Deep inside gas giants like Jupiter and Saturn, hydrogen and helium coexist under pressures millions of times greater than Earth's atmosphere. At those conditions, helium may separate from hydrogen and influence a planet's internal heat flow, structure and magnetic field. Understanding these processes and how these materials behave under extreme conditions is essential to building accurate models of planetary evolution.
New experimental results, published in Physical Review Research, reveal the behavior of helium at unprecedented pressures. The research, conducted by scientists at Lawrence Livermore National Laboratory (LLNL), the University of California, Berkeley, the French Commissariat à l'Énergie Atomique et aux Energies Alternatives (CEA) and the University of Rochester's Laboratory for Laser Energetics (LLE), shows that helium reacts differently from the predictions of most broad-range theoretical models.
Reaching warm dense helium
"Helium is an interesting material because, unlike hydrogen, its atoms do not form molecules," said LLNL physicist Marius Millot. "Therefore, we can isolate how pressure and temperature cause atoms to lose electrons, a process called ionization."
Because helium exists as individual atoms rather than molecules like hydrogen (H2), it has no dissociation step prior to ionization - i.e., it does not have to break apart first. This makes its response to extreme conditions simpler to study, without the transition of a covalent bond breaking. But to study helium's behavior, the team first had to solve its density problem. Gaseous helium is too diffuse to shock efficiently, while cryogenic liquid helium compresses too easily, making it an unsuitable starting point on its own for reaching warm dense helium conditions relevant to planetary interiors.
The team's solution was to pre-compress liquid helium in a diamond anvil cell (DAC). This device squeezes a tiny sample between two diamond tips to pressures of 7,600-12,000 atmospheres (0.76 to 1.2 gigapascals), where it becomes three times denser than cryogenic liquid helium.
"Using those initial densities, we use the OMEGA laser at the LLE to ablatively drive a large shock into the diamond anvil," said Jon Eggert, LLNL physicist.
LLNL and CEA have a long record of advancing DAC technology, and leveraging that expertise proved critical in their experiments. Redesigning the DAC with a thinner diamond window, just 200 micrometers, allowed the laser energy to couple more efficiently into the sample. The resulting shock pressures and temperatures reached between 270 and 360 gigapascals and 50 to 80 thousand Kelvin, nearly doubling the pressure and temperature of previous helium shock experiments.
Ionization confirmed at record pressures
During the experiments, velocity interferometry system for any reflector (VISAR) and streaked optical pyrometry (SOP) diagnostics simultaneously tracked how fast the shock wave traveled and how hot and reflective the shock front became. These diagnostics were analyzed by the paper's first author Michael Wadas, who joined the collaboration under the mentorship of Millot and Eggert as a KRELL Institute summer fellow.
Wadas' analyses showed greater compressibility and substantially higher reflectivity than several equation-of-state models had predicted. These results were consistent with helium undergoing continuous ionization - that is, the helium changed state gradually, not through a sharp transition. Conductivity values inferred from the temperature and reflectivity data confirmed that shocked helium in this regime behaves as an electrically conducting fluid. Encouragingly, the results also validated first-principles computer simulations - calculations built from fundamental physics rather than empirical fits - bringing theory and experiment into agreement for the first time for helium at these pressures and temperatures.
Looking ahead, the team will investigate the onset of electrical conductivity of helium at even higher density to reach the conditions found in the outer atmospheres of white dwarf stars.
"We plan to figure out if numerical simulation methods such as density functional theory are reliable to predict the conductivity of helium at those extreme conditions of pressure and temperature," said Millot.
By continuing to push the boundaries of what can be measured in the laboratory, LLNL researchers are bringing the extreme physics of planetary and stellar interiors within experimental reach.
This research was supported by LLNL's Laboratory Directed Research and Development program.
-Grace Jeng
