November 2, 2022
Thanks to their massive gravity, compact astrophysical objects like black holes and neutron stars are among the most voracious entities in the universe, gobbling up interstellar gas and even dragging matter away from nearby stars in what are known as accretion disks.
Scientists want to better understand this accretion process and the disk of ionized gas formed around these compact objects because it could provide clues to their nature and behavior—such as the spectacular “jets” that emerge from supermassive black holes at close to the speed of light—as well as the properties of astrophysical plasmas.
For example, quantitative analysis of the spectral features of iron observed in the x-ray reflection spectrum from the dense, innermost region of an accretion disk provides one of the best methods to infer the spin, or rotation, of the black hole that helps power the jets.
Directly observing accretion disks, however, is challenging because of their surrounding environment and the complexity of the system. That’s where well-characterized and -diagnosed laboratory experiments on facilities like NIF, the world’s largest and highest-energy laser, play a key role.
NIF experiments “provide a unique opportunity to create states of matter at extreme conditions of temperature, density, and radiation flux relevant to astrophysics, and thus to explore the universe in the laboratory,” said physics professor Roberto Mancini of the University of Nevada, Reno. Mancini is the principal investigator of a new NIF Discovery Science campaign designed to simulate the type of photoionized plasma found in accretion disks and the surrounding stellar wind.
“Laboratory experiments have become critical to guide our understanding as well as to test data interpretation and astrophysical models that have been developed only on a best-theory effort,” Mancini said.
Photoionized plasmas are created when matter is superheated on its way into a black hole or neutron star; high-intensity broadband x-ray and ultraviolet radiation knocks electrons out of their orbits, creating the mixture of ions and free electrons called plasma.
“The focus of this research,” Mancini said, “is to study the fundamental atomic and radiation physics of plasmas driven by an intense flux of x rays—i.e., iron photoionized plasmas—at conditions relevant to those encountered in accretion disks around black holes, x-ray binaries, and warm absorbers in active galactic nuclei.
“Iron is a key element for astrophysics,” he said. “The x-ray flux energy and duration needed to do the experiment are only available at NIF.”
Collaborating on the research are Stanford University, Caltech, the University of Kentucky, Imperial College London, Technion, NASA, and LLNL.
The Photoionized Plasma Campaign uses NIF’s high-energy laser beams to produce plasma in a type of steady state called photoionization equilibrium (PIE), where photoionization is balanced by radiative and dielectronic recombination.
Previous experiments at the Z pulsed-power facility at Sandia National Laboratories and other facilities have produced significant discrepancies with astrophysics codes. The NIF iron PIE experiments “will demonstrate iron PIE in the laboratory for the first time,” Mancini said, “thus making a critical contribution to understanding these discrepancies.”
The campaign conducted its first three experiments on Aug. 3. To achieve an x-ray drive longer than the equilibration time—about 10 to 30 nanoseconds (billionths of a second) in the lab—NIF drove three gold halfraums (half hohlraums), one at a time for 10 nanoseconds each, producing a 30-nanosecond drive onto a thin plastic-tamped iron-magnesium sample. Simulations predicted that the sample would heat and expand from half a micron to more than 1,000 microns in thickness.
The sample was backlit to produce an absorption spectrum recorded with the NIF Opacity Spectrometer. Analysis of the transmission spectrum provides plasma ionization and temperature data. Separately, a gated imager measured the sample expansion to infer its density and the DANTE-1 diagnostic measured the drive flux from the halfraums.
“The Aug. 3 experiments succeeded in establishing the experimental platform and “obtaining usable data on all three primary diagnostics,” said LLNL physicist Bob Heeter, the campaign’s responsible individual. The researchers expect the simultaneous flux, density, and absorption data to improve understanding of the potoionization/recombination rate balance, relevant not just to extreme astrophysical environments but also to laser-driven “non-LTE” (non-local thermodynamic equilibrium) plasmas generally.
The next experiments in the campaign are scheduled for May of next year.
“NIF Helps Unravel Mysteries of Heat Conduction in Turbulent Galaxy-Cluster Plasmas,” NIF & Photon Science News, March 9, 2022
“Laser Experiments Illuminate the Cosmos,” Science & Technology Review, December, 2016
Follow us on Twitter: @lasers_llnl