Laser-driven particle accelerator creates pairs of electron beams with different energies
Wakefield accelerator produces controllable energies in two pulses separated by only femtoseconds.
|Electron spectra depend on accelerator setting. Left: tuned to single bunch operation, Right: tuned to dual bunch operation while changing the energy of second bunch. (Image © Johannes Wenz)|
Stefan Karsch and his group at the Laboratory for Attosecond Physics (LAP), run jointly by the Max Planck Institute of Quantum Optics (MPQ) and Munich's Ludwig Maximilians University (LMU) in Garching, Germany, are making steady progress towards the goal of making small, practical laser particle accelerators for industry and medicine. As laser physicists, they are constantly in search of ever more efficient light-driven methods for the acceleration of subatomic particles. Karsch's work builds on the chirped pulse amplification (CPA) technique developed by Donna Strickland and Gérard Mourou to increase the intensity of ultrashort laser pulses (for which they won the Nobel Prize in Physics in 2018).
High-power laser pulses are at the heart of a particle-accelerator concept known as laser wakefield acceleration. When such a pulse is focused onto a gas jet, its wavefront detaches electrons from the gas molecules to form a plasma and its oscillating electric field then creates a plasma wave on which some electrons can surf and gain energy. Together, these effects can rapidly accelerate electron bunches to extremely high speeds over very short distances. A compact laser system can be used to accelerate electrons to velocities of up to 99.9999% of the speed of light within a distance of a few millimeters. These high-energy electron bunches can be used to investigate the ultrafast dynamic characteristic of the subatomic realm or to generate high-intensity x-rays for medical use.
But there is a problem with this approach: As a consequence of the extreme conditions in such a plasma accelerator, the plasma waves are prone to instabilities that are difficult to control. Now three members of the Karsch team – Johannes Wenz, Andreas Döpp, and Konstantin Khrennikov – have implemented two methods of controlling the trapping process of electrons in the wakefield. Their measurements demonstrate that this makes it possible to produce twin electron bunches with individually tunable energies.
This feat represents a significant advance in the control of laser-driven particle accelerators. The results lay the foundation for a new generation of experiments in ultrafast dynamics, as the new method generates paired electron bunches that are only a few femtoseconds apart. These electrons, or the synchrotron radiation associated with them, can therefore be used for pump-probe experiments on the rapid vibrational motions of molecules or other fast-paced aspects of atomic behavior. So far, such experiments have been restricted to a few compatible combinations of pump and probe sources. The advent of the new technique will provide bursts of electrons and/or multiple terahertz to gamma-ray region photon pulses for this purpose, which are also synchronized to the primary high-power laser pulse.
The Karsch group has already embarked on the construction of the next generation of their novel radiation source. With the ATLAS-3000 laser in LMU's new Center for Advanced Laser Applications (CALA), they are commissioning one of the most powerful lasers in the world. Potential medical applications of the newly acquired ability to create dual-energy electron bunches can now be explored, such as the development of compact, laser-driven x-ray sources for diagnostic purposes.