Advancing Plasma Science
Chuanfei Dong. Background image of flame nebula courtesy of NASA/JPL-Caltech
Image of starlight on exoplanet, courtesy of NASA/JPL-Caltech
Blowing in the stellar wind
Is there life beyond Earth in the cosmos? Astronomers looking for signs have found that our Milky Way galaxy teems with exoplanets, some with conditions that could be right for extraterrestrial life. Such worlds orbit stars in so-called “habitable zones,” regions where planets could hold liquid water that is necessary for life as we know it.
However, the question of habitability is highly complex. Researchers led by space physicist Chuanfei Dong of PPPL and Princeton University have recently raised doubts about water on — and thus potential habitability of — frequently cited exoplanets that orbit red dwarfs, the most common stars in the Milky Way.
In two papers in The Astrophysical Journal Letters and another in Proceedings of the National Academy of Sciences, the scientists develop models showing that the stellar wind — the constant outpouring of charged particles that sweep into space — could severely deplete the atmosphere of such planets over hundreds of millions of years, rendering them unable to host surface-based life as we know it.
“Traditional definition and climate models of the habitable zone consider only the surface temperature,” Dong said. “But the stellar wind can significantly contribute to the long-term erosion and atmospheric loss of many exoplanets, so the climate models tell only part of the story.”
Given the increased activity of red stars and the close-in location of planets in habitable zones, such findings indicate the high probability of dried-up surfaces on planets that orbit red stars that might once have held oceans that could give birth to life. The results could also modify the famed Drake equation, which estimates the number of civilizations in the Milky Way, by lowering the estimate for the average number of planets per star that can support life.
The researchers note that predicting the habitability of planets located light years from Earth is of course filled with uncertainties. Future missions like the James Webb Space Telescope, which NASA will launch in 2019 to peer into the early history of the universe, will therefore “be essential for getting more information on stellar winds and exoplanet atmospheres,” the authors say in a paper, “thereby paving the way for more accurate estimations of stellar-wind induced atmospheric losses.”
Igor Kaganovich and Charles Swanson
Feathers and whiskers help prevent short circuits in plasma devices
How can short-circuits in machines such as spacecraft thrusters, radar amplifiers, and particle accelerators that are fueled or operate with plasma be prevented? PPPL physicists have found a surprising way to keep such troublesome shorts from occurring. The researchers found that applying microscopic structures that resemble feathers and whiskers to the plasma can keep the machines operating at peak efficiency.
Physicists Charles Swanson and Igor Kaganovich calculated that introducing tiny feather- and whisker-like fibers into the plasma can trap electrons dislodged from the interior surfaces of the machines by other electrons that zoom in from the plasma. Trapping these dislodged electrons, called “secondary electron emissions” (SEE), prevents them from creating electric current that interferes with machine functions.
These findings build on previous experiments at PPPL involving plasma thrusters, showing that use of fibered textures can partially reduce the amount of secondary electron emission. Past research indicated that plasma with plain fibers called “velvet,” which lack feather-like branches, only trap about half of such electrons. “When we looked at velvet, we observed that it didn’t suppress SEE that well,” Swanson said. “So we added another set of fibers to suppress the remaining secondary electrons and that appears to work nicely.”
This research was funded by the Air Force Office of Scientific Research and follows experimental work on plasma-wall interactions and the effect of plasma thrusters on wall materials performed at PPPL by physicists and visiting students.
Scientists create first laboratory generation of high-energy shock waves
Throughout the universe, supersonic shock waves propel cosmic rays and supernova particles to velocities near the speed of light. The most high-energy of these astrophysical shocks occur too far outside the solar system to be studied in detail and have long puzzled astrophysicists. Shocks closer to Earth can be detected by spacecraft, which fly by too quickly to probe a wave’s formation.
Now a team of scientists has generated the first high-energy shock waves in a laboratory setting, opening the door to new understanding of these mysterious processes. “We have for the first time developed a platform for studying highly energetic shocks with greater flexibility and control than is possible with spacecraft,” said Derek Schaeffer, a physicist at PPPL and Princeton University and lead author of a paper in Physical Review Letters that outlines the experiments.
Schaeffer and colleagues, including PPPL physicist Will Fox, conducted their research on the Omega EP laser facility at the University of Rochester Laboratory for Laser Energetics. To produce the wave, scientists used a laser to create a high-energy plasma that expanded into a pre-existing magnetized plasma. The interaction created, within a few billionths of a second, a magnetized shock wave that expanded at a rate of more than 1 million miles per hour, congruent with shocks beyond the solar system. The rapid velocity represented a high “magnetosonic Mach number” and the wave was “collisionless,” emulating shocks that occur in outer space where particles are too far apart to frequently collide.
Researchers simulated the findings with a computer code called “PSC” that utilized data derived from the experiments. Results of the model agreed well with diagnostic images of the shock formation. Going forward, the laboratory platform will enable new studies of the relationship between collisionless shocks and the acceleration of astrophysical particles, paving the way to controlled laboratory investigations of high-Mach number shocks.
Physicist Derek Schaeffer
Image of a supernova remnant with shockwave seen as thin blue boundary at the edge. Credit: NASA, ESA, Zolt Levay
Northern lights illuminating the sky (Photo by NASA)
Physicists Jonathan Ng and Ammar Hakim
Team led by graduate student produces unique simulation of magnetic reconnection
Jonathan Ng, a Princeton University graduate student at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), has applied a fluid simulation that for the first time approximates the impact of the explosive process behind solar flares northern lights and space storms. The model could lead to improved forecasts of space weather that can shut down cell phone service and damage power grids, as well as to better understanding of the hot, charged plasma gas that fuels fusion reactions.
The new simulation captures the physics of magnetic reconnection, the breaking apart and snapping together of the magnetic field lines in plasma that occurs throughout the universe. The model treats plasma as a flowing liquid to create a more detailed picture of the reconnection process in the vastness of space where widely separated plasma particles rarely collide. Previous simplified fluid codes could not normally capture such kinetic effects. “This is the first application of this particular fluid model in studying reconnection physics in space plasmas,” said Ng, lead author of the findings reported in Physics of Plasmas.
Ng and coauthors approximated the kinetic effects of magnetic reconnection with a series of fluid equations based on plasma density, momentum and pressure. They concluded the process through a mathematical technique called “closure” that enabled them to describe the kinetic mixing of particles from non-local, or large-scale, regions. The completed results agreed better with kinetic models as compared with simulations produced by traditional fluid codes and could extend understanding of reconnection to whole regions of space to provide a more comprehensive view of the universal process.
Coauthoring the paper were physicists Ammar Hakim of PPPL and Amitava Bhattacharjee, head of the Theory Department at PPPL and a professor of astrophysical sciences at Princeton University, together with physicists Adam Stanier and William Daughton of Los Alamos National Laboratory.