Physicists in the US, Germany and Canada have built a miniature particle accelerator that uses terahertz radiation instead of radio waves to create pulses of high-energy electrons. A single accelerator module of the prototype is just 1.5 cm long and 1 mm thick, and the technology has the potential to create facilities that are much smaller than current radio-frequency (RF) accelerators. Potential applications include free-electron lasers, whereby the electrons are used to create coherent pulses of X-rays. However, the team cautions that much more work is needed to develop the technology so it can be used in medicine, particle physics and material science.

Terahertz radiation falls between the microwave and infrared portions of the electromagnetic spectrum (300 GHz–3 THz), and its production and detection are not without significant technical challenges. However, terahertz technologies have been improving steadily and some physicists are keen on using the radiation in much the same way that radio waves and microwaves are used to accelerate charged particles.

In this latest work Emilio Nanni and colleagues at the Massachusetts Institute of Technology (MIT), the Center for Free-Electron Laser Science (CFEL) at DESY in Germany and the University of Toronto have created a terahertz accelerator module with the aim of advancing experiments that use ultrafast electron diffraction to reveal the structure and dynamics of matter. Their prototype accelerator uses optically generated pulses centred at 450 GHz and a bandwidth of 200–800 GHz. The wavelength of this radiation is around 1000 times shorter than the electromagnetic radiation used by current particle accelerators – the Large Hadron Collider uses 400 MHz microwaves – everything else on the terahertz accelerator can also be 1000 times smaller.

Steep gradients

The terahertz accelerator module increased the energy of electrons fired into it by 7 keV. This is a boost of 2.3 MeV/m, which is modest compared to conventional accelerators that can achieve about 50 times that. But, it does show that terahertz accelerators are feasible and the researchers say that it should be possible to achieve accelerating gradients of around 1 GeV.

"If we can match the accelerating gradient available with radio-frequency sources (around 100 MeV/m), terahertz accelerators could be attractive for many applications because they can operate at higher repetition rates and are more efficient," explains Nanni. "Terahertz accelerators should be able to achieve much higher accelerating gradients than radio-frequency accelerators, which will reduce the size and cost of accelerators and improve the quality of the electron beams they produce."

Steven Jamison of the UK's Accelerator Science and Technology Centre (ASTeC), who wasn't involved in the research, says "While this demonstration of terahertz-driven acceleration is done with low-energy beams and significant challenges remain in scaling to higher energies and longer interaction lengths, it is an important first step to obtaining relativistic energy electrons with terahertz waves."

More power needed

The main barrier to faster accelerating gradients is the power of terahertz pulses that can be generated. "Considerable development is still necessary" in this area, explains team member Franz Kärtner, whose lab at MIT was used to test the prototype.

The researchers now plan to focus on developing a free-electron laser (FEL) based on terahertz technology, which they expect to be less than 1 m long. FELs fire high-speed electrons down an undulating path, which causes them to emit intense flashes of X-ray light. Currently, access to large-scale FELs is limited, but this would be "a low-cost system that can be integrated into laboratories with modest lasers" says Nanni.

Kärtner says that they are aiming to create a FEL that will "generate sub-femtosecond X-rays with a high brilliance". Such a system could be used to study the water splitting process in photosynthesis and other molecular processes, he adds.

Creating a FEL would require significant advancement of the terahertz technology. In particular, pulses that deliver around 20 mJ of terahertz energy would be needed. In contrast, their prototype accelerator gets by on 10 µJ. More powerful sources are available, and recently researchers in Switzerland and Russia have generated terahertz pulses with almost 1 mJ of energy.

The research is published in Nature Communications.