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 uncover 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 the cosmic jets.
“Understanding these plasma jets,” says Michael Bussmann, computational radiation physics group leader at the Helmholtz-Zentrum Dresden-Rossendorf Center (HZDR) in Germany, “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.”
In pursuit of the impossible, the team from HZDR used the Titan supercomputer at Oak Ridge National Laboratory in Tennessee to simulate billions of particles in two passing jet streams. The code’s run made the team a finalist for the Association of Computing Machinery’s 2013 Gordon Bell Prize
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. They then used radiative signatures, one of the clues that can be measured with 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 which directions are they traveling? Understanding plasma jet dynamics could reveal information about their objects of origin, such as active galactic nuclei.
“Our scientific questions are ‘Can we correlate the radiative signature with individual particles? Is there a chance to really see what’s happening inside the plasma just by looking at the radiation?'" says Bussmann. "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 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. 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,” Bussman notes. 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.
The “hot sources” of radiation emitted from a property of plasma turbulence known as the Kelvin-Helmholtz instability (KHI), which occurs when two passing streams of plasma collide.
“Calculations for kinetic KHI simulations are difficult because we have particles being accelerated by electromagnetic fields in plasmas, so they follow different paths,” Bussmann says. “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.”
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 says. “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,” says Bussmann.
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 high-performance computing applications. Both the plasma dynamics and emitted radiation were computed on the GPUs. “This task could not have been done without the accelerator architecture,” Bussmann says. “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, and by extension, we can use it as an input to predict the dynamics for different plasma jets we observe from Earth.”