Abstract-Cascaded Multicycle Terahertz-Driven Ultrafast Electron Acceleration and Manipulation

Dongfang Zhang, Moein Fakhari, Huseyin Cankaya, Anne-Laure Calendron, Nicholas H. Matlis, and Franz X. Kärtner

https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.011067

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with millimeter-scale, segmented interaction regions that are ideal for acceleration of electrons in the sub- to few-MeV range, where electron velocities vary significantly. However, in order to extend this technology to the multi-MeV range, an investigation of approaches supporting longer interaction lengths is needed. Here, we demonstrate first steps in electron acceleration and manipulation using dielectrically lined waveguides powered by temporally long, narrow-band, multicycle THz pulses that copropagate with the electrons. This geometry offers centimeter-scale single-stage interaction lengths and offers the opportunity to further increase interaction lengths by cascading acceleration stages that recycle the THz energy and rephase the interaction. We prove the feasibility of THz-energy recycling for the first time by demonstrating acceleration, compression, and focusing in two sequential ${\mathrm{Al}}_2{O}_3$-based dielectric capillary stages powered by the same multicycle THz pulse. Since the multicycle THz energy achievable using laser-based sources is currently a limiting factor for the maximum electron acceleration, recycling the THz pulses provides a key factor for reaching relativistic energies with existing sources and paves the way for applications in future ultrafast electron diffraction and free-electron lasers.

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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."

Abstract-Cascaded Multi-cycle terahertz driven ultrafast electron acceleration and manipulation

Dongfang Zhang, Moein Fakhari, Huseyin Cankaya, Anne-Laure Calendron, Nicholas H. Matlis, Franz X. Kärtner

https://arxiv.org/abs/1910.06639

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with mm-scale, segmented interaction regions that are ideal for acceleration of electrons in the sub- to few-MeV range where electron velocities vary significantly. However, in order to extend this technology to the multi-MeV range, investigation of approaches supporting longer interaction lengths is needed. Here, we demonstrate first steps in electron acceleration and manipulation using dielectrically-lined waveguides powered by temporally long, narrowband, multi-cycle THz pulses that co-propagate with the electrons. This geometry offers centimeter-scale single-stage interaction lengths and offers the opportunity to further increase interaction lengths by cascading acceleration stages that recycle the THz energy and rephase the interaction. We prove the feasibility of THz-energy recycling for the first time by demonstrating acceleration, compression and focusing in two sequential Al2O3-based dielectric capillary stages powered by the same multi-cycle THz pulse. Since the multi-cycle energy achievable using laser-based sources is currently a limiting factor for the maximum electron acceleration, THz energy recycling provides a key enabling factor for reaching relativistic energies with existing sources.

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."

Abstract-Segmented terahertz electron accelerator and manipulator (STEAM)

Dongfang Zhang, Arya Fallahi, Michael Hemmer, Xiaojun Wu, Moein Fakhari, Yi Hua, Huseyin Cankaya, Anne-Laure Calendron, Luis E. Zapata, Nicholas H. Matlis, Franz X. Kärtner

https://www.nature.com/articles/s41566-018-0138-z

Acceleration and manipulation of electron bunches underlie most electron and X-ray devices used for ultrafast imaging and spectroscopy. New terahertz-driven concepts offer orders-of-magnitude improvements in field strengths, field gradients, laser synchronization and compactness relative to conventional radiofrequency devices, enabling shorter electron bunches and higher resolution with less infrastructure while maintaining high charge capacities (pC), repetition rates (kHz) and stability. We present a segmented terahertz electron accelerator and manipulator (STEAM) capable of performing multiple high-field operations on the six-dimensional phase space of ultrashort electron bunches. With this single device, powered by few-microjoule, single-cycle, 0.3 THz pulses, we demonstrate record terahertz acceleration of >30 keV, streaking with <10 fs resolution, focusing with >2 kT m–1 strength, compression to ~100 fs as well as real-time switching between these modes of operation. The STEAM device demonstrates the feasibility of terahertz-based electron accelerators, manipulators and diagnostic tools, enabling science beyond current resolution frontiers with transformative impact.

Scientists create 'Swiss army knife' for electron beams

The mini accelerator STEAM (centre) is driven by Terahertz radiation (yellow, coming from both sides). It can accelerator, compress, focus and analyze the incident electron bunches (blue).

Pocket accelerator combines four functions in one device

DEUTSCHES ELEKTRONEN-SYNCHROTRON DESY

https://www.eurekalert.org/pub_releases/2018-04/ded-sc040318.php

DESY scientists have created a miniature particle accelerator for electrons that can perform four different functions at the push of a button. The experimental device is driven by a Terahertz radiation source and can accelerate, compress, focus and analyse electron bunches in a beam. Its active structures measure just a few millimetres across. The developers from the Center for Free-Electron Laser Science (CFEL) present their "Segmented Terahertz Electron Accelerator and Manipulator" (STEAM) in the journal Nature Photonics. Terahertz radiation is located between microwaves and the infrared in the electromagnetic spectrum.

One of the central features of the device is its perfect timing with the electron beam. The scientists achieve this by using the same laser pulse to generate an electron bunch and to drive the device. "To do this, we take an infrared laser pulse and split it up," explains first author Dongfang Zhang from the group of Franz Kärtner at CFEL. "Both parts are fed into nonlinear crystals that change the laser wavelength: For the generation of an electron bunch the wavelength is shifted into the ultraviolet and directed onto a photocathode where it releases a bunch of electrons. For STEAM the wavelength is shifted into the Terahertz regime. The relative timing of the two parts of the original laser pulse only depends on the length of the path they take and can be controlled very precisely."

This way, the scientists can control with ultra-high precision, what part of the Terahertz wave an electron bunch hits when it enters the device. Depending on the arrival time of the electron bunch, STEAM performs its different functions. "For instance, a bunch that hits the negative part of the Terahertz electric field is accelerated," explains Zhang. "Other parts of the wave lead to focusing or defocusing of the bunch or to a compression by a factor of ten or so." While compression means the electron bunch gets shorter in the direction of flight, focusing means it shrinks perpendicular to the direction of flight.

Additionally, STEAM allows to perform an analysis of the structure of the electron bunch along its path of flight. For this technique, called streaking, the incoming electron bunch is deflected sideways in such a way that it becomes smeared out perpendicular to the direction of flight. When this smeared out bunch hits a detector, it produces a profile of the bunch along its path of flight. Streaking is regularly used to analyse the bunch structures in particle accelerators. "STEAM is a kind of Swiss army knife for electron beams," says Zhang. To perform multiple functions, like compression and focusing, several units of the device can be combined.

Using Terahertz radiation also allows for the compact size of the STEAM device. "Terahertz radiation typically has a hundred times shorter wavelengths than the radio frequency radiation used in today's big particle accelerators. Therefore, all the structures in the device can shrink accordingly," explains Kärtner, who is lead scientist at DESY and professor at Universität Hamburg. Measuring just about two centimetres on the largest side, STEAM easily fits into a matchbox. "And that's just the size of the housing. The active structures are on a millimetre scale," adds Zhang.

The technology is still at an experimental stage. The developers see STEAM as a first step on the road to a future generation of compact, Terahertz driven particle accelerators. These could enable new applications and complement today's accelerators. Also, the pocket manipulator can already be utilised today: accelerator groups around the world are already considering it for bunch characterisation, as Kärtner points out: "STEAM can be used for future table-top accelerators, but its various functions are also interesting for existing machines."

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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

Segmented Terahertz Electron Accelerator and Manipulator (STEAM); Dongfang Zhang, Arya Fallahi, Michael Hemmer, Xiaojun Wu, Moein Fakhari, Yi Hua, Huseyin Cankaya, Anne-Laure Calendron, Luis E. Zapata, Nicholas H. Matlis and Franz X. Kärtner; Nature Photonics, 2018; DOI: 10.1038/s41566-018-0138-z