Scientists developing new optical systems is key to achieving unprecedented precision in high-power

2024-01-16

According to foreign media reports, the Berkeley Laboratory Laser Accelerator (BELLA) Center at the Lawrence Berkeley National Laboratory of the US Department of Energy has developed and tested an innovative optical system to accurately measure and control the position and pointing angle of high-power laser beams with new precision without interrupting or interfering with the laser beam. This new system will help users in the entire scientific community to make the most of high-power lasers.

This experimental validation work is led by Fumika Isono, a doctoral student from the Berkeley Laboratory and the University of California, Berkeley. Her research findings are described in a recent paper published in the Cambridge University Press journal High Power Laser Science and Engineering.

"Thisa huge advancement in measurement and control that will benefit high-power laser facilities worldwide," said Cameron Geddes, Director of the Accelerator Technology and Applied Physics Department (ATAP) at the Berkeley Laboratory. The BELLA Center is part of this department.

Interference free measurement

Some users with demanding application requirements know that laser beams move within a very small range to respond to even the vibrations and variability of controlled laboratory environments. Isono said, "Missing a target by just a few micrometers can create a difference between astonishing science and unnecessary supplementation of background noise." Minor directional shifts can also lead to unnecessary complexity. Thiswhere diagnostic sensors and feedback systems come into play.

Accurately measuring these parameters without intercepting the beam is crucial. Traditional methods either significantly reduce the power of the beam by intercepting its pulses (which is difficult for high-intensity, high-power beams), or errors occur due to inaccurate measurement of the transmitted beam. The innovative approach of BELLA Center includes segmenting and monitoring low-power precise copies of the main beam, which are reflected from the back surface of a specially designed final optical element in the beam line.

The core of this new method is a laser architecture with three key attributes. Firstly, it provides five high-power pulses and one thousand low-power pulses simultaneously per second, all of which follow the same path. Secondly, the design of the beam line has been optimized to match the size and divergence of high-power and low-power pulses. Finally, it replaced one of the reflective beam line mirrors with an innovative wedge-shaped mirror, which has a special coating on both the front and rear surfaces.

Almost all main beams are reflected off the front surface of the optical components without being significantly affected by other factors. A small portion of the beam (which may account for 1% of the input power) propagates through the front surface and reflects off from the back surface. This "witness beam" passes almost parallel to the main beam through any subsequent optical device and has sufficient shunt to facilitate the placement of measuring instruments. The final result is that the pointing angle and lateral position of the witness beam are related to the height of the main beam.

Isono said that the result is "a measurement that does not interfere with the main laser beam, but accurately tells us its situation.".

Benefits for BELLA Center and other locations

A recent goal of researchers is to use this diagnostic method as part of a feedback system to actively stabilize the lateral position and pointing angle of the laser. The preliminary research using a 100terawatt laser at the BELLA center is promising. This manuscript elaborates on the prospect of eliminating jitter in high-power 5Hz lasers by actively stabilizing low-power 1kHz laser pulse sequences. The vibration and motion of the laser beam were observed to occur on a scale of several tens of hertz, which is entirely within the range of practical feedback systems. It is expected that the position and angle of high-power laser pulse transmission will be improved five times.

The development of laser plasma particle accelerators (LPAs) is the main task of BELLA Center, which embodies the potential benefits of this innovation. LPAs generate ultra-high electric fields, which can accelerate charged particles very quickly, providing hope for the next generation of more compact and affordable accelerators, which can be used in various applications. Due to the fact that LPA accelerates within a thin hollow tube or capillary, they will greatly benefit from improved control of the position and pointing angle of the driving laser beam.

A direct application of BELLA Center is to use laser-driven plasma accelerators to provide electron beams to free electron lasers (FELs) - a device that can generate bright photon pulses with much higher energy and shorter wavelengths than visible light.

Isono said, "The oscillator, which is the magnetic array located at the core of the FEL, has very strict requirements for the acceptance of the electron beam, which directly affects the pointing angle and lateral fluctuations of the LPA driven laser."

The proposed kBELLA is the next generation laser system, which will combine high power and kilohertz repetition rate, and will be another possible application. "This work is not limited to laser plasma acceleration," said Eric Esarey, Director of BELLA Center. "It addresses a specific requirement of the entire high-power laser community, which is to demonstrate the relevant low-power copies of high-power pulses without obvious interference. In any application where high-power laser beams need to be transmitted with a certain degree of accuracy, this diagnostic device will bring significant changes. Think about laser particle collision experiments, or the interaction between lasers and micrometer level precision targets."

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