Nature Photonics: A milestone in laser technology

Extremely intense light pulses generated by free-electron lasers (FELs) are a versatile tool
in research.
Especially in the X-ray range, they can be used to analyze the atomic structure details of various materials and track basic ultrafast processes
with extreme precision.
Until now, electron accelerators like Germany's Euro XFEL have been based on traditional electron accelerators, which makes them both long and expensive
.
An international team led by France's synchrotron SOLEIL and Germany's Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now achieved a breakthrough in an affordable alternative solution: they were able to demonstrate laser lasers in ultraviolet conditions based on a still-young technology – laser plasma-accelerated seed FEL in ultraviolet conditions
.
In the future, this could allow for the creation of more compact systems, which would greatly expand the possible applications
of free-electron lasers.
The results of this collaborative study were published in the journal Nature Photonics (DOI: 10.
1038/s41566-022-01104w).

X-ray free-electron lasers are one of
the most powerful and sophisticated research machines in the world.
The principle is that with the help of strong radio waves, accelerators bring electrons close to the speed of
light.
These particles then swarm through the "waver"—a magnet arrangement that periodically alternates magnetic fields, forcing the electron beam to follow
a cyclotron path.
This causes the electron beam to regroup into many smaller clusters of electrons—microelectron beams—which together emit extremely powerful, laser-like pulses
of light.
This data can be used to decipher previously unknown material properties, or to track extremely fast processes, such as chemical reactions
that occur in quadrillionths of a second.

However, multibillion-dollar European XFEL and other similar infrastructure have a drawback: "They are hundreds of meters or even kilometers long," says
Professor Ulrich Schramm, director of the HZDR Institute of Radiation Physics.
That's why we're working on an alternative technology to make such facilities smaller and more cost-effective, so they can be closer to users
in universities and industry in the future.
"The basis for this is a new accelerator technology that is still under development – laser plasma acceleration
.

Dr.
Arie Irman, a physicist at HZDR, explains: "We use high-power lasers to emit short, ultra-powerful flashes of light into plasma, an ionized gas
consisting of negatively charged electrons and positively charged ions.
" "In plasma, pulses of light produce strong alternating electric field waves, similar to a ship's wake
.
" This wave can quickly accelerate electrons to higher velocities
over a short distance.
In principle, this could shrink the accelerator, which is now 100 meters long, to a length
of less than 1 meter.

Successful teamwork

In principle, electrons have long been accelerated by this technology
.
But until recently, although still in its early stages, it has become possible
, such rapid particle beams emitted from plasma accelerators through wavers and then converted into laser flashes.
To produce the first controllable FEL laser driven by plasma acceleration, HZDR collaborated
with experts from the French synchrotron SOLEIL.

Dr.
Marie-Emmanuelle Couprie, physicist at SOLEIL, said: "The plasma accelerator installed in Dresden is driven by a high-power laser DRACO, providing a high-quality, fast electron beam
。 "Behind it, we subsequently built a fluctuator and associated accelerator beamline, which had previously been worked with Lille's PhLAM in collaboration with Lille's PhLAM at the French plasma accelerator laboratory in Palesso, to optimize
electron beam transmission methods, the generation of fluctuator radiation, seed generation and molding including overlapping problems and methods.
"

In order to produce a FEL laser flash under ultraviolet (UV) conditions, researchers must address several fundamental questions
.
"We have to produce beams of particles that contain a lot of electrons," Irman explains
.
"At the same time, it is important that these electrons have as much energy
as possible.
"

To prevent the electron beam from diverging too quickly, a delicate method is used: so-called plasma lensing
.
In addition, the team employed a method called "seeding": in sync with the electron beam, they emitted external laser light pulses to the fluctuator, which is essential
to speed up the FEL process and improve the beam quality of the FEL laser flash.

A breakthrough in laser technology

With this device, the team finally achieved their goal: to demonstrate the ultrashort ultraviolet laser flash produced by the plasma-driven FEL
, as hoped.
"For 15 years, people in the advanced accelerator physics community have been dreaming of realizing such free-electron lasers," enthuses
Ulrich Schramm.
"You can imagine how happy
we are that we have achieved this now in Dresden.
" For Ali Illman, a dream has also come true: "Plasma-driven free-electron lasers have always been considered one of
the most important milestones in our field.
Through our experiments, we have now made tremendous progress
.
”

There are still many challenges to overcome
before plasma-based FEL can be put into practical use.
For example, while the Dresden device is capable of producing pulses of ultraviolet light, the research requires high-intensity X-ray flashes – for which electrons must be accelerated to higher energies
.

Schramm said: "This has been proven in principle in plasma acceleration, but so far the quality of the electron beam is still too poor and unstable
for X-ray free-energy lasers.
" "But with a new generation of high-power lasers, we hope to solve this problem
.
" If this effort succeeds, free-electron lasers could fit into the basement of the institute in the future — and thus be available to more research teams than they do today
.