The Crab Nebula, an interstellar cloud of gas and dust that formed in the wake of a supernova explosion, has a curious “kink” phenomenon that has been observed. Senior research scientist Chikang Li has been looking for answers ever since he became intrigued by this phenomenon, and he wants to experiment with the stars.
The Chandra X-ray observatory shows images of a jet of plasma pouring straight out from the neutron star at the center of the nebula. This jet appears to change direction every few years, without its structure changing. Scientists don’t know why this happens and have hypothesized that this behavior could be explained by magnetic fields with the right properties. This was not good enough for Li – he wanted proof.
Astrophysics is traditionally based on observation. After an observation is made, a theoretical model is typically built and some numerical simulations done. That’s about as far as you can go with this approach. Li was not happy with the status quo and wondered how an experiment could be designed on Earth to explain mysteries that are stretched over 13 light years of space and happen 6,500 light years away. He also wanted to know how one could go there and measure anything, and if an experiment can be done to test the model.
Li turned to the lab’s OMEGA laser to see if he could mimic the conditions in the Crab Nebula in order to prove (or disprove) the hypothesis that magnetic fields were responsible for the “kink in the crab”. If he were doing a fusion experiment, Li would focus the OMEGA’s multiple laser beams on a single pellet of hydrogen fuel. Instead, Li bounced lasers off two 3 x 3 mm foils hinged together at a 60-degree angle. He used two laser beams to heat each side, thereby generating plumes, or plasma bubbles. Li knew that these plumes would immediately expand and collide in the middle plane between the two foils to form a jet. The plumes react this way due to their density and heat.
Although laboratory generated jets and astrophysical jets have very different sizes, Li was aware that the critical dimensionless parameters are similar, ensuring that the fundamental physics is the same. The two types of jets share enough physical properties to allow Li to scale his laboratory experiments to conditions in the crab nebula. The same principle is used when one conducts experiments for an airplane in a wind tunnel.
Whereas it takes a few years to form the kink in the nebula jet, a jet is created in the laboratory experiment in one nanosecond (billionth of a second). The jet then spreads for another five to six nanoseconds. When he considers the speed of the experiments, Li laughs. He notes that he has to generate, diagnose, characterize and quantify a multitude of data in a very short time. Li’s division invented a mono-energetic proton radiography (MPR) diagnostic in 2005. He now uses this device to measure the magnetic fields generated by the experiment. This allows him to make a radiograph of the fields through the deflection of the protons.
From the quantitative measurements, Li has been able to prove that the nebula jet behavior is governed by weak magnetic fields. Some of these fields circle around the jet, creating the instability responsible for the directional change, while others are oriented along the jet, keeping its structure mostly straight. Nature Communications recently published an article with these results.
Richard Petrasso, the head of the HEDP division, noted the importance of Li’s work. He explains that through Li’s development of the MPR diagnostic used to map transient magnetic fields in the laboratory and his understanding of instabilities, Li has been able to explore and explain such puzzling phenomena as the jetting in the Crab Nebula for the first time.
Li is now extending this methodology to a range of other astrophysical phenomena with the help of graduate students working under his supervision. The phenomena they are exploring includes the observation of collisionless shocks and their associated magnetic fields, and the turbulent generation and amplification of magnetic fields.
With the team being able to create measurable astrophysical conditions on earth using the National Ignition Facility and OMEGA lasers, the sky is no longer the limit!