In fields including biology, materials science, and particle physics, particle accelerators are critical instruments. Researchers are always seeking for more powerful techniques to accelerate particles in order to improve existing equipment and expand experiment capacity. Dielectric laser acceleration (DLA) is one such strong technique.
Particles are accelerated in this method by focusing ultra-short laser pulses on a nanophotonic structure, which creates an optical near-field. Researchers from the Chair of Laser Physics at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have successfully guided electrons down a vacuum channel, which is an important component of particle accelerators, using this approach. The photonic nanostructure channel’s fundamental design was created by TU Darmstadt, a collaboration partner. Their findings have now been published in the journal Nature.
Staying focused –
All accelerator technologies confront the issue of maintaining charged particles within the appropriate spatial and time bounds as they spread because charged particles prefer to drift further apart as they spread. As a result, particle accelerators may be up to 10 kilometers long and take years to prepare and build before they can be used, not to mention the significant financial commitment.
Dielectric laser acceleration, or DLA, makes advantage of ultra-fast laser technology and improvements in semi-conductor manufacturing to reduce the size of accelerators to a few millimeters or centimeters. A potential approach: Experiments have already shown that DLA outperforms currently available technology by at least 35 times. This implies that the length of a prospective accelerator may be cut in half.
It was unknown if these figures could be scaled up for longer and longer buildings until now. Prof. Dr. Peter Hommelhoff of FAU’s Chair of Laser Physics has led a team of scientists in a key step in adapting DLA for use in fully functional accelerators. Their research is the first to propose a method for guiding electron pulses over great distances.
Technology is key –
‘Alternating phase focusing’ (APF) is a technique that dates back to the early days of accelerator theory. It is impossible to concentrate charged particles in all three dimensions at the same time due to a fundamental rule of physics.
This can be prevented by concentrating electrons in various dimensions alternatively. Electrons are first focused using a modulated laser beam, then ‘drift’ through another short passage with no forces acting on them before being accelerated and steered onward.
The researchers from FAU and TU Darmstadt used a colonnade of oval pillars with small gaps at regular intervals to create repeating macro cells in their experiment. Depending on the delay between the incoming laser, the electron, and the gap that produces the drifting section, each macro cell has a focusing or defocusing impact on the particles.
At the optical or fem-to-second ultra-timescale, this arrangement enables for exact electron phase space control (a fem-to-second corresponds to a millionth of a billionth of a second). The beam current through the structure increases when a laser is shone on the structure in the experiment. If a laser is not utilized, the electrons will not be directed and will eventually smash against the channel walls.
‘It’s very exciting,’ says FAU physicist Johannes Illmer, co-author of the publication. ‘By way of comparison, the large Hadron collider at CERN uses 23 of these cells in a 2450 metre long curve. Our nanostructure uses five similar-acting cells in just 80 micrometers.’
When can we expect to see the first DLA accelerator?
‘The results are extremely significant, but for us it is really just an interim step,’ explains Dr. Roy Shiloh, ‘and our final goal is clear: we want to create a fully-functional accelerator on a microchip.’
The worldwide ‘accelerator on a chip’ (ACHIP) cooperation, of which the authors are members, is driving work in this field. The cooperation has previously demonstrated that APF can be modified in principle to achieve electron beam acceleration. Complex, three-dimensional APF installations may thus serve as the foundation for future particle accelerator technologies.
“We have to capture the electrons in all three dimensions if we are to be able to accelerate them over longer distances without any losses,” explains Dr. Uwe Niedermayer from TU Darmstadt, and co-author of the publication.