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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. 

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.

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