International Team Conducts First Collisionless Shock Experiment on NIF

The first NIF Discovery Science experiment designed to create and study fully formed collisionless shocks, such as those responsible for the properties of many astrophysical phenomena including supernova remnants, gamma-ray bursts, jets from active galactic nuclei, and cosmic ray acceleration, was performed on July 29.

Studying astrophysics with laboratory experiments can help answer questions about micro-physics in astrophysical objects that are far beyond the reach of direct measurements. Most shock waves in astrophysics are collisionless from high plasma flow velocities—they form due to plasma instabilities and self-generated magnetic fields. Laser-driven plasma experiments can study the micro-physics of plasma interaction and instability formation (known as filamentary Weibel instability) under controlled conditions. NIF is the only facility that can create the proper plasma conditions to generate fully formed collisionless shocks and strong magnetic fields.

The NIF experiment, conducted by an international team of physicists comprising the Astrophysical Collisionless Shock Experiments with Lasers (ACSEL) campaign, builds on simulations and a number of previous experiments at the University of Rochester’s OMEGA Laser Facility. It was designed to investigate high-Mach-number non-relativistic collisionless shock formations. The experiment also collected data for the study of self-generated magnetic fields from the Weibel instability in counter-streaming plasma flows, and magnetic field generation and amplification in turbulent flows.

Supported by LLNL’s Hye-Sook Park and Steven Ross, the ACSEL team used the NIF lasers to irradiate the inner surface of two deuterated plastic foils doped with iron and nickel to create high-velocity counter-streaming plasmas. All 60 requested NIF beams delivered 307 kilojoules (kJ) of 3ω (ultraviolet) light to the targets in a 64.5-terawatt (TW) peak power pulse. The two resulting plasmas interacted at high velocity in a collisionless shock.

Neutron-yield diagnostics and x-ray spectral and imaging diagnostics were tested to evaluate the interaction region of the two counterpropagating plasma discs. Stimulated Raman scattering was measured from four laser probe beams.

"The experiment yielded excellent results," Park said. "The team observed a high number of neutrons and observed that neutrons came at a relatively late time, which may indicate that they were produced in a shock. We also observed strong x-ray brightening from hot plasmas in the center of the experiment that had never been seen previously. The backscatter measurements delivered good results as well."

Park added that the suite of diagnostics in the experiment performed extremely well, producing a copious amount of data that is now being processed. "With the neutron yield, the delayed neutron production and the x-ray brightening, we are studying whether these signals could be consistent from the shock," she said. "However, the team needs to confirm these results with physics ‘controlled reference’ shots with a single disc and non-deuterated discs. These shots are planned for this fall. Self-generated magnetic field measurements from the collisionless shock will be done when proton backlighter capability is available on NIF next year."

The ACSEL collaboration is led by LLNL, Princeton University, Osaka University, and Oxford University, with many other universities participating.