Mapping the self-generated magnetic fields due to thermal Weibel instability
Published in Proceedings of the National Academy of Sciences, 2022
Abstract: The origin of the seed magnetic field that is amplified by the galactic dynamo is an open question in plasma astrophysics. Aside from primordial sources and the Biermann battery mechanism, plasma instabilities have also been proposed as a possible source of seed magnetic fields. Among them, thermal Weibel instability driven by temperature anisotropy has attracted broad interests due to its ubiquity in both laboratory and astrophysical plasmas. However, this instability has been challenging to measure in a stationary terrestrial plasma because of the difficulty in preparing such a velocity distribution. Here, we use picosecond laser ionization of hydrogen gas to initialize such an electron distribution function. We record the 2D evolution of the magnetic field associated with the Weibel instability by imaging the deflections of a relativistic electron beam with a picosecond temporal duration and show that the measured k-resolved growth rates of the instability validate kinetic theory. Concurrently, self-organization of microscopic plasma currents is observed to amplify the current modulation magnitude that converts up to ~1% of the plasma thermal energy into magnetic energy, thus supporting the notion that the magnetic field induced by the Weibel instability may be able to provide a seed for the galactic dynamo.
Significance: Weibel instability driven by temperature anisotropy is thought to be an important mechanism for self-magnetization of many laboratory and astrophysical plasmas, yet its unambiguous demonstration remains a challenge. This work employs an experimental platform that allows us to “design” highly anisotropic electron velocity distributions using optical-field ionization of hydrogen gas and measure the subsequent self-organization of plasma currents and magnetic fields driven by Weibel instability with unprecedented spatiotemporal resolution using ultrafast electron probing. As the plasma thermalizes, a significant amount of electron energy is converted into magnetic energy, which supports the hypothesis that the Weibel instability may provide the seed that is amplified by the galactic dynamo to produce microgauss-level magnetic fields that exist in the cosmos.
Recommended citation: Chaojie Zhang, Yipeng Wu, Mitchell Sinclair, Audrey Farrell, Kenneth A. Marsh, Irina Petrushina, Navid Vafaei-Najafabadi, Apurva Gaikwad, Rotem Kupfer, Karl Kusche, Mikhail Fedurin, Igor Pogorelsky, Mikhail Polyanskiy, Chen-Kang Huang, Jianfei Hua, Wei Lu, Warren B. Mori, Chan Joshi, "Mapping the self-generated magnetic fields due to thermal Weibel instability," PNAS 119(50), e2211713119 (2022).
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