NSE-CSE PhD Thesis Defense | Zhuo Liu

Zhuo Liu, NSE-CSE PhD Thesis Defense Announcement

Thesis Title: Collisionless Plasma Physics below the Ion Scales: Instabilities, Reconnection, and Turbulence

Date:  Thursday, November 6th, 2025
Time: 2-4 PM ET
Location: NW17-218 / Zoom

Thesis Committee:

• Thesis advisor: Nuno F.G. Loureiro, Professor, Nuclear Science and Engineering, Herman Feshbach (1942) Professor of Physics, MIT
• Thesis reader: Muni Zhou, Assistant Professor, Department of Physics and Astronomy, Dartmouth College
• Committee member:  Jack Hare, Assistant Professor, School of Electrical and Computer Engineering, Cornell University
• Defense Chair:  Sophia Henneberg, Norman Rasmussen Career Development Professor, Assistant Professor, Nuclear Science and Engineering, MIT

Abstract:

Weakly collisional plasmas dissipate and reorganize energy primarily at and below ion kinetic scales, where wave-particle interactions, magnetic reconnection, and turbulence are intricately coupled.  This thesis advances a first-principles understanding of several fundamental processes via numerical simulations and analytical theories.

The first part of this thesis reexamines the long-standing hypothesis that ion-acoustic turbulence (IAT) provides the anomalous resistivity,  potentially responsible for fast, collisionless magnetic reconnection. Using first-principles Vlasov-Poisson simulations, we show that IAT-driven resistivity is transient and weak, insufficient to sustain reconnection electric fields or regulate outflows, thereby ruling out IAT as a possible source of anomalous resistivity for reconnection events.

The remainder of the thesis focuses on reconnection-mediated decaying turbulence below the ion scale.  To understand magnetic reconnection in this regime, we develop the first analytical theory of magnetic reconnection in sub-ion scales, which predicts reconnection rates that increase with decreasing structure size.  These predictions are verified through gyrofluid and fully kinetic particle-in-cell (PIC) simulations across a range of values of plasma $\beta$ and system sizes, establishing the first self-consistent theoretical framework for reconnection below ion scales.  

Building on this foundation, we investigate the interplay between magnetic reconnection and turbulence in strongly magnetized, low-$\beta$ plasmas.  We show that sub-ion-scale reconnection-mediated turbulence differs fundamentally from its magnetohydrodynamic (MHD) counterpart.   A new, time-dependent scaling theory of the sub-ion energy inverse cascade is derived and validated through gyrokinetic-reduced simulations, revealing that small-scale magnetic structures can undergo successive coalescence, enabling magnetic energy to transfer inversely toward system-size scales.  

Finally, we extend the study of decaying turbulence to the high-$\beta$, weakly magnetized regime relevant to magnetogenesis problem.  PIC simulations initialized with magnetic fields relevant to a Weibel-saturated state demonstrate that pressure-anisotropy-driven microinstabilities, particularly the firehose instability, remove magnetic tension, suppress reconnection-mediated coalescence, and ultimately arrest the inverse cascade.  This identifies a microphysical mechanism that limits the growth of large-scale magnetic fields from small-scale seeds and delineates the plasma conditions under which inverse transfer is either inhibited or permitted.

Together, these studies elucidate how kinetic turbulence, microinstabilities, and electron-scale reconnection collectively determine energy dissipation pathways and magnetic-field evolution in weakly collisional plasmas. The results have broad implications for heliospheric, magnetospheric, and astrophysical systems, including solar wind turbulence, planetary magnetotail dynamics, intracluster-medium heating, and the origin of cosmic magnetic fields.