Particle physics is witnessing a transformation with the advent of laser-plasma accelerators. These compact powerhouses redefine how we accelerate particles, offering a smaller, more efficient alternative to traditional facilities that often stretch across kilometres. The ability to house a powerful particle accelerator in the basement of a university institute could soon be a reality, thanks to advances in laser-plasma technology.
The Basics of Laser-Plasma Acceleration
At the core of this technology is a simple yet powerful concept. Laser-plasma accelerators utilize intense laser pulses to ionize a gas, creating a plasma—a hot mix of electrons and ions. As the laser pulse travels through this plasma, it pushes the lighter electrons forward, leaving behind a positively charged “bubble.” By injecting electrons into this bubble, they can be rapidly accelerated, reaching speeds that would typically require vast and expensive conventional accelerators. Remarkably, this process occurs over just a few centimetres, compared to the hundreds of meters needed in traditional setups.
Applications and Challenges
One of the most promising applications of laser-plasma accelerators is in free electron lasers (FELs). FELs require precisely bundled electron bunches that travel close to the speed of light through an undulator—a series of magnets that make the electrons follow a zigzag path. This motion causes the electrons to emit powerful laser-like X-rays or ultraviolet (UV) light, which can be used to observe processes occurring in mere quadrillionths of a second, such as chemical reactions.
Despite the immense potential of laser-plasma accelerators, significant challenges exist. The electron bunches produced by these accelerators must be finely controlled and exhibit specific properties to generate UV or X-ray light effectively. Achieving this level of precision has proven difficult, especially when accurately measuring these ultra-fast, ultra-small electron bunches.
A Breakthrough in Measurement
Researchers at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have recently made a significant breakthrough that could propel the development of laser-plasma acceleration technology forward. Dr. Maxwell LaBerge, a postdoctoral researcher at HZDR, has developed a novel method to measure these elusive electron bunches with unprecedented precision.
The method involves shooting electron bunches from a plasma accelerator onto a thin metal foil at nearly the speed of light. When these electrons hit the foil, they cause the surface electrons of the foil to move, generating a signal similar to that of a transmitting antenna. By detecting this signal, researchers can reconstruct the shape and structure of the electron bunches in detail. This process, known as Coherent Optical Transition Radiation (COTR), offers a new way to analyze and control the properties of electron bunches.
The HZDR team has explored different methods of injecting electrons into the plasma bubble using this innovative technique. Their findings reveal that different injection techniques produce varying forms of electron bunches, highlighting the potential of this method to fine-tune and control the bunches more precisely. Improved control over these electron bunches directly translates to brighter, more stable light in FELs, paving the way for more powerful and efficient X-ray and UV lasers.
The Future of Compact Accelerators
The development of laser-plasma accelerators represents a significant leap forward in making advanced research tools more accessible. Conventional FELs, like the European XFEL in Hamburg, rely on traditional linear accelerators that span several kilometres and come with hefty price tags. As a result, access to these facilities is limited, with only a select few research teams worldwide able to secure the necessary beamtime.
In contrast, laser-plasma accelerators could be constructed to be both compact and cost-effective, allowing even university institutes to house their FELs. This would democratize access to cutting-edge research tools, enabling more teams to conduct experiments that were previously out of reach.
Since 2021, research groups in Shanghai, Frascati, and Dresden have demonstrated the feasibility of FELs based on laser-plasma accelerators. Their progress, detailed in a review article in the journal Nature Photonics, outlines the current state of development and the challenges that remain.
Among these challenges are improving the quality and stability of the accelerated electron bunches and minimizing the energy spread within them. As research continues, advancements in diagnostic methods like the one developed by LaBerge and his team will be crucial in overcoming these obstacles.
Conclusion
Laser-plasma accelerators are on the brink of revolutionizing particle physics and related fields. By offering a compact, efficient, and cost-effective alternative to traditional accelerators, they could bring powerful research tools into the hands of many more scientists and researchers. With ongoing advancements in measurement and control techniques, the dream of having a high-performance particle accelerator in the basement of a university—or even in smaller laboratories—is becoming increasingly attainable. The future of particle acceleration is bright, compact, and full of potential.
Maxwell LaBerge et al, Revealing the three-dimensional structure of microbunched plasma-wakefield-accelerated electron beams, Nature Photonics (2024). DOI: 10.1038/s41566-024-01475-2
