Shock waves created in the lab mimic supernova particle accelerators


Date:
June 8, 2020
Source:
DOE/SLAC National Accelerator Laboratory
Summary:
Scientists have found new details about how supernovas boost
charged particles to nearly the speed of light.



FULL STORY ==========================================================================
When stars explode as supernovas, they produce shock waves in the plasma surrounding them. So powerful are these shock waves, they can act as
particle accelerators that blast streams of particles, called cosmic rays,
out into the universe at nearly the speed of light. Yet how exactly they
do that has remained something of a mystery.


==========================================================================
Now, scientists have devised a new way to study the inner workings of astrophysical shock waves by creating a scaled-down version of the shock
in the lab. They found that astrophysical shocks develop turbulence
at very small scales -- scales that can't be seen by astronomical
observations -- that helps kick electrons toward the shock wave before
they're boosted up to their final, incredible speeds.

"These are fascinating systems, but because they are so far away it's
hard to study them," said Frederico Fiuza, a senior staff scientist at
the Department of Energy's SLAC National Accelerator Laboratory, who led
the new study. "We are not trying to make supernova remnants in the lab,
but we can learn more about the physics of astrophysical shocks there
and validate models." The injection problem Astrophysical shock waves
around supernovas are not unlike the shockwaves and sonic booms that form
in front of supersonic jets. The difference is that when a star blow up,
it forms what physicists call a collisionless shock in the surrounding
gas of ions and free electrons, or plasma. Rather than running into
each other as air molecules would, individual electrons and ions are
forced this way and that by intense electromagnetic fields within the
plasma. In the process, researchers have worked out, supernova remnant
shocks produce strong electromagnetic fields that bounce charged particles across the shock multiple times and accelerate them to extreme speeds.

Yet there's a problem. The particles already have to be moving pretty
fast to be able to cross the shock in first place, and no one's sure what
gets the particles up to speed. The obvious way to address that issue,
known as the injection problem, would be to study supernovas and see
what the plasmas surrounding them are up to. But with even the closest supernovas thousands of light years away, it's impossible to simply point
a telescope at them and get enough detail to understand what's going on.



========================================================================== Fortunately, Fiuza, his postdoctoral fellow Anna Grassi and colleagues had another idea: They'd try to mimic the shock wave conditions of supernova remnants in the lab, something Grassi's computer models indicated could
be feasible.

Most significantly, the team would need to create a fast, diffuse shock
wave that could imitate supernova remnant shocks. They would also need
to show that the density and temperature of the plasma increased in
ways consistent with models of those shocks -- and, of course, they
wanted to understand if the shock wave would shoot out electrons at very
high speeds.

Igniting a shock wave To achieve something like that, the team went
to the National Ignition Facility, a DOE user facility at Lawrence
Livermore National Laboratory. There, the researchers shot some of the
world's most powerful lasers at a pair of carbon sheets, creating a pair
of plasma flows headed straight into each other.

When the flows met, optical and X-ray observations revealed all the
features the team were looking for, meaning they had produced in the
lab a shock wave in conditions similar to a supernova remnant shock.

Most importantly, they found that when the shock was formed it
was indeed capable of accelerating electrons to nearly the speed of
light. They observed maximum electron velocities that were consistent
with the acceleration they expected based on the measured shock
properties. However, the microscopic details of how these electrons
reached these high speeds remained unclear.



========================================================================== Fortunately, the models could help reveal some of the fine points,
having first been benchmarked against experimental data. "We can't see
the details of how particles get their energy even in the experiments,
let alone in astrophysical observations, and this is where the simulations really come into play," Grassi said.

Indeed, the computer model revealed what may be a solution to the electron injection problem. Turbulent electromagnetic fields within the shock wave itself appear to be able to boost electron speeds up to the point where
the particles can escape the shock wave and cross back again to gain even
more speed, Fiuza said. In fact, the mechanism that gets particles going
fast enough to cross the shock wave seems to be fairly similar to what
happens when the shock wave gets particles up to astronomical speeds,
just on a smaller scale.

Toward the future Questions remain, however, and in future experiments
the researchers will do detailed measurements of the X-rays emitted
by the electrons the moment they are accelerated to investigate how
electron energies vary with distance from the shock wave. That, Fiuza
said, will further constrain their computer simulations and help them
develop even better models. And perhaps most significantly, they will
also look at protons, not just electrons, fired off by the shock wave,
data which the team hopes will reveal more about the inner workings of
these astrophysical particle accelerators.

More generally, the findings could help researchers go beyond the
limitations of astronomical observations or spacecraft-based observations
of the much tamer shocks in our solar system. "This work opens up a
new way to study the physics of supernova remnant shocks in the lab,"
Fiuza said.


========================================================================== Story Source: Materials provided by
DOE/SLAC_National_Accelerator_Laboratory. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. F. Fiuza, G. F. Swadling, A. Grassi, H. G. Rinderknecht,
D. P. Higginson,
D. D. Ryutov, C. Bruulsema, R. P. Drake, S. Funk, S. Glenzer,
G. Gregori, C. K. Li, B. B. Pollock, B. A. Remington, J. S. Ross,
W. Rozmus, Y.

Sakawa, A. Spitkovsky, S. Wilks, H.-S. Park. Electron acceleration
in laboratory-produced turbulent collisionless shocks. Nature
Physics, 2020; DOI: 10.1038/s41567-020-0919-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200608134410.htm

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