This Tiny Particle Accelerator Recycles Energy With Terahertz Waves

A team of scientists built a miniature double particle accelerator that recycles laser energy initially fed into the system to boost the energy of electrons for a second go, according to a new study published in the journal Physical Review X.

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Particle Accelerator Recycles Energy

Making use of narrowband terahertz radiation lying between infrared and radio frequencies in the electromagnetic spectrum, the device itself is a single, 1.5-centimeter long and 0.79-millimeter in diameter object. Dongfang Zhang and his colleagues from the Center for Free-Electron Laser Science (CFEL) at DESY unveiled their experimental accelerator in the journal Physical Review X.

The tiny stature of the novel device is possible because of the short wavelength of terahertz radiation. "Terahertz-based accelerators have emerged as promising candidates for next-generation compact electron sources," said Franz Kärtner, head of the CFEL group that built the device, and Lead Scientist at DESY. Scientists have completed successful experiments with terahertz accelerators before, which allowed applications where large particle accelerators are neither feasible nor necessary. "However, the technique is still in an early stage, and the performance or experimental terahertz accelerators has been limited by the relatively short section of interaction between the terahertz pulse and the electrons," said Kärtner, according to phys.org.

Terahertz waves save laser energy

To operate the new device, the team used a longer pulse composed of many cycles of terahertz waves. They were able to extend the interaction section with the particles using this multicycle pulse to great success. "We feed the multicycle terahertz pulse into a waveguide that is lined with a dielectric material," said Zhang, according to phys.org. 

The new device didn't produce a large acceleration in the lab. But the team might prove their concept by showing how electrons gain energy in a waveguide. "It is a proof of concept. The electrons' energy increased from 55 to about 56.5 kilo electron volts," said Zhang, to phys.org. "A stronger acceleration can be achieved by using a stronger laser to generate the terahertz pulses."

This arrangement was mainly intended for non-relativistic physics, which means electrons have speeds that are not quite as fast as light itself. Notably, non-relativistic physics allows for the recycling of the terahertz pulse for a second stage of acceleration. "Once the terahertz pulse leaves the waveguide and enters the vacuum, its speed is reset to the speed of light," Zhang explained to phys.org. "This means, the pulse overtakes the slower electron bunch in a couple of centimeters. We placed a second waveguide at just the right distance that the electrons enter it together with the terahertz pulse which is again slowed down by the waveguide. In this way, we generate a second interaction section, boosting the electrons' energies further."

Within the lab experiment, only a fraction of the terahertz pulse could be recycled in this way. But the experiment showed that recycling energy in paticle accelerators is feasible in principle, and Zhang believes the recycled fraction might be substantially increased. Nicholas Mattlis, senior scientist and the team leader of the project of the CFEL group, stressed: "Our cascading scheme will greatly lower the demand on the required laser system for electron acceleration in the non-relativistic regime, opening new possibilities for the design of terahertz-based accelerators."