Probing the Mysteries of Cosmic Magnetic Fields

The universe is awash in magnetic fields that fill galaxies like our Milky Way and even galactic clusters—the largest structures in the universe. But how were these magnetic fields created in the early universe, and how did they grow to the size and strength seen in galaxies and galactic clusters today? It’s a mystery that a new NIF Discovery Science campaign known as Turbulent Dynamo, or TDyno, is hoping to help solve.

The goal of the TDyno campaign, led by a team from the University of Oxford in the United Kingdom, is to study how small "seed" magnetic fields are amplified under the kind of turbulent plasma conditions that exist throughout the cosmos. The process by which this happens is called the turbulent dynamo mechanism. In everyday life, dynamos are machines that convert mechanical energy into electric energy. This is the way scientists believe seed magnetic fields in the early universe are amplified to the point where the plasma’s magnetic energy is on par with its kinetic energy, as is observed today.

"NIF is the only laser on Earth that can create plasmas that are hot and fast enough, and that last long enough, to produce a turbulent dynamo of the kind scientists think occurs in galaxy clusters—the Holy Grail of laboratory plasma astrophysics," said experimentalist Jena Meinecke, a junior research fellow at the University of Oxford. "We’re accessing a plasma regime that no one has had access to before—it’s really exciting science.”

The first three NIF TDyno experiments were conducted on Aug. 4, capping a Discovery Science shot week that included 13 experiments in five separate high energy density science campaigns (see "A Big Week for NIF Discovery Science"). The shot day’s goal was to create a turbulent plasma and measure its properties. The TDyno campaign is closely related to the collisionless shock campaign being conducted on NIF by the Astrophysical Collisionless Shock Experiments with Lasers (ACSEL) collaboration. Collisionless shocks are thought to be one of the mechanisms that can create seed magnetic fields, which the turbulent dynamo can then amplify.

The NIF TDyno experiments build on earlier experiments conducted by the Oxford team, led by physicist Gianluca Gregori, at the Laboratory for the Use of Intense Lasers (LULI) near Paris in 2012. These experiments tested the theory that astrophysical magnetic fields could have been seeded by asymmetric shock waves due to misaligned temperature and pressure gradients, known as the Biermann battery process, during galaxy formation (see "Generation of scaled protogalactic seed magnetic fields in laser-produced shock waves," Nature, Jan. 26, 2012). The researchers trained intense, short-duration laser pulses on the end of a thin carbon rod (like the tip of a needle) held inside a helium-filled chamber to create shock waves. As the shock wave moved through the plasma in the chamber, a magnetic field was generated. Scientists believe this is what occurs in galactic clusters. While the initial seed fields are miniscule, simulations show that they could be amplified over about 700 million years to the strength we see in galactic clusters through the turbulent dynamo mechanism.

"Over time what happens is that these (original) magnetic fields are amplified," Meinecke said, "and so all the magnetic fields we see today in our universe may be attributed to these primordial magnetic fields. Turbulent dynamo is the mechanism that is thought to amplify fields to the levels that we observe today. The goal of the NIF experiments is to recreate in a scaled sense these incredible dynamics of the universe. We can then study the history of the magnetic fields in our universe."

Extensive simulations using FLASH—a computer code that can model laser-produced hot plasmas, radiation, and magnetic fields—were used to help design the NIF experiments. "The simulations we did were crucial to designing a laboratory experiment that would create plasma conditions extreme enough for the turbulent dynamo to operate," said Petros Tzeferacos, the associate director of the Flash Center for Computational Science at the University of Chicago where the FLASH code was developed.

"We gained confidence in the simulations by showing that FLASH correctly predicted the plasma properties in earlier TDyno experiments we conducted at the Omega Laser Facility at the Laboratory for Laser Energetics at the University of Rochester. The FLASH simulations of the NIF experiment successfully predicted the moment when the two plasma flows collide, and therefore the best times to fire the diagnostics."

In the Aug. 4 NIF experiments, plasma flows were created by two plastic foils facing each other, each one heated by 281, then 237, and finally 355 kilojoules of laser energy. Passing the flows through plastic grids and having them collide produced a highly turbulent plasma. A tiny hollow laser-irradiated sphere known as an "exploding pusher" target filled with a mixture of deuterium and helium-3 (D³He) created a monoenergetic proton source to diagnose the turbulent flow. The D³He target was developed in collaboration with the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center.

Previous TDyno experiments have been conducted on the Vulcan Laser Facility in the UK and the Omega Laser Facility, but NIF & Photon Science physicist Hye-Sook Park, the LLNL liaison to the TDyno team, said the NIF experiments were expected to produce the conditions relevant to galactic clusters for the first time.

"It is so wonderful to see that the first set of experiments was finally carried out successfully," she said. "Gianluca Gregori, the principal investigator, and I have waited a long time to start these laboratory astrophysics experiments on NIF; our first NIF proposal was submitted in 2010. It is also very rewarding to see that many junior researchers like Jena Meinecke are so enthusiastic about NIF."

Meinecke praised the professionalism of the NIF Team for completing all three hoped-for shots in one day. "The Control Room team was just on fire in the evening," she said. "Everything got done so quickly, and it really shows you how efficient the NIF Team is.

"All three shots produced good data," she said, "and we will compare them with our simulations as soon as possible. We used x-ray cameras to image the turbulent structures, polarimetry and proton radiography to probe the magnetic fields, backscatter diagnostics, and proton self-emission."

Significantly, she said, excellent data were obtained by the polarimetry diagnostic, which measures the rotation of the polarization of a NIF laser beam as it passes through a magnetized region of plasma, for the first time for a NIF Discovery Science experiment.

"We are hoping to see a very turbulent plasma that has magnetic fields embedded within it," she said. "Eddies stretch, twist, and fold the magnetic field, and when that happens you get the turbulent dynamo; at least that is the prediction. The x-ray images show a very turbulent structure and they show that the turbulence grows. We are hopeful that our data will show that we achieved the turbulent dynamo mechanism.

"Seeing our work come to life on the National Ignition Facility is incredibly rewarding and exciting," Meinecke said. "The entire team has worked so hard, for years."

NIF Discovery Science Leader Bruce Remington credited the TDyno experiments and the other Discovery Science campaigns with "bringing new ideas, new science, and new diagnostic techniques" to NIF. "We benefit greatly by working with" NIF’s users, he said. "We inevitably have a better facility and better science coming out of NIF when we have new people coming in with new thoughts, new creative instincts and new challenges for us to try to achieve."

The TDyno campaign on NIF is scheduled to resume next February, with the third shot day six months after that.