They are members of the new Center for
Astrophysical Plasma Properties (UTexas, UNevada, UArizona, WVU, Sandia Labs,
Lawrence Livermore Lab, NASA) and they share each precious Z shot to acquire
the laboratory data they use to benchmark theoretical models and to confirm or
refute hypothesis of processes taking place in our own galaxy. The
at-astrophysical-parameters approach has, as recently as last month, been
recognized by a new 5-year Center of Excellence, funded by the U.S. National
Nuclear Security Administration Stockpile Stewardship Academic Alliance, called
CAPP, an outgrowth of the ongoing Z Astrophysical Plasma Properties (ZAPP)
collaboration in which Koepke's group already belongs. The lab astrophysics
Center of Excellence benefits ongoing WVU Physics and Astronomy research
through partial support for graduate students, participation in Center
activities, and access to the Center’s intellectual resources, for example, to
the computational physics simulation codes that are key to spectroscopically
interpreting all of experiment's target-emission and target-absorption data. .
Plasma, which is ionized gas, is the term used
to describe ordinary gas that has had its neutral atoms separated into the
constituent positively charged ions and negatively charged electrons. Koepke
launched the WVU Plasma Physics Laboratory in 1987 and the group has grown to 4
regular faculty, 5 research faculty, 2 postdocs, and over 20 students engaged
internationally on a wide range of grand-challenge topics.
The ZAPP collaboration began in 2009 and has
been awarded approximately 10 dedicated and 10 ride-along shot opportunities
per year. ZAPP has demonstrated the capability to conduct up to five physics
experiments per Z shot, each designed by university investigators collaborating
with Sandia scientists. This multi-topic efficiency is crucial to the success
of the research and it maximizes scientific impact from expensive Z Facility
time.
The Z Facility drives 26 mega-amperes of current
through a parallel array of 360 11.4-micron-diameter tungsten wires during 100
nanoseconds (amounting to 220 terawatts of power). This requires 85 kV of
voltage and 1.6 mega-joules of energy.
“The wires heat and vaporize into tungsten gas
and the sudden compression of surrounding magnetic field energy pressurizes the
gas into a tungsten plasma that emits a 3-nanosecond blast of x-rays at maximum
plasma compression. The current is driven along one axis, the z axis, and the
compression pinches the plasma-laden magnetic-field lines to near-ignition
temperature and density, so we refer to this device as a z-pinch,” explains Koepke.
Surrounding this fist-size volume of extreme matter are 5 experiments,
conducted simultaneously with the x-ray blast that energizes the 5 target
samples into target plasma, for addressing unresolved mysteries associated with
characteristic properties and atomic kinetics of astrophysical plasma. The lab
data, so acquired, are unprecedented in terms of the harsh conditions and
precisely because those conditions have the same characteristic parameters as
the astrophysical conditions being studied.
"As an astronomer, I am used to looking at
these stars from light-years away," says Don Winget of the University of
Texas at Austin, ZAPP collaborator and CAPP Director. One of the primary
methods astronomers use to study a distant object is to analyze its spectrum of
light as it reaches Earth. "So it was a remarkable moment the first time
we took a spectrum from a distance of just 5 centimeters," said Winget.
That first time was in April 2010. Since then, the ZAPP collaboration has been
refining its ability to make precision measurements of a laboratory-recreated
white dwarf surface, active galactic nucleus, black-hole accretion disk,
neutron star, and the Sun’s interior, in order to improve interpretations of
data from spaced-borne instruments.
X-ray generation, propagation, heating, and
ionization contribute to the formation and evolution of many astrophysical and
laboratory plasmas. Spectroscopy measurements of these plasmas provide
intrinsically-interesting atomic and plasma physics. In addition, the interpretation
of laser fusion and astrophysics observations often must rely on
radiation-hydrodynamic simulations.
Realistic simulations depend on an accurate
interpretation of radiation absorption and re-emission, properties controlled
by the plasma particles and measured by sophisticated diagnostic instruments.
Approximations for radiation transport must often be used and the suitability
of these approximations can be tested in radiation science experiments.
Plasma spectroscopy is a key diagnostic and the
information quality is only as good as the atomic and plasma physics models
used to interpret the data. The importance of reliable radiation science
information is highest when multiple radiation hydrodynamic phenomena are
intermingled. In laser fusion or astrophysics measurements, isolating one
effect from another is often not an option and those effects must be unraveled
when interpreting the data. Prior radiation science experiments on isolated
phenomena bolsters confidence when interpreting integrated results.
"X-ray bursts emitted by the Z-pinch plasma
at Sandia’s Z Facility investigates radiation science at the world's intensity
frontier," Koepke points out. "To obtain enough plasma in the proper
fashion requires an experimental facility that can generate a large amount of
energy over a short time. Not many facilities can provide that, but the Z
machine can. The Z was built to study nuclear weapons, but it now allots about
15 percent of its experimental time to academic research." For further information,
contact Dr. Mark Koepke, mkoepke@wvu.edu, 304-293-4912.
U.S. Energy Dept's $7M promotes ZAPP to CAPP, benefitting ongoing WVU Physics and Astronomy research
A
dozen scientists gather each afternoon on Albuquerque’s Kirtland Air Force
Base, in Sandia National Laboratories, awaiting the bright flash and
unmistakable floor jolt that accompanies the daily lightning bolt known as a Z
shot. WVU physics professor Mark Koepke, his postdoc, and his students take
advantage of approximately twenty Z shots per year to feed their passion for
studying the physical principles that govern extreme astrophysical environments
through the laboratory study of high-energy-density physics.