The “hot sources” of radiation emitted from a property of plasma turbulence known as the Kelvin-Helmholtz instability, which occurs when two, passing streams of plasma collide. Visualization by Dave Pugmire, ORNL.

Radiation measured from Earth can help scientists characterize far-away plasma dynamics

Outshining the black holes they surround, the bright, hot centers of galaxies known as active galactic nuclei can spew jets of plasma thousands of light-years long. These streams of plasma create an effect often seen in popular images—galaxies speared through the heart by intense light. Such jets are also associated with stars and other astronomical phenomenon.

Although physicists are keen to learn the fluid-like mechanics taking place in jets of roiling protons and electrons, there is one problem: distance. Millions or billions of light-years stretch between scientists here on Earth and cosmic jets, and observing individual electromagnetic particles through a telescope is impossible—impossible, but no less desirable.

“Understanding these plasma jets can help explain what is happening to the matter in these objects—how it is accelerated to such high energies and other fundamental physics out of our reach,” said Michael Bussmann, HZDR–Dresden Computational Radiation Physics group leader.

In pursuit of the impossible, a team from Germany’s HZDR–Dresden used Titan, the most powerful supercomputer in the United States located at Oak Ridge National Laboratory, to simulate billions of particles in two passing jet streams. The code’s run on Titan, the Oak Ridge Leadership Computing Facility’s 27-petaflop, hybrid CPU/GPU Cray XK7 machine, earned the team a finalist nomination for the Association of Computing Machinery’s 2013 Gordon Bell Prize. The prize recognizes outstanding achievements in high-performance computing (HPC) applications, specifically codes that redefine what is possible in HPC. This year’s winner will be announced at the supercomputing conference SC13 held November 17–22.

An unlikely couple

By modeling a well-known property of plasma turbulence called the relativistic Kelvin-Helmholtz instability (KHI), which occurs where passing plasma jets collide, researchers were able to make out patterns of particle behavior—the inner workings of these far-away objects.

Then they used radiative signatures, one of the clues we can measure with the help of a telescope, to correlate plasma dynamics with radiation emitted during turbulence. If the jets simulated on Titan were flung far into space, particles disrupted by the KHI could not be observed from Earth, but the radiation they put off could.

Ultimately the KHI tells physicists about the properties of passing plasma jets through comparison: Is one plasma stream denser than the other? What are their velocities? In what directions are they traveling?

And understanding plasma jet dynamics could reveal information about their objects of origin, such as active galactic nuclei.

“Our scientific question was, ‘Can we correlate the radiative signature with individual particles?’ ” Bussman said. “Is there a chance to really see what’s happening inside the plasma just by looking at the radiation? We are very limited in our tools to connect plasma dynamics to what we observe, and this is where simulation comes in.”

Indeed, the results from Titan show radiation can be a diagnostic for the plasma dynamics taking place far beyond our reach.

Like wind causing ripples on the surface of a lake, plasma turbulence has often been viewed as a hydrodynamics question because the local electromagnetic field in plasma creates similar currents. However, while similar, the two are not exactly the same thing.

Hydrodynamics, or fluid, simulations generate smooth patterns of movement by averaging velocity, density, and other parameters around a point. Until now KHI simulations have largely been done with hydrodynamics calculations.

“The KHI is so fundamental—it occurs in all fluid systems—so it’s a very good test system to see if we can see it in the radiation patterns,” he explained.

But to simulate radiation emitting from the jets as well, the team had to perform kinetic simulations, which follow the path of individual particles and require many more unique calculations than hydrodynamics simulations.

“Calculations for kinetic KHI simulations are difficult because we have particles being accelerated by electromagnetic fields in plasmas, so they follow different paths,” Bussmann said. “And every time particles collide, there is a slight change in direction. They slow down or speed up, and then they radiate at a given frequency.”

Colors distinguish electron acceleration, or the change in an electron’s speed and direction, occurring in passing plasma streams. Red indicates large electron acceleration leading to strong radiation emission.

Colors distinguish electron acceleration, or the change in an electron’s speed and direction, occurring in passing plasma streams. Red indicates large electron acceleration leading to strong radiation emission. Visualization by Dave Pugmire, ORNL.

Creating chaos

Using a particle-in-cell code that computes the interaction between charged particles, the team modeled two streams of unmagnetized hydrogen plasma totaling 75 billion particles per simulation, including protons and electrons. Further calculations simulated the entire spectrum of radiation streaming from the jets in 481 directions.

When simulations of the jets’ charged particles began running, researchers saw chaos—at first.

“There is a lot of complex substructure, and it looks like particles are moving randomly. It looks chaotic, but as the simulation continues to run, we see structures emerge,” Bussmann said. “What we see at the interface between the two streams where turbulence occurs looks like mushrooms or whirlpools.”

These patterns are coming to light primarily because, on Titan, the KHI simulations achieved a resolution not previously obtained.

“To our knowledge, this simulation on Titan is 46 times larger, and the spatial resolution is 4.2 times higher than the largest kinetic KHI simulation to date,” Bussmann said.

Most of this scalability is due to Titan’s GPUs, or graphics processing units, which are incredibly fast at the repetitive calculations that compose many HPC applications. Both the plasma dynamics and emitted radiation were computed on the GPUs.

“We believe that this task could not have been done without the accelerator architecture,” Bussmann said.  “In order to compute the particle motion and radiation together, one needs a high bandwidth and computing power. We take the trajectory of each of the several billion particles and then use this data to calculate the radiation emitted in hundreds of directions for all relevant wavelengths.”

With the data from Titan, researchers can begin to apply the results to actual plasma jets. The spectrum of radiative signatures emanating from the simulated jets provides a measurement stick of sorts against which scientists can calibrate for observed objects.

“We know every spectrum and every direction of the radiation from the Titan simulations, and we can use this information to map the radiative signatures to different objects,” Bussmann said. “By extension, we can use it as an input to predict the dynamics for different plasma jets we observe from Earth.”

Turns out, maybe Earth isn’t such a bad place to study black holes and quasars after all, no matter the distance.—Katie Elyce Jones