Small-scale X-rays challenge the large electron accelerators
Laser-driven X-ray radiation is a new technology providing shorter pulse duration and cheaper sources compared to large conventional facilities, but at much lower photon numbers. The present work published in Nature Physics, made in Lund with the cooperation of Umeå physicist László Veisz, significantly reduces the divergence of these X-rays. This is a fundamental requirement for any future applications in science, industry and medicine.
Text: Ingrid Söderbergh
In the first helium gas jet (green) an intense laser pulse (red) accelerated an electron pulse (blue) to relativistic velocity. Then in a second nitrogen gas jet (light blue) an electron lens refocuses the electrons that efficiently emit collimated and ultrashort (few-femtosecond) X-ray radiation (purple) as shown in the magnified image. Such a laboratory-scale X-ray source allows high-temporal resolution when filming the fastest dynamical processes in research, industry, and medicine.
X-ray radiation allows the investigation of structure and composition of materials, biomolecules, and even cells. Correspondingly, it plays a fundamental role in research, industry, and medicine. Laser-driven X-rays are emitted by high-energy electrons, typically from a conventional electron accelerator like the huge MAX IV facility outside Lund in Sweden.
Recently, alternative table top (size is from millimetre to meter) electron accelerators are being developed utilizing lasers. The advantage with these are that they possess much higher accelerating fields (1 000 times) that allows correspondingly shorter and cheaper accelerators. Furthermore, the duration of the generated electron bunches is also shorter. In all, this looks like a promising future accelerator technology.
The new technique is about very intense laser pulses that are focused in a gas jet, where they ionize the gas producing a mixture of electrons and ions, a so-called plasma. Furthermore, they also generate plasma waves, i.e., electron density oscillations.
“These plasma waves are efficient miniature electron accelerators with super strong accelerating electric fields”, explains László Veisz, leader of the study and professor at the Department of Physics at Umeå University.
The relativistic electrons accelerating and oscillating in the plasma waves, emit X-ray radiation that is up to 100 times shorter than their conventional large-scale counterparts that are normally larger than 100 m in size. The plasma waves under different conditions can act as strong electron lenses that refocus divergent electron beams.
A team of physicists from the universities in Lund and Gothenburg together with Professor László Veisz at the Relativistic Attosecond Physics Laboratory at Umeå University realized a novel form of this laser-based X-ray source based on the plasma lens emitting an order of magnitude better collimated (parallel) X-rays, a characteristic that is a fundamental requirement for applications.
This allows the producing of collimated few-femtosecond X-ray pulses and application of small X-ray optics. This permits possible future applications, such as filming via time-resolved X-ray diffraction and absorption spectroscopy.
“We did this successful finding in the laser laboratory by accelerating electrons by a laser in a first gas jet and then refocusing them by a lens formed in a second gas one. During this refocusing unprecedentedly collimated and ultrashort X-rays are emitted, optimal for application requiring moderate number of photons” says László Veisz.