Artist rendering of accelerator tube

Proof-of-concept for cascaded terahertz accelerator using long pulses. The mini-accelerator uses terahertz radiation that can be recycled for a second stage of acceleration. [Image: DESY, Science Communication Lab]

Once is not enough, according to the title of a sizzling 1970s romance novel—and now, too, for researchers working to deliver compact zing with tabletop particle accelerator tools. OSA Fellow Franz X. Kärtner’s group at Germany’s DESY (Deutsches Elektronen-Synchrotron) particle accelerator facility has developed the proof-of-concept for a miniature particle accelerator device that can recycle laser energy fed into the system to give the energy of accelerated electrons a second boost (Phys. Rev. X, doi: 10.1103/10.1103/PhysRevX.10.011067).

Toward mini-accelerators

“Terahertz-driven electron acceleration has recently emerged as a promising approach for developing compact accelerators,” explains co-author Dongfang Zhang. “Scaling accelerators to terahertz frequencies enables order-of-magnitude increases in field-induced breakdown thresholds, allowing for greater acceleration gradients. Meanwhile, it allows intrinsic synchronization between the electrons, the driving fields, and any probe lasers,” he says.

Terahertz (THz) radiation lies between infrared and radio frequencies in the electromagnetic spectrum. The new device is just 1.5 cm long and 0.79 mm in diameter, a miniature size made possible by THz radiation’s short wavelength.

As described in their paper, the researchers assembled a 53-keV phototriggered DC gun as an injector for the multi-staged, dielectrically lined waveguide (DLW)-based electron accelerator and manipulator, which is powered by narrowband, multicycle THz pulses.

The electron beam from the DLW device, the authors write, is analyzed by a segmented THz electron accelerator and manipulator (STEAM) device (used as a streak camera, to measure the electron bunch length with a resolution of approximately 150 fs), and by a tunable electromagnetic dipole coupled with a microchannel plate to measure the energy and deflection of the beam. An ytterbium laser creates UV pulses for photoemission in the gun, multicycle THz pulses to drive the DLW device, and single-cycle THz to drive the STEAM device.

Feeding the energy booster

The way it works, Zhang says, is that the researchers feed a multicycle THz pulse into the DLW, which reduces the speed of the pulse. Then, electrons are shot into the central part of the waveguide to travel along with the pulse, creating the first electron energy boost.

Because the electrons travel in the nonrelativistic regime (less than the speed of light), this enables recycling of the THz pulse for a second stage of acceleration, Zhang says.

As he describes it, once the THz pulse leaves the waveguide, it enters a vacuum, its speed is reset to the speed of light, and the pulse overtakes the electrons in just a few centimeters. The pulse then encounters another waveguide that slows its speed, creating an interaction section to boost the electron energies a second time.

Efficient acceleration

One of the current limitations of THz-driven accelerators for tabletop high-energy electron sources, according to Zhang, is the conversion efficiency of current laser-based THz generation, which is less than 1%. “This cascading scheme,” he says, “will greatly lower the demand on the required laser system for electron acceleration in the nonrelativistic regime.” Zhang adds that the setup also allows for implementing “multiple functions using a single multicycle THz pulse.”

Terahertz-driven accelerators are still in the early stages of development, says Zhang, who describes the DESY team’s new device as a proof-of-concept that shows electrons can gain energy in the waveguide—from 55 to about 56.5 keV, in this case. While that’s not a big gain, he says, a stronger laser could deliver stronger acceleration.

Game-changing approach?

Although the electron energy gains achieved thus far are still low, the team nonetheless believes that the new THz generations schemes, high-power-laser development and coupling efficiencies of the THz pulse could be game changers for the technology concept.

“It’s an alternative, compact, relativistic electron source for future ultrafast electron diffractometers and free-electron lasers,” Zhang says, “that can provide exceptional temporal and spatial resolution.” He cites possibilities for physics and chemistry, such as studies of the dynamics of photosynthesis, as well as potential applications like medical imaging.

The technique is relatively simple, Zhang says, and can be readily reproduced. “Several groups in the world are working to develop different schemes for effective THz-driven electron acceleration,” he says.