A DESY team has built a two-stage mini-accelerator that recycles part of the laser energy that is fed in, thereby pushing the accelerated particles a second time. The device works with so-called terahertz radiation from the wavelength range between infrared light and radio waves. Each individual accelerator tube is only 1.5 centimeters long and 0.79 millimeters in diameter. Lead author Dongfang Zhang and his colleagues from Center for Free-Electron Laser Science (CFEL) at DESY present their experimental particle accelerator in the journal “Physical Review X”.

Terahertz radiation is around a thousand times shorter than the radio waves used in conventional particle accelerators, which means that all components can be much smaller. The still experimental devices currently do not achieve the energy and do not offer the high number of particles of large accelerators, but should enable applications in which large particle accelerators are not possible or not necessary.

“Terahertz-based accelerators have emerged as promising candidates for next-generation compact electron sources,” explains Franz Kärtner, lead scientist at DESY and head of the CFEL group that built the experimental device. “However, the technology is still in an early phase, and the performance of experimental terahertz accelerators is limited by the relatively short region of interaction between the terahertz pulse and the electrons.”

In contrast to previous experiments, the researchers feed their double accelerator with longer pulses from several cycles of terahertz waves. This significantly extends the interaction region with the particles to be accelerated. “We feed the multi-cycle terahertz pulse into a waveguide that is lined with a dielectric material,” explains Zhang. The speed of the terahertz pulse decreases within this waveguide. The researchers shoot an electron packet into the center of the waveguide so that it flies right through with the pulse. “This procedure extends the interaction region between the terahertz pulse and the electron packet to the centimeter range – compared to a few millimeters in previous experiments,” explains Zhang.

Although the experimental setup in the laboratory did not deliver much acceleration, the team was able to show that the electrons gain energy. “It is proof of the feasibility. The energy of the electrons rose from 55 to 56.5 kilo electron volts, ”reports Zhang. “A stronger acceleration can be achieved with a stronger laser, with which the terahertz pulses are generated.”

The structure has been designed primarily for the so-called non-relativistic area, in which the electrons are not yet approaching the speed of light. This enables recycling of the terahertz pulse for a second acceleration level. “As soon as the terahertz pulse leaves the waveguide and flies into a vacuum, its decelerated speed in the waveguide increases again to the speed of light,” explains Zhang. “This means that the pulse overtakes the slower electron package within a few centimeters. We have now placed the second waveguide at exactly the right distance so that the electrons cross it again together with the terahertz pulse, the speed of which is slowed down again in the second waveguide. This creates a second interaction region,

In a laboratory experiment, only a small part of the terahertz pulse could be recycled in this way. However, the experiment shows that this recycling is fundamentally possible, and Zhang is confident that the amount recycled can still be increased significantly. The team leader of the project from the CFEL group, Nicholas Matlis, emphasizes: “Our multi-stage process will significantly reduce the requirements for the laser system required for non-relativistic electron acceleration, which opens up new possibilities for the design of terahertz-based accelerators.”

Publication : Cascaded Multi-cycle terahertz driven ultrafast electron acceleration and manipulation; Dongfang Zhang, Moein Fakhari, Huseyin Cankaya, Anne-Laure Calendron, Nicholas H. Matlis and Franz X. Kärtner; “Physical Review X”, 2020; DOI: 10.1103 / PhysRevX.10.011067