Among the intriguing issues in plasma
physics are those surrounding X-ray pulsars—collapsed stars that orbit around a
cosmic companion and beam light at regular intervals, like lighthouses in the
sky. Physicists want to know the strength of the magnetic field and density of
the plasma that surrounds these pulsars, which can be millions of times greater
than the density of plasma in stars like the sun.
Researchers at the U.S. Department of
Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have developed a
theory of plasma
waves that can infer these properties in greater detail than in standard
approaches. The new research analyzes the plasma surrounding the pulsar by
coupling Einstein's theory of relativity with quantum mechanics, which describes the motion of subatomic particles such as
the atomic nuclei—or ions—and electrons in plasma. Supporting this work is the
DOE Office of Science.
Quantum field theory
The key insight comes from quantum
field theory, which describes charged particles that are
relativistic, meaning that they travel at near the speed of light.
"Quantum theory can describe certain details of the propagation of waves
in plasma," said Yuan Shi, a graduate student in the Princeton Program in
Plasma Physics and lead author of a paper published July 29 in the journal Physical
Review A. Understanding the interactions behind the propagation can then
reveal the composition of the plasma.
Shi developed the paper with assistance
from co-authors Nat Fisch, director of the Program in Plasma Physics and
professor and associate chair of astrophysical sciences at Princeton
University, and Hong Qin, a physicist at PPPL and executive dean of the School
of Nuclear Science and Technology at the University of Science and Technology
of China. "When I worked out the mathematics they showed me how to apply
it," said Shi.
In pulsars, relativistic particles in
the magnetosphere, the magnetized atmosphere that surrounds the body, absorb
light waves, and this absorption displays peaks against a blackbody background.
"The question is, what do these peaks mean?" asks Shi. Analysis of
the peaks with equations from special relativity and quantum field theory, he
found, can determine the density and field strength of the magnetosphere.
Combining physics
techniques
The process combines the techniques of
high-energy physics, condensed matter physics, and plasma
physics. In high-energy physics, researchers use quantum field
theory to describe the interaction of a handful of particles. In condensed
matter physics, people use quantum mechanics to describe the states of a large
collection of particles. Plasma physics uses model equations to explain the
collective movement of millions of particles. The new method utilizes aspects
of all three techniques to analyze the plasma waves
in pulsars.
The same technique can be used to infer
the density of the plasma and strength of the magnetic field created by
inertial confinement fusion experiments. Such experiments use lasers to ablate—or
vaporize —a target that contains plasma fuel. The ablation then causes an
implosion that compresses the fuel into plasma and produces fusion reactions.
Standard formulas
give inconsistent answers
Researchers want to know the precise
density, temperature and field strength of the plasma that this process
creates. Standard mathematical formulas give inconsistent answers when lasers
of different color are used to measure the plasma parameters. This is because
the extreme density of the plasma gives rise to quantum effects, while the high
energy density of the magnetic field gives rise to relativistic
effects, says Shi. So formulations that draw upon both fields are needed to
reconcile the results.
For Shi, the new technique shows the
benefits of combining physics disciplines that don't often interact. Says he:
"Putting fields together gives tremendous power to explain things that we
couldn't understand before."
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