Tiny double terahertz accelerator recycles energy

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. Credit: DESY, Science Communication Lab

by Deutsches Elektronen-Synchrotron
https://phys.org/news/2020-03-tiny-recycles-energy.html

A team of DESY scientists has built a miniature double particle accelerator that can recycle some of the laser energy fed into the system to boost the energy of the accelerated electrons a second time. The device uses narrowband terahertz radiation which lies between infrared and radio frequencies in the electromagnetic spectrum, and a single accelerating tube is just 1.5 centimetres long and 0.79 millimetres in diameter. Dongfang Zhang and his colleagues from the Center for Free-Electron laser Science (CFEL) at DESY present their experimental accelerator in the journal Physical Review X.

The miniature size of the device is possible due to the short wavelength of terahertz radiation. "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 device. Scientists have successfully experimented with terahertz accelerators before, which could enable applications where large particle accelerators are just not feasible or necessary. "However, the technique is still in an early stage, and the performance of experimental terahertz accelerators has been limited by the relatively short section of interaction between the terahertz pulse and the electrons," says Kärtner.

For the new device, the team used a longer pulse comprising many cycles of terahertz waves. This multicycle pulse significantly extends the interaction section with the particles. "We feed the multicycle terahertz pulse into a waveguide that is lined with a dielectric material", says Zhang. Within the waveguide, the pulse's speed is reduced. A bunch of electrons is shot into the central part of the waveguide just in time to travel along with the pulse. "This scheme increases the interaction region between the terahertz pulse and the electron bunch to the centimetre range—compared to a few millimetres in earlier experiments," reports Zhang.

The device did not produce a large acceleration in the lab. However, the team could prove the concept by showing that the electrons gain energy in the waveguide. "It is a proof of concept. The electrons' energy increased from 55 to about 56.5 kilo electron volts," says Zhang. "A stronger acceleration can be achieved by using a stronger laser to generate the terahertz pulses."

The set-up is mainly designed for the non-relativistic regime, meaning the electrons have speeds that are not so close to the speed of light. Interestingly, this regime enables a 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," explains Zhang. "This means, the pulse overtakes the slower electron bunch in a couple of centimetres. 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."

In the lab experiment, only a small fraction of the terahertz pulse could be recycled this way. But the experiment shows that recycling is possible in principle, and Zhang is confident that the recycled fraction can be substantially increased. Nicholas Mattlis, senior scientist and the team leader of the project in the CFEL group, emphasises: "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."

Experimental mini-accelerator achieves record energy

The two-stage miniature accelerator is operated with terahertz radiation (shown here in red). In a first step (left) the electron bunches (shown in blue) are compressed, in a second step (right) they are accelerated. The two individual elements are each about two centimeters wide. Credit: DESY, Gesine Born

https://phys.org/news/2019-07-experimental-mini-accelerator-energy.html\

Scientists at DESY have achieved a new world record for an experimental type of miniature particle accelerator: For the first time, a terahertz powered accelerator more than doubled the energy of the injected electrons. At the same time, the setup significantly improved the electron beam quality compared to earlier experiments with the technique, as Dongfang Zhang and his colleagues from the Center for Free-Electron Laser Science (CFEL) at DESY report in the journal Optica. "We have achieved the best beam parameters yet for terahertz accelerators," said Zhang.

"This result represents a critical step forward for the practical implementation of terahertz-powered accelerators," emphasized Franz Kärtner, who heads the ultrafast optics and X-rays group at DESY. Terahertz radiation lies between infrared and microwave frequencies in the electromagnetic spectrum and promises a new generation of compact particle accelerators. "The wavelength of terahertz radiation is about a hundred times shorter than the radio waves currently used to accelerate particles," explained Kärtner. "This means that the components of the accelerator can also be built to be around a hundred times smaller." The terahertz approach promises lab-sized accelerators that will enable completely new applications for instance as compact X-ray sources for materials science and maybe even for medical imaging. The technology is currently under development.

Since terahertz waves oscillate so fast, every component and every step has to be precisely synchronized. "For instance, to achieve the best energy gain, the electrons have to hit the terahertz field exactly during its accelerating half cycle," explained Zhang. In accelerators, particles usually do not fly in a continuous beam, but are packed in bunches. Because of the fast-changing field, in terahertz accelerators these bunches have to be very short to ensure even acceleration conditions along the bunch.

"In previous experiments the electron bunches were too long", said Zhang. "Since the terahertz field oscillates so quickly, some of the electrons in the bunch were accelerated, while others were even slowed down. So, in total there was just a moderate average energy gain, and, what is more important, a wide energy spread, resulting in what we call poor beam quality." To make things worse, this effect strongly increased the emittance, a measure for how well a particle beam is bundled transversally. The tighter, the better—the smaller the emittance.

To improve the beam quality, Zhang and his colleagues built a two-step accelerator from a multi-purpose device they had developed earlier: The Segmented Terahertz Electron Accelerator and Manipulator (STEAM) can compress, focus, accelerate and analyze electron bunches with terahertz radiation. The researchers combined two STEAM devices in line. They first compressed the incoming electron bunches from about 0.3 millimetres in length to just 0.1 millimetres. With the second STEAM device, they accelerated the compressed bunches. "This scheme requires control on the level of quadrillionths of a second, which we achieved," said Zhang "This led to a fourfold reduction of the energy spread and improved the emittance sixfold, yielding the best beam parameters of a terahertz accelerator so far."

The net energy gain of the electrons that were injected with an energy of 55 kiloelectron volts (keV) was 70 keV. "This is the first energy boost greater than 100 percent in a terahertz powered accelerator," emphasised Zhang. The coupled device produced an accelerating field with a peak strength of 200 million Volts per metre (MV/m) - close to state-of-the-art strongest conventional accelerators. For practical applications this still has to be significantly improved. "Our work shows that even a more than three times stronger compression of the electron bunches is possible. Together with a higher terahertz energy, acceleration gradients in the regime of gigavolts per metre seem feasible," summarized Zhang. "The terahertz concept thus appears increasingly promising as a realistic option for the design of compact electron accelerators."

Laser trick produces high-energy terahertz pulses

From the color difference of two slightly delayed laser flashes (left) a non-linear crystal generates an energetic terahertz pulse (right).CREDIT DESY, Lucid Berlin

https://www.eurekalert.org/pub_releases/2019-06/ded-ltp061319.php

A team of scientists from DESY and the University of Hamburg has achieved an important milestone in the quest for a new type of compact particle accelerator. Using ultra-powerful pulses of laser light, they were able to produce particularly high-energy flashes of radiation in the terahertz range having a sharply defined wavelength (colour). Terahertz radiation is to open the way for a new generation of compact particle accelerators that will find room on a lab bench. The team headed by Andreas Maier and Franz Kärtner from the Hamburg Center for Free-Electron Laser Science (CFEL) is presenting its findings in the journal Nature Communications. CFEL is jointly run by DESY, the University of Hamburg and the Max Planck Society.

The terahertz range of electromagnetic radiation lies between the infrared and microwave frequencies. Air travellers may be familiar with terahertz radiation from the full-body scanners used by airport security to search for objects hidden beneath a person's garments. However, radiation in this frequency range might also be used to build compact particle accelerators. "The wavelength of terahertz radiation is about a thousand times shorter than the radio waves that are currently used to accelerate particles," says Kärtner, who is a lead scientist at DESY. "This means that the components of the accelerator can also be built to be around a thousand times smaller." The generation of high-energy terahertz pulses is therefore also an important step for the AXSIS (frontiers in Attosecond X-ray Science: Imaging and Spectroscopy) project at CFEL, funded by the European Research Council (ERC), which aims to open up completely new applications with compact terahertz particle accelerators.

However, chivvying along an appreciable number of particles calls for powerful pulses of terahertz radiation having a sharply defined wavelength. This is precisely what the team has now managed to create. "In order to generate terahertz pulses, we fire two powerful pulses of laser light into a so-called non-linear crystal, with a minimal time delay between the two," explains Maier from the University of Hamburg. The two laser pulses have a kind of colour gradient, meaning that the colour at the front of the pulse is different from that at the back. The slight time shift between the two pulses therefore leads to a slight difference in colour. "This difference lies precisely in the terahertz range," says Maier. "The crystal converts the difference in colour into a terahertz pulse."

The method requires the two laser pulses to be precisely synchronised. The scientists achieve this by splitting a single pulse into two parts and sending one of them on a short detour so that it is slightly delayed before the two pulses are eventually superimposed again. However, the colour gradient along the pulses is not constant, in other words the colour does not change uniformly along the length of the pulse. Instead, the colour changes slowly at first, and then more and more quickly, producing a curved outline. As a result, the colour difference between the two staggered pulses is not constant. The difference is only appropriate for producing terahertz radiation over a narrow stretch of the pulse.

"That was a big obstacle towards creating high-energy terahertz pulses," as Maier reports. "Because straightening the colour gradient of the pulses, which would have been the obvious solution, is not easy to do in practice." It was co-author Nicholas Matlis who came up with the crucial idea: he suggested that the colour profile of just one of the two partial pulses should be stretched slightly along the time axis. While this still does not alter the degree with which the colour changes along the pulse, the colour difference with respect to the other partial pulse now remains constant at all times. "The changes that need to be made to one of the pulses are minimal and surprisingly easy to achieve: all that was necessary was to insert a short length of a special glass into the beam," reports Maier. "All of a sudden, the terahertz signal became stronger by a factor of 13." In addition, the scientists used a particularly large non-linear crystal to produce the terahertz radiation, specially made for them by the Japanese Institute for Molecular Science in Okazaki.

"By combining these two measures, we were able to produce terahertz pulses with an energy of 0.6 millijoules, which is a record for this technique and more than ten times higher than any terahertz pulse of sharply defined wavelength that has previously been generated by optical means," says Kärtner. "Our work demonstrates that it is possible to produce sufficiently powerful terahertz pulses with sharply defined wavelengths in order to operate compact particle accelerators."

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DESY is one of the world's leading particle accelerator centres. Researchers use the large?scale facilities at DESY to explore the microcosm in all its variety - ranging from the interaction of tiny elementary particles to the behaviour of innovative nanomaterials and the vital processes that take place between biomolecules to the great mysteries of the universe. The accelerators and detectors that DESY develops and builds at its locations in Hamburg and Zeuthen are unique research tools. DESY is a member of the Helmholtz Association, and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 per cent) and the German federal states of Hamburg and Brandenburg (10 per cent).

Reference:

Spectral Phase Control of Interfering Chirped Pulses for High-Energy Narrowband Terahertz Generation; Spencer W. Jolly, Nicholas H. Matlis, Frederike Ahr, Vincent Leroux, Timo Eichner, Anne-Laure Calendron, Hideki Ishizuki, Takunori Taira, Franz X. Kärtner, and Andreas R. Maier; Nature Communications, 2019; DOI: 10.1038/s41467-019-10657-4

Abstract-Analysis of terahertz generation using tilted pulse fronts

Koustuban Ravi and Franz Kärtner

Fig. 1 A tilted-pulse-front setup comprised of a diffraction grating and an imaging system. The angularly dispersed pulse produces a tilted pulse front (red ellipse) with tilt angle γ, resulting in terahertz radiation propagating at an angle γ with respect to the direction of pump pulse propagation.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-3-3496

A spatio-temporal analysis of terahertz generation by optical rectification of tilted pulse fronts is presented. Closed-form expressions of terahertz transients and spectra in two spatial dimensions are furnished in the undepleted limit. Importantly, the analysis incorporates spatio-temporal distortions of the optical pump pulse such as angular dispersion, group velocity dispersion due to angular dispersion, spatial and temporal chirp, as well as beam curvature. The influence of the radius of curvature on the tilt angle is shown. Furthermore, the impact of group velocity dispersion due to angular dispersion on terahertz frequency, conversion efficiency and peak field is revealed. In particular, the deterioration of terahertz frequency, efficiency and field at large pump bandwidths and beam sizes by group velocity dispersion due to angular dispersion is expressed analytically.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-High efficiency terahertz generation in a multi-stage system

Lu Wang, Arya Fallahi, Koustuban Ravi, and Franz Kärtner

Fig. 2 Schematic illustration of the simulated geometry: the dark thick arrows represent the polarization direction of both pump and terahertz beams. The polarization direction is aligned with the extraordinary optical axis of PPLN. The origin of the cylindrical coordinate is at the center of the beam. r and z represent the transverse and propagation directions, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-23-29744&origin=search

We describe a robust system for laser-driven narrowband terahertz generation with high conversion efficiency in periodically poled Lithium Niobate (PPLN). In the multi-stage terahertz generation system, the pump pulse is recycled after each PPLN stage for further terahertz generation. By out-coupling the terahertz radiation generated in each stage, extra absorption is circumvented and effective interaction length is increased. The separation of the terahertz and optical pulses at each stage is accomplished by an appropriately designed out-coupler. To evaluate the proposed architecture, the governing 2-D coupled wave equations in a cylindrically symmetric geometry are numerically solved using the finite difference method. Compared to the 1-D calculation which cannot capture the self-focusing and diffraction effects, our 2-D numerical method captures the effects of difference frequency generation, self-phase modulation, self-focusing, beam diffraction, dispersion and terahertz absorption. We found that the terahertz generation efficiency can be greatly enhanced by compensating the dispersion of the pump pulse after each stage. With a two-stage system, we predict the generation of a 17.6 mJ terahertz pulse with total conversion efficiency ηtotal = 1.6% at 0.3 THz using a 1.1 J pump laser with a two-lines spectrum centered at 1 μm. The generation efficiency of each stage is above 0.8% with the out-coupling efficiencies above 93.0%.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Designing chirped aperiodically poled structures for high-energy single-cycle terahertz generation

Alireza Yahaghi, Koustuban Ravi, Arya Fallahi, and Franz Kärtner

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-34-3-590&origin=search

We introduce a collinear scheme for the highly efficient generation of broadband single or few-cycle high-power terahertz (THz) pulses using optical rectification. For this purpose, two concepts are introduced and thoroughly analyzed. The first concept is the generation of chirped broadband terahertz pulses using chirped aperiodically poled electro-optic crystals. The second concept involves the compression of the terahertz pulses extracted from the aperiodically poled structure using a chirped mirror. An illustrative design employing cryogenically cooled aperiodically poled lithium niobate crystals and the appropriate chirped mirrors is presented. It is shown that the presented design allows the optical pulse to be re-used in subsequent generation stages, resulting in optical-to-THz conversion efficiencies in excess of 5% for terahertz radiation centered around ∼0.3  THz$\sim 0.3\mathrm{THz}$. Analytic solutions and numerical calculations are presented. In order to design a crystal with the optimum conversion efficiency, we take advantage of binary optimization techniques. This work paves the way for the generation of broadband terahertz radiation with several mJ of pulse energy using large-area aperiodically poled structures.

Toward X-ray movies

This illustration shows a miniature electron gun driven by terahertz radiation. A UV pulse (blue) back-illuminates the gun photocathode, producing a high-density electron bunch inside the gun. The bunch is immediately accelerated by ultra-intense terahertz pulses to energies approaching 1 kiloelectronvolt. These high-field optically-driven electron guns can be utilized for ultrafast electron diffraction or injected into the accelerators for X-ray light sources.

Courtesy of W. Ronny Huang

Low-power tabletop source of ultrashort electron beams could replace car-size laboratory devices

http://news.mit.edu/2016/tabletop-ultrashort-electron-beams-x-ray-movies-1122
Larry Hardesty | MIT News Office

Ultrashort bursts of electrons have several important applications in scientific imaging, but producing them has typically required a costly, power-hungry apparatus about the size of a car.

In the journal Optica, researchers at MIT, the German Synchrotron, and the University of Hamburg in Germany describe a new technique for generating electron bursts, which could be the basis of a shoebox-sized device that consumes only a fraction as much power as its predecessors.

Ultrashort electron beams are used to directly gather information about materials that are undergoing chemical reactions or changes of physical state. But after being fired down a particle accelerator a half a mile long, they’re also used to produce ultrashort X-rays.

Last year, in Nature Communications, the same group of MIT and Hamburg researchers reported the prototype of a small “linear accelerator” that could serve the same purpose as the much larger and more expensive particle accelerator. That technology, together with a higher-energy version of the new “electron gun,” could bring the imaging power of ultrashort X-ray pulses to academic and industry labs.

Indeed, while the electron bursts reported in the new paper have a duration measured in hundreds of femtoseconds, or quadrillionths of a second (which is about what the best existing electron guns can manage), the researchers’ approach has the potential to lower their duration to a single femtosecond. An electron burst of a single femtosecond could generate attosecond X-ray pulses, which would enable real-time imaging of cellular machinery in action.

“We’re building a tool for the chemists, physicists, and biologists who use X-ray light sources or the electron beams directly to do their research,” says Ronny Huang, an MIT PhD student in electrical engineering and first author on the new paper. “Because these electron beams are so short, they allow you to kind of freeze the motion of electrons inside molecules as the molecules are undergoing a chemical reaction. A femtosecond X-ray light source requires more hardware, but it utilizes electron guns.”

In particular, Huang explains, with a technique called electron diffraction imaging, physicists and chemists use ultrashort bursts of electrons to investigate phase changes in materials, such as the transition from an electrically conductive to a nonconductive state, and the creation and dissolution of bonds between molecules in chemical reactions.

Ultrashort X-ray pulses have the same advantages that ordinary X-rays do: They penetrate more deeply into thicker materials. The current method for producing ultrashort X-rays involves sending electron bursts from a car-sized electron gun through a billion-dollar, kilometer-long particle accelerator that increases their velocity. Then they pass between two rows of magnets — known as an “undulator” — that converts them to X-rays.

In the paper published last year — on which Huang was a coauthor — the MIT-Hamburg group, together with colleagues from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg and the University of Toronto, described a new approach to accelerating electrons that could shrink particle accelerators to tabletop size. “This is supposed to complement that,” Huang says, about the new study.

Franz Kärtner, who was a professor of electrical engineering at MIT for 10 years before moving to the German Synchrotron and the University of Hamburg in 2011, led the project. Kärtner remains a principal investigator at MIT’s Research Laboratory of Electronics and is Huang’s thesis advisor. He and Huang are joined on the new paper by eight colleagues from both MIT and Hamburg.

Subwavelength confinement

The researchers’ new electron gun is a variation on a device called an RF gun. But where the RF gun uses radio frequency (RF) radiation to accelerate electrons, the new device uses terahertz radiation, the band of electromagnetic radiation between microwaves and visible light.

The researchers’ device, which is about the size of a matchbox, consists of two copper plates that, at their centers, are only 75 micrometers apart. Each plate has two bends in it, so that it looks rather like a trifold letter that’s been opened and set on its side. The plates bend in opposite directions, so that they’re farthest apart — 6 millimeters — at their edges.

At the center of one of the plates is a quartz slide on which is deposited a film of copper that, at its thinnest, is only 30 nanometers thick. A short burst of light from an ultraviolet laser strikes the film at its thinnest point, jarring loose electrons, which are emitted on the opposite side of the film.

At the same time, a burst of terahertz radiation passes between the plates in a direction perpendicular to that of the laser. All electromagnetic radiation can be thought of as having electrical and magnetic components, which are perpendicular to each other. The terahertz radiation is polarized so that its electric component accelerates the electrons directly toward the second plate.

The key to the system is that the tapering of the plates confines the terahertz radiation to an area — the 75-micrometer gap — that is narrower than its own wavelength. “That’s something special,” Huang says. “Typically, in optics, you can’t confine something to below a wavelength. But using this structure we were able to. Confining it increases the energy density, which increases the accelerating power.”

Because of that increased accelerating power, the device can make do with terahertz beams whose power is much lower than that of the radio-frequency beams used in a typical RF gun. Moreover, the same laser can generate both the ultraviolet beam and, with a few additional optical components, the terahertz beam.

According to James Rosenzweig, a professor of physics at the University of California at Los Angeles, that’s one of the most attractive aspects of the researchers’ system. “One of the main problems you have with ultrafast sources like this is timing jitter between, say, the laser and accelerating field, which produces all sorts of systematic effects that make it harder to do time-resolved electron diffraction,” Rosezweig says.

“In the case of Kärtner’s device, the laser produces the terahertz and also produces the photoelectrons, so the jitter is highly suppressed. You could do pump-probe experiments where the laser is the driver and the electrons would be the probe, and they would be more successful than what you have right now. And of course it would be a very small-sized and modest-cost device. So it might turn out to be very important as far as that scenario goes.”

The researchers’ work was funded by the U.S. Air Force Office of Scientific Research and by the European Research Council. Ronny Huang was supported by a National Defense Science and Engineering Graduate fellowship.

Scientists create mini linac prototype, could alter radiation therapy

                               Terahertz accelerator modules easily fit into two fingers.
                                   Credit: DESY/Heiner Mueller-Elsner                                                                                  

by Lisa Chamoff 
http://www.dotmed.com/news/story/27370

A team of scientists has built a prototype for a miniature particle accelerator with a single module that is 1.5 centimeters long and 1 millimeter thick, which could enable new diagnostic imaging and radiation therapy techniques. 

The researchers, part of the Hamburg-based Center for Free-Electron Laser Science — a joint enterprise of DESY, the Max Planck Society and the University of Hamburg — presented the prototype in the journal Nature Communications. The prototype, which was set up in DESY scientist Franz Kärtner's lab at the Massachusetts Institute of Technology (MIT), uses terahertz radiation instead of radio frequency structures, which the scientists say has the potential to miniaturize the entire accelerator by at least a factor of 100. 

“The compact accelerator we are building enables the construction of very bright and potentially fully coherent X-ray sources, which enable also new medical diagnostic imaging techniques, like phase contrast imaging and potentially also new radiation therapy techniques like image-guided small tumor radiation therapy and micro-beam radiation therapy or brain tumors,” Kärtner, a professor at the University of Hamburg and at MIT, as well a member of the Hamburg Centre for Ultrafast Imaging, told HCB News. “Currently, high brightness X-ray beams are only available from large synchrotron or free-electron laser facilities, which is not practical for health care. A compact highly coherent X-ray source would address this shortcoming.” 

For the prototype, the physicists used a type of electron gun to fire fast electrons into an accelerator module that was tailored to be used with terahertz radiation, which was fed into the module, further accelerating the electrons. The prototype was able to increase the energy of the particles by 7 kiloelectronvolts, according to the scientists. While not a particularly large acceleration, the experiment demonstrated that the principle works in practice, said co-author Arya Fallahi of CFEL in a press release. 

"The theory indicates that we should be able to achieve an accelerating gradient of up to one gigavolt per meter,” Fallahi said in the release. This is more than 10 times what can be achieved with the top conventional accelerator modules currently available, the scientists said. Plasma accelerators could product higher accelerations, but the scientists said this experimental technology requires much more powerful lasers than the ones needed for terahertz accelerators. 

Recently, scientists from the European Organization for Nuclear Research (CERN) built a miniature linear accelerator made up of four modules that are each roughly 20 inches long, for a total size of a little more than 6.5 feet. They doubled the operating frequency used for the radiofrequency quadrupole (RFQ), a linear accelerator component used in the acceleration of low-velocity ion beams. 

Kärtner said his group’s prototype works at an even higher frequency, roughly 100 times the usual frequency of 1.3 gigahertz, and generates electrons, which Kärtner said is good for making accelerator-driven X-ray sources. 

The scientists are looking to make a 20 mega-electronvolt accelerator within the next three to four years, Kärtner said.

Team shrinks particle accelerator: Prototype demonstrates feasibility of building terahertz accelerators

http://phys.org/news/2015-10-team-particle-prototype-feasibility-terahertz.html

An interdisciplinary team of researchers has built the first prototype of a miniature particle accelerator that uses terahertz radiation instead of radio frequency structures. A single accelerator module is just 1.5 centimetres long and one millimetre thick. The terahertz technology holds the promise of miniaturising the entire set-up by at least a factor of 100, as the scientists surrounding DESY's Franz Kärtner from the Center for Free-Electron Laser Science (CFEL) point out. They are presenting their prototype, that was set up in Kärtner's lab at the Massachusetts Institute of Technology (MIT) in the U.S., in the journal Nature Communications. The authors see numerous applications for terahertz accelerators, in materials science, medicine and particle physics, as well as in building X-ray lasers. CFEL is a cooperation between DESY, the University of Hamburg and the Max Planck Society.

In the electromagnetic spectrum, terahertz radiation lies between infrared radiation and microwaves. Particle accelerators usually rely on electromagnetic radiation from the radio frequency range; DESY's particle accelerator PETRA III, for example, uses a frequency of around 500 megahertz. The wavelength of the terahertz radiation used in this experiment is around one thousand times shorter. "The advantage is that everything else can be a thousand times smaller too," explains Kärtner, who is also a professor at the University of Hamburg and at MIT, as well as being a member of the Hamburg Centre for Ultrafast Imaging (CUI), one of Germany's Clusters of Excellence.

For their prototype the scientists used a special microstructured accelerator module, specifically tailored to be used with terahertz radiation. The physicists fired fast electrons into the miniature accelerator module using a type of electron gun provided by the group of CFEL Professor Dwayne Miller, Director at the Max Planck Institute for the Structure and Dynamics of Matter and also a member of CUI. The electrons were then further accelerated by the terahertz radiation fed into the module. This first prototype of a terahertz accelerator was able to increase the energy of the particles by seven kiloelectronvolts (keV).

"This is not a particularly large acceleration, but the experiment demonstrates that the principle does work in practice," explains co-author Arya Fallahi of CFEL, who did the theoretical calculations. "The theory indicates that we should be able to achieve an accelerating gradient of up to one gigavolt per metre." This is more than ten times what can be achieved with the best conventional accelerator modules available today. Plasma accelerator technology, which is also at an experimental stage right now, promises to produce even higher accelerations, however it also requires significantly more powerful lasers than those needed for terahertz accelerators.

The physicists underline that terahertz technology is of great interest both with regard to future linear accelerators for use in particle physics, and as a means of building compact X-ray lasers and electron sources for use in materials research, as well as medical applications using X-rays and electron radiation. "The rapid advances we are seeing in terahertz generation with optical methods will enable the future development of terahertz accelerators for these applications," says first author Emilio Nanni of MIT. Over the coming years, the CFEL team in Hamburg plans to build a compact, experimental free-electron X-ray laser (XFEL) on a laboratory scale using terahertz technology. This project is supported by a Synergy Grant of the European Research Council.