing 786 J of energy into a 154 fs pulse at a repetition rate of one shot per minute, the amplifier characterization campaign demonstrated how Phasics' SID4 wavefront sensors played a central role in monitoring wavefront evolution and guiding adaptive optics (AO) correction strategies.

Figure 2. Setup of the 180 mm and 300 mm amplifier modules in the L4 ATON system.
Image credit: THRILL Project Report (ELI Beamlines, 2024).

During the amplifier characterization campaign, the Phasics' SID4 wavefront sensor was integrated into the diagnostic beamline to deliver real-time wavefront measurements and enable closed-loop adaptive correction.
With a phase sensitivity better than 2 nm RMS, SID4 offers high spatial resolution together with a wide dynamic range, making it well suited for resolving complex wavefront structures in high-energy amplifier stages.
In the 180 mm amplifier module (PA1), the SID4 was positioned as shown in Figure 3:

Figure 3. Test configuration for the 180 mm amplifier (PA1) module. Image credit: THRILL Project Report (ELI Beamlines, 2024).

A small diagnostic portion of the amplified beam was extracted through a vacuum window, attenuated, and then split into several channels to measure the beam profile, pulse energy, and wavefront at the same time. This layout limited additional aberrations from downstream relay optics and ensured accurate wavefront retrieval.

The SID4 sensor was coupled to a deformable mirror through Phasics' OASYS control software to form a closed-loop adaptive optics system. Using predefined mirror modes and modal reconstruction, the loop corrected low order aberrations in real time and maintained beam quality once thermal equilibrium was reached.

Wavefront evolution was evaluated using orthogonal mode decomposition to separate slow low order thermal distortions from higher order residual structure. Low order terms, mainly arising from alignment drift and environmental fluctuations, were effectively reduced by the AO loop, while the remaining high order components defined the steady state wavefront.

Under high repetition operation, the SID4 captured both single-shot variations and slower thermal drift, providing quantitative insight into how heat load affected beam quality over time. With adequate cooling and energy management, the wavefront converged predictably and stayed highly reproducible. Figure 4 shows the wavefront RMS evolution at various repetition rates.

Figure 4. Comparison of the wavefront evolution of the 180 mm amplifier for different flow rates through 4 amplifier modules with no active wavefront correction at 1 shot per minute repetition rate.  Image credit: THRILL Project Report (ELI Beamlines, 2024).

At higher repetition rates, the wavefront RMS increased at the start of operation as thermal load accumulated, then settled to a stable value after roughly 20–30 minutes. At lower repetition rates, the wavefront remained stable from the beginning.

These results show the balance between thermal diffusion, coolant flow, and the limits of adaptive optics correction. Higher flow rates provided a slightly faster approach to equilibrium, but all conditions converged after about 30 minutes, indicating that a 2 L/min flow rate is adequate for routine operation.

From a cold start, the system produced a clean focus with a sharp far field peak. As the amplifier warmed, astigmatism and mid spatial frequency structure developed, broadening the focal spot and introducing rings, which reduced the peak intensity.

Figure 5. Comparison of spatial profiles of the beam in nearfield vs far field in cold versus hot state of the 180 mm amplifier at 1 shot per 3 minutes repetition rate and pre-corrected wavefront. Image credit: THRILL Project Report (ELI Beamlines, 2024).

For the 300 mm amplifier (PA2), adaptive correction remained active throughout the test campaign. The SID4 sensor monitored the deformable mirror response and quantified the remaining wavefront error.

Figure 6 shows the wavefront RMS evolution at a repetition rate of one shot per minute with the correction loop engaged. Low order aberrations increased over the first ten shots as the amplifier warmed, then settled once thermal balance was reached. High order residuals stayed below roughly 0.2 λ RMS, which reflects the correction limit of the system.

The stability and repeatability of the SID4 allowed these small fluctuations to be tracked with high confidence level, providing a clear picture of long term system performance.

Figure 6. Wavefront RMS evolution at one shot per minute with active AO correction (300 mm amplifier). Image credit: THRILL Project Report (ELI Beamlines, 2024).

Figure 7 shows that although the far field pattern exhibits some thermal induced broadening, the beam profile remains largely unchanged. This indicates a stable spatial gain distribution with no evidence of thermal imbalance, even during repeated shots.

Figure 7. Top: Spatial profiles in nearfield for cold state(above)/ of focused beam for cold state(below)  (a) and a sequence of three consecutive shots (b, c, d) from the 1 / 2 min repetition rate measurement. Image credit: THRILL Project Report (ELI Beamlines, 2024).

Both amplifier stages showed stable spatial gain across all tested energies and repetition rates. This confirms the strength of the L4 ATON amplifier chain and the reliability of its adaptive optics control.

Across the full test campaign, Phasics' SID4 wavefront sensor was a key part of the diagnostic system. It provided accurate wavefront tracking under different thermal conditions and repetition rates, and supplied the data needed to keep the amplifiers performing well during long, high repetition operations.

We congratulate the ELI Beamlines team on this major achievement. We are glad that SID4 contributed to the project, and we look forward to supporting the next steps of the L4 ATON program.

Have a similar project in mind?
Contact Phasics to discuss how SID4 wavefront sensing can support your high power or ultrafast laser systems.
We’re here to help you implement reliable diagnostics in the most demanding environments.

📩 Get in touch here or send email to contact@phasics.com

 

Reference

ELI‑Beamlines, GSI, & HZDR. (2024). Report on the characterization of ATON laser amplifier (D4.1, Version 7.1).