In the depths of space, there are celestial bodies where extreme conditions
prevail: Rapidly rotating neutron stars generate super-strong magnetic fields.
And black holes, with their enormous gravitational pull, can cause huge,
energetic jets of matter to shoot out into space. An international physics
team with the participation of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
has now proposed a new concept that could allow some of these extreme
processes to be studied in the laboratory in the future: A special setup of
two high-intensity laser beams could create conditions similar to those found
near neutron stars. In the discovered process, an antimatter jet is generated
and accelerated very efficiently. The experts present their concept in the
journal Communications Physics.
The basis of the new concept is a tiny block of plastic, crisscrossed by
micrometer-fine channels. It acts as a target for two lasers. These
simultaneously fire ultra-strong pulses at the block, one from the right,
the other from the left. "When the laser pulses penetrate the sample, each
of them accelerates a cloud of extremely fast electrons," explains HZDR
physicist Toma Toncian. "These two electron clouds then race toward each
other with full force, interacting with the laser propagating in the
opposite direction." The following collision is so violent that it produces
an extremely large number of gamma quanta -- light particles with an energy
even higher than that of X-rays.
The swarm of gamma quanta is so dense that the light particles inevitably
collide with each other. And then something crazy happens: According to
Einstein's famous formula E=mc2, light energy can transform into matter. In
this case, mainly electron-positron pairs should be created. Positrons are
the antiparticles of electrons. What makes this process special is that
"very strong magnetic fields accompany it," describes project leader Alexey
Arefiev, a physicist at the University of California at San Diego. "These
magnetic fields can focus the positrons into a beam and accelerate them
strongly." In numbers: Over a distance of just 50 micrometers, the particles
should reach an energy of one gigaelectronvolt (GeV) -- a size that usually
requires a full-grown particle accelerator.
Successful computer simulation
To see whether the unusual idea could work, the team tested it in an
elaborate computer simulation. The results are encouraging; in principle,
the concept should be feasible. "I was surprised that the positrons that
were created in the end were formed into a high-energy and bundled beam in
the simulation," Arefiev says happily. What's more, the new method should be
much more efficient than previous ideas, in which only a single laser pulse
is fired at an individual target: According to the simulation, the "laser
double strike" should be able to generate up to 100,000 times more positrons
than the single-treatment concept.
"Also, in our case, the lasers would not have to be quite as powerful as in
other concepts," Toncian explains. "This would probably make the idea easier
to put into practice." However, there are only few places in the world where
the method could be implemented. The most suitable would be ELI-NP (Extreme
Light Infrastructure Nuclear Physics), a unique laser facility in Romania,
largely funded by the European Union. It has two ultra-powerful lasers that
can fire simultaneously at a target -- the basic requirement for the new
method.
First tests in Hamburg
Essential preliminary tests, however, could take place in Hamburg
beforehand: The European XFEL, the most powerful X-ray laser in the world,
is located there. The HZDR plays a major role in this large-scale facility:
It leads a user consortium called HIBEF, which has been targeting matter in
extreme states for some time. "At HIBEF, colleagues from HZDR, together with
the Helmholtz Institute in Jena, are developing a platform that can be used
to experimentally test whether the magnetic fields actually form as our
simulations predict," explains Toma Toncian. "This should be easy to analyze
with the powerful X-ray flashes of the European XFEL."
For astrophysics as well as nuclear physics, the new technique could be
exceedingly useful. After all, some extreme processes in space are also
likely to produce vast quantities of gamma quanta, which then quickly
materialize again into high-energy pairs. "Such processes are likely to take
place, among others, in the magnetosphere of pulsars, i.e. of rapidly
rotating neutron stars," says Alexey Arefiev. "With our new concept, such
phenomena could be simulated in the laboratory, at least to some extent,
which would then allow us to understand them better."
Reference:
Yutong He, Thomas G. Blackburn, Toma Toncian, Alexey V. Arefiev. Dominance
of γ-γ electron-positron pair creation in a plasma driven by high-intensity
lasers. Communications Physics, 2021; 4 (1)
DOI: 10.1038/s42005-021-00636-x
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Physics